Previous seminars in Kapitza Institute for Physical Problems

A Josephson relation for e/3 and e/5 fractionally charged anyons

23 May 2019 in 11:30

D. Christian Glattli (Nanoelectronics Group, Service de Physique de l’Etat Condensé, CEA Saclay, France)

Anyons occur in two-dimensional electron systems as excitations with fractional charge in the topologically ordered states of the fractional quantum Hall effect (FQHE). Their dynamics are of utmost importance for topological quantum phases and possible decoherence-free quantum information approaches, but observing these dynamics experimentally is challenging. Here, we report on a dynamical property of anyons: the long-predicted [1] Josephson relation f_J = e*V/h for charges e* = e/3 and e/5, where e is the charge of the electron and h is Planck’s constant [2]. The relation manifests itself as marked signatures in the dependence of photo-assisted shot noise (PASN) [3-4] on voltage V when irradiating contacts at microwaves frequency f_J [4]. The validation of FQHE PASN models indicates a path toward realizing time-resolved anyon sources based on levitons. The method may be of interest to provide a demonstration of anyonic statistics, a pre-requisite for topological quantum computing. [1] X. G. Wen, Edge transport properties of the fractional quantum Hall states and weak-impurity scattering of a one-dimensional charge-density wave, Phys. Rev. B 44, 5708–5719 (1991). [2] M. Kapfer, P. Roulleau, M. Santin, I. Farrer, D. A. Ritchie, and D. C. Glattli, A Josephson relation for fractionally charged anyons, Science 363, 846–849 (2019). [3] G. B. Lesovik and L. S. Levitov, Noise in an ac biased junction: Nonstationary Aharonov-Bohm effect, Phys. Rev. Lett. 72, 538–541 (1994). [4] C. de C. Chamon, D. E. Freed, and X. G. Wen, Tunneling and quantum noise in one-dimensional Luttinger liquids, Phys. Rev. B 51, 2363–2379 (1995).

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On energy landscape of elastic manifolds pinned by random potentials

11 April 2019 in 11:30

Yan Fyodorov (King's College London)

We consider an elastic manifold of internal dimension d and length L pinned in a N dimensional random potential and confined by an additional parabolic potential of curvature μ. First we consider N = d = 1 case of a directed polymer and show how counting stationary points of its energy provides an upper bound on the strength of the depinning force. Then we consider a mean-filed limit: first N→∞ at fixed L^d, and then L→∞, and find the mean spectral density ρ(λ) of the Hessian matrix K at the absolute minimum of the energy functional. We show that for a confinement curvature μ exceeding a critical value, the so-called "Larkin mass", the system is replica-symmetric and the Hessian spectrum is always gapped (from zero). The gap vanishes quadratically at approaching the Larkin mass. For smaller curvatures the replica symmetry breaking (RSB) occurs and the Hessian spectrum is either gapped or extends down to zero, depending on whether RSB is 1-step or full. In the 1-RSB case the gap vanishes in all d as fourth power of the distance to the transition. In the full RSB case the gap is identically zero. The presentation will be based on Y.V. Fyodorov et al., Ann. Phys. 397, 1–64 (2018) and Y.V. Fyodorov and P. Le Doussal, arXiv:1903.07159.

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Unconventional magnons and their impact on spin pumping transport

6 December 2018 in 11:30

Wolfgang Belzig (University of Konstanz, Germany), Akashdeep Kamra (NTNU Trondheim, Norway)

Exotic quasiparticles have been observed in complex spin systems exhibiting spin ice rules, skyrmions and so on. Here, we discuss the emergence of novel quasiparticles, mediated by magnetic dipolar interactions that have been hiding in simpler spin systems with uniformly ordered ground states. Amongst other properties, these quasiparticles exhibit a spin ranging from zero to above 1ℏ [1]. Of particular interest is our finding that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin-1 magnons [2]. These unusual properties originate from a competition between quantum mechanical squeezing (increasing the spin) and hybridization (decreasing the spin).
We then present a theoretical study of spin transport across a ferrimagnet/non-magnetic conductor interface, when a magnetic eigenmode is driven into a coherent state. In the simple case of ferromagnets with non-integer “effective spin” above 1ℏ, we show that spin-current noise measurement can reveal this fundamental quantum phenomenon [1]. This is in full analogy to the effective charge known e.g. in the fractional quantum Hall regime, which has been experimentally determined via shot noise measurements.
Furthermore, we extend our model to continuously encompasses systems from ferromagnets to antiferromagnets [3] and include novel dissipation terms [4], thereby allowing analytical results for the full range of materials within a unified description. We also allow arbitrary interfaces (disordered and asymmetric). The obtained spin current expression includes intra- as well as cross-sublattice terms. We find that the cross-sublattice terms, disregarded in previous studies, play an important role and result in qualitative changes to our understanding of spin pumping in antiferromagnets. The dc current is found to be sensitive to the asymmetry in interfacial coupling between the two sublattice magnetizations and the mobile electrons, especially for antiferromagnets.
References:
[1] A. Kamra and W. Belzig, Super-Poissonian shot noise of squeezed-magnon mediated spin transport, Phys. Rev. Lett. 116, 146601 (2016).
[2] A. Kamra, U. Agrawal, and W. Belzig, Noninteger-spin magnonic excitations in untextured magnets, Phys. Rev. B 96, 020411(R) (2017).
[3] A. Kamra and W. Belzig, Spin pumping and shot noise in ferrimagnets: bridging ferro- and antiferromagnets, Phys. Rev. Lett. 119, 197201 (2017).
[4] A. Kamra, R. E. Troncoso, W. Belzig, and A. Brataas, Gilbert damping phenomenology for two-sublattice magnets, arXiv:1808.04385 (to appear in Phys. Rev. B).

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Josephson and non-Josephson emission from Bi₂Sr₂CaCu₂O_8+δ mesa structures

22 November 2018 in 11:30

Vladimir Krasnov (Stockholm University, Sweden)

Mesa structures made of Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. When voltage is applied to such mesas, they can generate various types of waves (photons, phonons, polaritons, magnons, plasmons) in a variety of different ways. In this talk I will overview both Josephson and non-Josephson mechanisms of emission, including the standard ac-Josephson effect for emission of electromagnetic waves [1], segnetoelectric effect for emission of phonons and polaritons [2] and non-equilibrium quasiparticle relaxation and recombination for emission of any type of bosons, having strong electron-boson interaction and participating in pairing [3]. Those effects are important both for applied research, e.g., creation of tunable, compact, continuous wave and monochromatic THz source with a frequency span in the whole THz gap region and beyond 0.1-15 THz [1,2], and for fundamental understanding of the mechanism of pairing in high-temperature superconductors [3].
References:
[1] E. A. Borodianskyi and V.M. Krasnov, Josephson emission with frequency span 1–11 THz from small Bi₂Sr₂CaCu₂O_8+δ mesa structures, Nature Commun. 8, 1742 (2017).
[2] S. O. Katterwe, H. Motzkau, A. Rydh, and V. M. Krasnov, Coherent generation of phonon-polaritons in Bi₂Sr₂CaCu₂O_8+x intrinsic Josephson junctions, Phys. Rev. B 83, 100510(R) (2011).
[3] V.M. Krasnov, S.O. Katterwe, & A. Rydh, Signatures of the electronic nature of pairing in high-Tc superconductors obtained by non-equilibrium boson spectroscopy, Nature Commun. 4, 2970 (2013).

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Velocity distribution functions and intermittency in one-dimensional randomly forced Burgers turbulence

25 October 2018 in 11:30

Viktor Dotsenko (Universite Jussieu, Paris)

The problem of one-dimensional randomly forced Burgers turbulence is considered in terms of (1+1) directed polymers. In the limit of strong turbulence (which corresponds to the zero temperature limit for the directed polymer system) using the replica technique a general explicit expression for the joint distribution function of two velocities separated by a finite distance is derived. In particular, it is shown that at length scales much smaller than the injection length of the Burgers random force the moments of the velocity increment exhibit typical strong intermittency behavior. Literature: J.Stat.Mech., 083302 (2018); arXiv:1804.08294.

Post-Ehrenfest many-body quantum interferences in ultracold atoms far-out-of-equilibrium

6 September 2018 in 11:30

Denis Ullmo (LPTMS, CNRS-Universite Paris-Sud)

Recent experimental progress with ultracold atomic gases has made it possible to investigate in exquisite detail the far out-of-equilibrium many-body quantum dynamics of isolated systems. This dynamics necessarily generates interferences beyond an Ehrenfest time scale, where quantum and classical expectation values diverge. Theoretically speaking, the heavily-relied-upon truncated Wigner approximation leaves out these interferences. In this talk, I will present a semiclassical theory which bridges classical and quantum concepts in many-body bosonic systems and properly incorporates such missing quantum effects. For mesoscopically populated Bose-Hubbard systems, this theory captures post-Ehrenfest quantum interference phenomena very accurately.

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Spontaneous Time-Reversal Breaking and Edge Reconstruction in Quantum Hall and Topological Insulators

5 July 2018 in 17:10

Yigal Meir (Department of Physics, Ben Gurion University of the Negev, Israel)

Topological states, such as the quantum Hall state or the quantum spin Hall state, are usually bulk insulators, with helical edge states that may carry current. The edge structure of such two dimensional systems is usually studied with sharp boundary conditions in spite of the fact that the confining potential in physical systems is expected to be smooth. It is shown that such a smooth confining potential may lead to edge reconstruction and formation of additional edge states. This is demonstrated explicitly for the case of integer and fractional quantum Hall systems, explaining several recent experimental puzzles. Moreover, the effect is also manifested in two-dimensional topological insulators (TIs), which are predicted to support helical edge modes that come in counter-propagating pairs, due to the time-reversal symmetry (TRS). The TRS protection of these edge states led to various suggested applications of TIs, ranging from spintronics to quantum computation. Here edge reconstruction leads to spontaneous TRS breaking, a finite Hall resistance at zero magnetic field and possible spin current. Such spontaneous TRS breaking may have important implications on transport properties and possible applications.

Spin liquids from Majorana Zero Modes in a network of Cooper Boxes

5 July 2018 in 16:00

Yuval Oreg (Weizmann Institute of Science, Israel)

Spin liquid phases are insulating states of matter with unique properties. In certain cases the phase hosts edge modes, end modes, and emergent non-abelian quasiparticles. The latter is a key element in several suggestions for topological quantum computation. In this talk, I`ll describe a proposal to construct a platform for creating effective spin models using semiconductor nanowires. The wires are tuned to the topological regime; with Majorana zero modes on each end. We group them into three-wires building blocks called hexons, each containing six Majorana zero modes. In the presence of a strong charging energy, the hexon becomes a Cooper box that is equivalent to two spin-1/2 degrees of freedom. This structure enables a flexible control (using local gates only) of the couplings between the Majorana zero modes. This tuning of the Hamiltonian governing the low energy effective spins, provides us with a path of simulating interacting spin-models in one- and two-dimensions. I will describe several examples including realizations of different phases of 1/2 Heisenberg spin chains, topological spin phases on a two dimensional Fisher lattice and their experimental signature.

Large deviations of surface height in the Kardar-Parisi-Zhang equation

31 May 2018 in 11:30

Baruch Meerson (Hebrew University of Jerusalem)

The Kardar-Parisi-Zhang (KPZ) equation describes an important universality class of nonequilibrium stochastic growth. There has been a surge of recent interest in the one-point probability distribution P(H,t) of height H of the evolving interface at time t in one dimension. I will show how one can use the optimal fluctuation method (OFM) to evaluate P(H,t) for different initial conditions and in different dimensions. In one dimension the central part of the short-time height distribution is Gaussian, but the tails are non-Gaussian and strongly asymmetric. One interesting initial condition is an ensemble of Brownian interfaces, where we found a singularity of the large deviation function of the height at a critical value of |H|. This singularity results from a breakdown of mirror symmetry of the optimal path of the system, and it has the character of a second-order phase transition. At d>2 the OFM is valid, in the weak-coupling regime, at all times. Here the long-time height distribution P(H) is time-independent, and we use the OFM to determine the Gaussian body and strongly asymmetric non-Gaussian tails of P(H).

Duality in Power-Law Localization in Disordered One-Dimensional Systems

22 March 2018 in 11:00

V.E. Kravtsov (Abdus Salam ICTP, Trieste)

The transport of excitations between pinned particles in many physical systems may be mapped to single-particle models with power-law hopping, 1/r^α. For randomly spaced particles, these models present an effective peculiar disorder that leads to surprising localization properties. We show that in one-dimensional systems almost all eigenstates (except for a few states close to the ground state) are power-law localized for any value of α > 0. Moreover, we show that our model is an example of a new universality class of models with power-law hopping, characterized by a duality between systems with long-range hops (α < 1) and short-range hops (α > 1), in which the wave function amplitude falls off algebraically with the same power γ from the localization center. The paper has just been published: X. Deng, V. E. Kravtsov, G. V. Shlyapnikov, and L. Santos, Phys. Rev. Lett. 120, 110602 (2018).

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Quantum simulation of Luttinger liquid physics in Josephson transmission lines

25 January 2018 in 11:30

Vladimir Manucharyan (University of Maryland)

Here we present the first quantum simulator for an impurity scattering in interacting 1D wires. The simulator consists of a transmission line made out of more than 30,000 Josephson junctions serving as a high-impedance media for microwave photons and a small phase slip Josephson junction playing the role of a back-scattering impurity. The system can be described by a boundary sine-Gordon model where the interaction strength is defined as g = Z/R_q with Z being the transmission line impedance and R_q = 6.5 kOhm the resistance quantum. By measuring scattering amplitudes and a spectrum of inelastically scattered microwave photons we can find the first and higher order correlation functions related to an AC conductance of the impurity. The controllability of the transmission line parameters and the finite size of the system allow us to fabricate lines with impedances exceeding R_q while keeping the phase slip rate of the line’s junctions very low. It gives us the unique opportunity to test Luttinger liquid physics at both sides of the critical point g = 1. A similar experimental setup can be used to simulate a Kondo impurity.

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Interaction induced topological phases in one dimension

14 September 2017 in 11:30

Sam Carr (University of Kent, England)

We consider a one-dimensional single channel quantum wire with a spin gap but gapless charge excitations. We show that the spin gap can be generated in two different ways, one of which has non-trivial topological properties. This topology manifests itself in two ways - firstly in the existence of gapless single-particle edge states, and secondly in an insensitivity of the wire to weak impurities reminiscent of the helical edge states of two dimensional topological insulators. We will demonstrate a number of ways such a phase of matter can be engineered, including spin-orbit quantum wires or two coupled helical edge states. If time permits, we will also touch on recent work concerning strong impurities in such systems. References: [1] Kairanis and Carr, PRB 92, 035139 (2015). [2] Santos, Gutman and Carr, PRB 93, 235436 (2016). [3] Kainaris, Santos, Gutman and Carr, Forschr. Phys. 1600054 (2017). [4] Kainaris, Carr and Mirlin, coming soon.

Together we can: transport properties of electron viscous flows

27 April 2017 in 11:30

L. Levitov (MIT)

Strongly interacting electrons can move in a neatly coordinated way, reminiscent of the movement of viscous fluids. In viscous electron flows interactions give rise to a collective behavior, facilitating transport and allowing conductance to exceed the fundamental Sharvin-Landauer ballistic limit G_ball. This talk will describe a theory of the ballistic-to-viscous crossover in a constriction exhibiting the ballistic transport at T = 0 but governed by electron hydrodynamics at elevated temperatures. An approach based on quasi-hydrodynamic variables predicts an additive relation G = G_ball + G_vis, where the viscous contribution G_vis dominates over G_ball in the hydrodynamic limit. We will also discuss recent measurements of electron transport through graphene constrictions, finding that conductance below 150 K increases with increasing temperature. The measurements help to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, this work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behavior for designing graphene-based devices.

Replica approach with one step replica symmetry breaking in the problem of Anderson localization on the Bethe lattice

29 December 2016 in 11:30

V.E. Kravtsov (Abdus Salam ICTP, Trieste)

We formulate general criteria for localized, extended ergodic and extended non-ergodic (multifractal) phases in disordered quantum systems and apply these criteria to the problem of Anderson localization on Bethe lattice, random regular graphs and generalized Rosenzweig-Porter random matrix ensemble. We focus on the replica symmetry breaking approach to the problem and show how this approach provides a natural classification of phases and phase transitions.

Finite-temperature conductance of strongly interacting quantum wire with a helical nuclear spin order

15 December 2016 in 11:30

Pavel Aseev (University of Basel)

We study the temperature dependence of the electrical conductance of a clean strongly interacting quantum wire in the presence of a helical nuclear spin order. The nuclear spin helix opens a temperature-dependent partial gap in the electron spectrum. Using a bosonization framework we describe the gapped electron modes by sine-Gordon-like kinks. We predict an internal resistivity caused by an Ohmic-like friction these kinks experience via interacting with gapless excitations. As a result, the conductance rises from G=e^2/h at temperatures below the critical temperature when nuclear spins are fully polarized to G=2e^2/h at higher temperatures when the order is destroyed, featuring a relatively wide plateau in the intermediate regime. The theoretical results are compared with the experimental data for GaAs quantum wires obtained recently by Scheller et al. [Phys. Rev. Lett. 112, 066801 (2014)]. (arXiv:1611.10238)

Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi₂Se₃

1 December 2016 in 11:30

S.V. Zaitsev-Zotov (Kotel’nikov IRE RAS, Moscow)

Results of detailed study of the topological insulator Bi₂Se₃ surface state energy structure in the vicinity of surface steps by scanning tunneling microscopy (STM) and spectroscopy (STS) methods are described. Increase of the chemical potential in the vicinity of the step edge is observed. The value of the increase is found to correlate with the step height and is caused by redistribution of electron wave functions between outer and inner edges of surface steps, as it is known for usual metals, as well as by presence of dangling bonds on the step. Smaller value of the shift and its larger characteristic length reflect specifics of the helical surface states. This increase is accompanied by enhancement of the relative value of the differential tunneling conductance, dI/dV, at the Dirac point and thereby produces an illusion of appearance of edge states. We show that the enhancement is reproduced in the framework of the tunneling model, which takes into account the dependence of the tunneling gap transparency on the voltage. References: [1] N.I. Fedotov, S.V. Zaitsev-Zotov, JETP Letters, 104, #11 (2016) (in press); arXiv:1609.08294. [2] N.I. Fedotov, S.V. Zaitsev-Zotov, arXiv:1609.08911.

Dissipation-induced topological insulators: A recipe

22 September 2016 in 11:30

Moshe Goldstein (Tel-Aviv University)

It has recently been realized that driven-dissipative dynamics, which usually tends to destroy subtle quantum interference and correlation effects, could actually be used as a resource. By proper engineering of the reservoirs and their couplings, one may drive a system towards a desired quantum-correlated steady state, even in the absence of internal Hamiltonian dynamics. An intriguing class of quantum phases is characterized by topology, including the quantum Hall effect and topological insulators and superconductors. Which of these noninteracting topological states can be achieved as the result of purely dissipative Lindblad-type dynamics? Recent studies have only provided partial answers to this question. In this talk, I will present a general recipe for the creation, classification, and detection of states of the integer quantum Hall and 2D topological insulator type as the outcomes of coupling a system to reservoirs, and show how the recipe can be realized with ultracold atoms and other quantum simulators. The mixed states so created can be made arbitrarily close to pure states, and the construction may be generalized to other topological phases.

Spiraling energy dispersion of arc states in Weyl semimetals

15 September 2016 in 11:30

Anton Andreev (University of Washington, Seattle,USA)

Weyl semimetals are recently discovered materials in which the valence and conduction bands touch at isolated points (Weyl nodes) in the Brillouin zone. This gives rise to unusual electronic properties of these materials. In particular, Weyl semimetals host peculiar surface electron states whose Fermi lines are shaped as open arcs. I will show that static electric fields that are necessarily present near the crystal surface result in a spiraling structure of Fermi arcs. The winding angle of the spiral is controlled by the chirality of the Weyl node and the magnitude of the surface potential. I will also discuss magnetoresistance of a pn-junction in a Weyl semimetal.

Random matrix approach to the jamming transition

9 June 2016 in 11:30

Ya.M. Bel’tyukov (FTI im. A.F. Ioffe RAN, Sankt-Peterburg)

Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can jam into a solid-like disordered state. In this state each particle has a certain equilibrium position and can vibrate around it. In this talk, I will show that the dynamical matrix M describing harmonic oscillations in such media can be represented in the form M = AA^T. The rows of the matrix A correspond to the degrees of freedom of individual granules and its columns correspond to elastic contacts between granules. This representation allows to apply the random matrix theory and estimate the vibrational density of states. The resulting vibrational density of states is approximately constant over a wide frequency range which is determined mostly by the ratio of the number of degrees of freedom and the total number of elastic contacts in the system. The results are in a good agreement with numerical experiments performed by various authors.

Majorana fermion from Landau quantization in 2D topological superconductors

2 June 2016 in 11:30

A.V. Rozhkov (MFTI; ITPE RAN; VNIIA im. Dukhova)

We study the generation of Majorana fermions in a two-dimensional topological superconductor placed in a transverse magnetic field B. A topological insulator/superconductor heterostructure and a two-dimensional p-wave superconductor are discussed. It is demonstrated that in these systems a single vortex creates two Majorana fermions, one hosted at the vortex core. The wave function of the second Majorana state is localized in the superconductor volume along a circle of radius r^∗∝B⁻¹ centered at the vortex core. In the case of many vortices, the sensitivity of r^∗ to the magnetic field B may be used to control the coupling between the Majorana fermions. The latter property could be an asset for quantum computations.

Electro-Convective Instability in Concentration Polarization

28 April 2016 in 11:30

Boris Zaltzman (Blaustein Institutes for the Desert Research, Ben-Gurion University of the Negev)

DC ionic current in a binary electrolyte passing through a perm-selective interface (electrode, ion exchange membrane, micro-nano-channel junction) is a basic element of many electrochemical engineering and micro-fluidic processes, such as electrodeposition, electrodialysis and protein pre-concentration. Such current passage is diffusion-limited in the sense that it induces a decrease of electrolyte concentration towards the interface, known as concentration polarization (CP), whose expression is the saturation of current upon increasing voltage at some value known as the limiting current. Upon a further increase of voltage, this saturation is followed by a relatively rapid current increase — the so-called over-limiting conductance (OLC) regime. The mechanism of OLC remained unexplained for a long time. Only recently was it shown that in open systems OLC is due to the destruction of the diffusion layer by a micro-scale vortical flow which spontaneously develops as a result of Electro-Convective Instability near the limiting current and provides an additional ionic transport mechanism. This instability has attracted interest of both theoreticians and experimentalists. Commonly, it has been attributed to non-equilibrium electro-osmosis (EO) related to the extended space charge (ESC), which develops at the limiting current. One reason for this attribution was the realization that for a perfectly perm-selective solid, neither equilibrium EO nor bulk electric force can yield Electro-Convective Instability. On the other hand, it has been shown that non-equilibrium EO can. In the early studies, perfect perm-selectivity was assumed for the sake of simplicity. The subsequent studies of various time-dependent and nonlinear features of EO instability continued to make this assumption, while recognizing that the ESC is essentially identical for a perfect and non-perfect interface. Recently, we reported that relaxing the assumption of perfect perm-selectivity allows for equilibrium Electro-Convective Instability, irrespectively of any ESC effects. Generally, electro-convection is driven by the electric force acting both upon the space charge of an interfacial electric double layer (EDL) and the residual space charge of the quasi-electroneutral bulk. A slip-like flow induced by the former is EO, whereas the flow induced by the latter is bulk electroconvection. There are two regimes of EO that correspond to different states of the EDL and are controlled by the non-equilibrium voltage drop across it: equilibrium EO and non-equilibrium EO. While both regimes result from the action of a tangential electric field upon the space charge of the EDL, the former relates to the charge of the equilibrium EDL, whereas the latter relates to the ESC of the non-equilibrium EDL. For a perfectly perm-selective interface CP under the equilibrium EO slip condition is stable. So it is for bulk electro-convection in a low molecular electrolyte. As was recently shown, imperfect perm-selectivity makes equilibrium instability possible, driven by either equilibrium EO or bulk electro-convection, or both. Thus, depending on perm-selectivity, the Electro-Convective Instability in CP may be either driven or inhibited by several factors, such as equilibrium or non-equilibrium EO, bulk electroconvection, diffusio-osmosis, etc. The first results on identifying and analyzing the interplay of these major ’surface’ and ’bulk’ factors for varying perm-selectivity of the interface will be presented.

Electronic phase transitions induced by electric or optical impacts

4 February 2016 in 11:30

Serguei Brazovskii

Controlled transformations of electronic states or even of whole phases are achievable today by impacts of very strong electric fields and/or the ultrafast optical pumping. The experimental success is coming from ferroelectrically and ionically enhanced field effect in high-Tc superconductors, induced metallization in oxides of transition metals and in organic materials, field-effect superconductivity in MBE monolayers. The techniques of the femto-second optical pumping span from the purely optical setups to the newer time-resolved photoemission spectroscopy and to the latest time-sliced diffraction. The tested electronic phases include: superconductivity, charge density waves, charge ordering, ferroelectricity, magnetic phases, Peierls and Mott insulators. A super goal is to attend “hidden” states which are inaccessible and even unknown under equilibrium conditions. Such a bistable switching has been achieved in a “polaronic Wigner-crystalline Mott insulator” 1T-TaS2. After the experimental review, some time will be devoted to a phenomenological theory (collaboration with N. Kirova). Thus, the modeling of the quasi-condensate of excitons interacting with the order parameter recovers the dynamical realization of the “excitonic insulator” state and spacio-temporal patterns with self-focusing, domains segregation, and local dynamical phase transitions.

Recent identifications of microscopic solitons in quasi 1D electronic systems and generalisations to higher dimensions

10 December 2015 in 11:30

Serguei Brazovskii

We review a progress in experiments and theory, elucidating the role of microscopic solitons in quasi-1D electronic systems with a symmetry breaking. The new interest rises from studies of the «electronic ferroelectricity» in organic conductors, and from nano-scale tunneling experiments in Charge Density Wave (CDW) materials. Individual solitons have been visually captured in recent STM experiments. On this basis we extrapolate to a picture of combined topological excitations in general strongly correlated systems: from doped antiferromagnets to strong coupling and spin-polarized superconductors. At more macroscopic scales, we recover the electronic vortices generated in mesa-junctions, and domain walls evolving in femtosecond pump-probe experiments.

Thermal transport in the disordered electron liquid

12 November 2015 in 11:30

Georg Schwiete (SPICE and Johannes Gutenberg Universität Mainz, Germany)

In this talk, I will present a theoretical study of thermal transport in the disordered two-dimensional electron liquid. At temperatures smaller than the impurity scattering rate, in the diffusive regime, thermal conductivity acquires non-analytic quantum corrections. Our approach to this problem is based on an analysis of the heat density-heat density correlation function. To this end, Luttinger’s gravitational potentials are introduced in the action as sources that couple to the heat density. In a two-stage procedure, a renormalization group calculation based on the Keldysh non-linear sigma model in the presence of Luttinger’s gravitational potentials is supplemented with a perturbative study of scattering processes induced by the Coulomb interaction in the sub-temperature energy range. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. As an application, I intend to discuss thermal transport on the metallic side of the metal-insulator transition in Si MOSFETs. References: G. Schwiete and A.M. Finkel’stein, PRB 90, 060201 (2014); PRB 90, 155441 (2014); arXiv:1509.02519; arXiv:1510.06529.

The Essentially Entangled Component of Multipartite Mixed Quantum States, its Properties and an Efficient Algorithm for its Extraction

22 October 2015 in 11:30

V.M. Akulin, G.M. Kabatyanski, A.Mandilara

We introduce a density matrix decomposition of a multipartite quantum system of a finite dimension into two density matrices: a separable one and an essentially entangled one, which contains no product states components. This convex decomposition can be achieved in practice with the help of an algorithm based on linear programming, which in the general case scales polynomially with the dimension of the system. We prove that the rank of the essentially entangled component is always lower than that of the initial density matrix and we give an upper bound for this rank. We illustrate the algorithm at an example of a composed system of total dimension 12 undergoing loss of coherence due to classical noise and we trace the time evolution of its essentially entangled component. We suggest a geometric description of entanglement dynamics and show how it explains the well-known phenomena of sudden death and revival of multipartite entanglement. Interestingly, while we observe a weight loss of the essentially entangled component with time, its average entanglement content is not affected by the coherence loss.

Occurrence of flat bands in strongly correlated Fermi systems and high-T_c superconductivity of electron-doped compounds

26 February 2015 in 11:30

V.A. Khodel (Kurchatov Institute & Washington University)

We consider a class of strongly correlated Fermi systems that possess interaction-induced flat bands, pinned to the Fermi surface. We demonstrate that in such systems, the fundamental Landau equation, connecting the single-particle spectrum to the quasiparticle momentum distribution, fails. We propose a method, allowing to rectify drawbacks of Landau theory and, with the aid of the Pitaevskii identity, generalize equations obtained to apply the method to electron systems of solids. The emergent non-Fermi-liquid behavior, derived from the theory constructed, is compared with relevant experimental data on two-dimensional liquid He-3, heavy-fermion metals and electron-doped high-T_c compounds.

Topological Valley Currents in Gapped Dirac Materials

19 February 2015 in 11:30

L. Levitov (MIT)

Gapped 2D Dirac materials, in which inversion symmetry is broken by a gap-opening perturbation, feature a unique valley transport regime. The system ground state hosts dissipationless persistent valley currents existing even when topologically protected edge modes are absent or when they are localized due to edge roughness. Topological valley currents in such materials are dominated by bulk currents produced by electronic states just beneath the gap rather than by edge modes. Dissipationless currents induced by an external bias are characterized by a quantized half-integer valley Hall conductivity. The under-gap currents dominate magnetization and the charge Hall effect in a light-induced valley-polarized state.

Dissipation of turbulence in superfluid 4He in the limit of zero temperature

30 October 2014 in 11:30

Andrei Golov (School of Physics and Astronomy, The University of Manchester, UK)

We will review Manchester experiments on quantum turbulence in superfluid 4He generated by various means. The main focus is on the limit of zero temperatures, in which the turbulence is fully represented by tangled quantized vortex lines. In this regime, the dynamics of the vortex lines span length scales from the size of container to nearly atomic scale, at which energy is transmitted to phonons. A quantum cascade of energy, involving individual quantized vortex lines (and hence, having no analogs in classical turbulence), is necessary for the energy of large-scale flow to be transferred to the dissipative short scales. The processes maintaining the cascade are believed to be: hydrodynamic interactions of vortex lines, reconnections of vortex lines, interaction of excitations such as small vortex loops and Kelvin waves on vortex lines. The dissipative length scale can be increased (i.e. the quantum cascade curtailed) by increasing temperature above 0.4 K due to the scattering of thermal excitations by vortex lines. Three different means of generating turbulence will be discussed (each having an analog in classical turbulence, thus allowing comparison with classical results): (a) With the towed grid, we produce turbulence that is nearly homogeneous and isotropic. Studying the free decay allows the rate of dissipation to be quantified [6]. (b) An unsteady rotation of a square-shaped container allows to create anisotropic turbulence — that decays more slowly [1,5,6]. In the presence of the steady background rotation, the following phenomena are observed: steady polarization of vortex lines, non-zero threshold for turbulence onset, resonances of inertial waves [4]. (c) With an immersed jet of variable duration and intensity (created by a current of injected electrons), a cross-over from ultraquantum («non-structured» or «Vinen») turbulence to quasiclassical («structured» or «Kolmogorov») turbulence is observed [2,3]. (References: [1] P. M. Walmsley, A. I. Golov, H. E. Hall, A. A. Levchenko and W. F. Vinen, Dissipation of quantum turbulence in the zero-temperature limit, Phys. Rev. Lett. 99, 265302 (2007). [2] P. M. Walmsley and A. I. Golov, Quantum and quasiclassical types of superfluid turbulence, Phys. Rev. Lett. 100, 245301 (2008). [3] A. I. Golov, P. M. Walmsley, P. Tompsett, Charged Tangles of Quantized Vortices in Superfluid 4He, J. Low Temp. Phys. 161, 509-525 (2010). [4] P. M. Walmsley and A. I. Golov, Rotating quantum turbulence in superfluid 4He in the T=0 limit, Phys. Rev. B 86, 060518(RC) (2012). [5] Paul Walmsley, Dmitry Zmeev, Fatemeh Pakpour, and Andrei Golov, Dynamics of quantum turbulence of different spectra, PNAS 111 (Supplement 1) 4691-4698 (2014). [6] D. E. Zmeev, P. M. Walmsley, A. I. Golov, P. V. E. McClintock, S. N. Fisher, and W. F. Vinen, Turbulence in superfluid 4He generated by various means (in preparation).)

Electron-electron interaction effects in monotonic and oscillatory magnetotransport in 2D electron systems

23 October 2014 in 11:30

V.M. Pudalov (LPI)

Studying the oscillatory magnetorestance in crossed fields, we found that the product (m^*T_D) that determines damping of quantum oscillations, to the first approximation is equal in the majority and minority subbands even though the spin polarization degree amounts to about 66%. This result confirms theory predictions that the interaction takes place at high energies > E_F rather than within narrow strip of energies E_F ± T. To the next approximation, we revealed a difference in the damping factor of the two spin subbands, which causes skewness of the oscillation lineshape. The difference, quantified with the skew factor can be as high as 20%. The skew factor decreases as B_∥ or temperature grow, or B_⊥ decreases; for low electron densities and high in-plane field the skew factor even changes sign. In contrast to the conventional theory, the product (m^*T_D) varies with perpendicular field and (nonmonotonically) with temperature. These dependencies explain notable scattering of the m^*(n) values experimentally obtained under assumption of the T-independent (m^*T_D). The monotonic magnetoconductance in the in-plane field δσ(B,T) was found to scales in a sharp contrast to the theory predictions: whereas below a density dependent temperature T<T^* it scales as theory predicted, (B²/T), at higher temperatures T>T^*, it scales as (B²/T²). The latter dependence, hence, mimics the behavior anticipated for the low-temperature diffusive regime of interaction. These functional dependencies are at odd with interaction quantum corrections. The crossover temperature T^*(n) correlates well with the inflection point at the strong σ(T) dependence in zero field, inherent for high mobility samples. Our data thus point at the existence of an energy scale T^* beyond the E_F. The results also call in question the previous attempts to exploit the parallel field MR as a tool to determine Fermi liquid coupling constants, F₀^a and γ₂, and to plot the two-parameter phase diagrams for interacting and disordered 2D electron systems.

Entropy measurements in 2D systems

18 September 2014 in 11:30

A.Yu. Kuntsevich (LPI)

In this talk I am going to discuss the experimental technique to measure entropy-per-electron in gated 2D systems, the we recently developed. We applied this technique to study magnetooscillations in Si and GaAs-based 2D systems and zero-perpendicular field effects. The latter originate from: (i) non-degeneracy of the Fermi system and (ii) electron-electron correlations. In particular, we explore enhanced effective mass in non-degenerate strongly correlated plasma regime E_F < T < U (where U is the Coulomb energy e² / ‹ r ›).

Can liquid water unmix?

26 June 2014 in 11:30

M.A. Anisimov (University of Maryland, USA)

Twenty years ago Poole et al. suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of metastable liquid-liquid separation of lower-density and higher-density water [1]. This phenomenon is viewed as “water’s polyamorphism”. In this presentation I explain how supercooled water can unmix and describe a phenomenological model in which liquid water at low temperatures is viewed as athermal “solution” of two hydrogen-bond network structures with different entropies and densities [2,3]. Alternatively to lattice-gas models, in which fluid phase separation is driven by energy, the phase transition in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the “reaction” equilibrium constant. The order parameter, the extent of “reaction”, coupled with density and entropy, while the ordering field is a combination of temperature and pressure. The model gives the best representation of all available experimental data for supercooled ordinary and deuteraded water and predicts the location of density maxima at the locus of a near-constant fraction of the lower-density structure. I will also discuss the lessons we can learn on real water from simulations of popular water-like models, such as mW and ST2 [4,5]. ( References: [1] P. H. Poole, F. Sciortino, U. Essmann, and H. E. Stanley, Nature 360, 324 (1992). [2] C. E. Bertrand and M. A. Anisimov, Phys. Chem. B 115, 14099 (2011). [3] V. Holten and M. A. Anisimov, Sci. Rep. 2, 713 (2012). [4] V. Holten, D. T. Limmer, V. Molinero, and M. A. Anisimov, J. Chem. Phys. 138, 174501, (2013). [5] V. Holten, J. C. Palmer, P. H. Poole, P. G. Debenedetti, and M. A. Anisimov, J. Chem. Phys. 140, 104502 (2014). )

New results on dark energy in the early and present Universe and the observational status of the inflationary scenario

19 June 2014 in 11:30

Alexei A. Starobinsky

The measurement of a small deviation of the primordial spectrum of scalar (density) perturbations in the Universe from the exactly flat (or, Harrison-Zeldovich) one in the WMAP and Planck experiments confirms the general prediction of the early Universe scenario with the de Sitter (inflationary) stage preceding the radiation dominated stage (the hot Big Bang) and strongly restricts the class of viable inflationary models. The announced discovery of primordial gravitational wave background through the measurement of the B-mode of linear polarization of the cosmic microwave background radiation (CMB) in the range of multipoles l = (50 – 150) in the BISEP2 experiment [1] confirms another general prediction [2] of this scenario, as well as produces the direct evidence for the existence of a very strongly curved space-time in the past of our Universe and the necessity of quantization of gravitational waves. Still the BISEP2 result is partially contaminated by foregrounds (mainly by polarized galactic dust emission) and requires confirmation of its blackbody character by measurements at other wavelengths. Moreover, comparison of BISEP2 data with the temperature and E-mode polarization data earlier obtained in the WMAP and Planck experiments shows that the inflationary stage is not so simple and may not be described by a one-parametric model. In particular, the primordial spectrum of scalar perturbations generated during inflation is not of a power-law form [3], mainly due to the ~10% depression of the angular anisotropy power spectrum in the multipole range 20 < l < 40. A class of models describing this feature which implies existence of some scale (i.e. new physics) during inflation is proposed [4]. Furthermore, account of additional wiggles in the spectrum at l = 22 and 40 requires further complication of the inflaton potential [5] by introducing sharp features of the type suggested by previous studies [6]. ( References: [1] P.A.R. Ade et al. [BISEP2 Collaboration], arxiv:1403.3985. [2] A.A. Starobinsky, JETP Lett. 30, 682 (1979). [3] D.K. Hazra, A. Shafieloo, G.F. Smoot, A.A. Starobinsky. Ruling out the power-law form of the scalar primordial spectrum. JCAP, in press (2014); arXiv:1403.7786. [4] D.K. Hazra, A. Shafieloo, G.F. Smoot, A.A. Starobinsky. Whipped inflation. ArXiv:1404.0360. [5] D.K. Hazra, A. Shafieloo, G.F. Smoot, A.A. Starobinsky. Wiggly whipped inflation. ArXiv:1405.2012. [6] A.A. Starobinsky, JETP Lett. 55, 489 (1992). )

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Spinful fermionic ladders at incommensurate filling: Phase diagram, local perturbations, and ionic potentials

24 February 2014 in 10:00

Boris Narozhny (Karlsruhe Institute of Technology, Germany)

We study the effect of external potential on transport properties of the fermionic two-leg ladder model. The response of the system to a local perturbation is strongly dependent on the ground state properties of the system and especially on the dominant correlations. We categorize all phases and transitions in the model (for incommensurate filling) and introduce ’’hopping-driven transitions’’ that the system undergoes as the inter-chain hopping is increased from zero. We also describe the response of the system to an ionic potential. The physics of this effect is similar to that of the single impurity, except that the ionic potential can affect the bulk properties of the system and in particular induce true long range order.

Nonequilibrium transport in high Landau levels of 2D systems: recent developments

13 February 2014 in 15:00

M.A. Zudov (School of Physics and Astronomy, University of Minnesota, USA)

Over the last decade, magneto transport in very high Landau levels of high-mobility 2D electron systems (2DES) hosted in GaAs/AlGaAs quantum wells revealed a variety of new intriguing phenomena. Two prime examples of these phenomena are microwave-induced resistance oscillations (MIRO) and associated zero-resistance states which emerge when a high-mobility 2DES is irradiated by microwave radiation. Another prominent effect is Hall field-induced resistance oscillations (HIRO) which appear in differential resistivity when a system is driven by a dc field. Both MIRO and HIRO originate from inter-Landau level transitions owing to photon absorption and/or impurity scattering. In this talk I will discuss our recent experimental studies on i) Shubnikov-de Haas oscillations in conventional high-mobility 2D electron gas in GaAs/Al_0.24Ga_0.76As quantum wells irradiated by sub-terahertz (up to 0.4 THz) radiation, ii) MIRO and HIRO in Al_xGa_1-xAs/Al_0.24Ga_0.76As quantum wells with x up to 0.0078, and, finally, iii) observation of MIRO in a moderate-mobility 2D hole gas hosted in a pure Ga/Si_0.2Ge_0.8 quantum well.

Probing interactions with thermal transport

6 February 2014 in 11:30

Karen Michaeli (Weizmann Institute of Science)

Thermal and thermoelectric conductivities are ideal probes of interaction effects in correlated electron systems. This is because, in contrast to an electric current, a heat current can be transmitted also by neutral quasiparticles. For instance, energy can be carried by excitations that mediate interactions between other quasiparticles. In my talk I will present two examples of the dramatic effect of interactions on thermal and thermoelectric transport phenomena. The first is the Nernst effect in the vicinity of the superconducting phase transition. I will discuss the anomalous behavior of the Nernst effect near the magnetic-field-induced quantum critical phase transition. The second example is thermal conductivity in spin liquids. Spin liquids can form in the vicinity of the Mott metal-insulator transition when the charge is gapped while the spin degrees of freedom strongly fluctuate. These low energy excitations, dubbed spinons, can conduct heat. The spinons also exhibit a magnetic interaction that leads to non-Fermi liquid behavior. I will show that even in the absence of disorder this strong interaction provides an efficient relaxation mechanism for the heat current.

Composite fermion state of spin-orbit coupled bosons

21 November 2013 in 11:30

Alex Kamenev (Univ. of Minnessota, USA)

Recent experiments in cold atomic gases produced bosons with spin-orbit coupling. Their dispersion relation exhibits a minimum along a circle in the momentum space. It poses a problem of Bose condensation of particles with the degenerate minimum of the kinetic energy. We argue that in 2D the Bose condensation does not occur. Instead, a correlated state, similar to Laughlin liquid in the fractional quantum Hall effect, is formed.

Electrodynamics of strongly disordered superconductors

14 November 2013 in 11:30

T. M. Klapwijk

Superconductors with a high resistivity in the normal state are of interest for experiments on quantum phase slip devices and radiation-detection, such as hot-electron bolometers and, more recently, microwave kinetic inductance detectors. In addition they are excellent candidate-materials to be used in experiments, which require a superconductor in the presence of a high magnetic field, such as in experiments which combine semiconductors with superconductors. These materials have also been of interest, for several decades already, to demonstrate and analyze the so-called superconductor-insulator transition. Examples are NbN, NbTiN, TiN, MoN, etc. The new interest provides new experimental data to be understood and stimulates further theoretical analysis. In my talk I will present an overview of the new applications as well as new experimental data on the electrodynamics originating from our work with superconducting resonators. [1] E. F. C. Driessen, P. C. J. J. Coumou, R. R. Tromp, P.J. de Visser, and T. M. Klapwijk, Phys. Rev. Lett. 167, 360 (2012); [2] P. C. J. J. Coumou, E. F. C. Driessen, J. Bueno, C.Chapelier, and T.M. Klapwijk, Phys. Rev. B 88, 180505 R, (2013).

Anderson localization at the edge of 2D topological insulator

7 November 2013 in 11:30

Vladimir Yudson (Institute for Spectroscopy RAS, Troitsk)

Spins of chiral electrons moving along the 1D helical edge of a 2D topological insulator are tightly bound to their momenta. This suppresses electron backscattering by the usual potential disorder and protects the system from Anderson localization. We study effects of electron interactions with a disordered chain of spin (Kondo) impurities located near the helical edge. Assuming the electron-spin couplings of random anisotropies, we map this system to the problem of the pinning of the charge density wave by the disordered potential. This mapping shows that anisotropic couplings of chiral electrons with spin impurities lead to Anderson localization of the helical edge states. [B.L. Altshuler, I.L. Aleiner, and V.I. Yudson, Phys. Rev. Lett. 111, 086401 (2013)]

Hydrodynamics of active polar fluids

14 October 2013 in 11:30

Prof. K. Kruse (University of Saarlandes, Germany)

Many vital cellular processes are driven by the cytoskeleton, a network of filamentous polymers present in every living cell. The filaments have two structurally different ends making them polar objects and interact with a large number of associated proteins. To fulfill its various tasks, the cytoskeleton is kept out of thermodynamic equilibrium by energy that is fed into the system on the level of its constituents. Notably, the assembly of cytoskeletal filaments and the action of molecular motors that can generate stresses in the network depend on the local release of chemical energy. From a physical point of view, the cytoskeleton can be classified as an active polar fluid. In this talk, we will present a hydrodynamic description of active polar fluids. Using this theory, we will analyze the spontaneous emergence of flows as well as the stability of topological point defects. If time permits, we will also discuss the actin cortex as an example of a biologically relevant structure. We propose to view its formation as an active analogue of a (pre-)wetting transition.

Conductivity of suspended graphene at the Dirac point

3 October 2013 in 11:30

V.Yu. Kachorovskii (A.F. Ioffe Physical-Technical Institute)

We study transport properties of clean suspended graphene at the Dirac point. In the absence of the electron-electron interaction, the main contribution to resistivity comes from interaction with flexural (out-of-plane deformation) phonons. We find that the phonon-limited conductivity scales with the temperature as T^−η, where η is the critical exponent (equal to ≈ 0.7 according to numerical studies) describing renormalization of the flexural phonon correlation functions due to anharmonic coupling with the in-plane phonons. The electron-electron interaction induces an additional scattering mechanism and also affects the electron-phonon scattering by screening the deformation potential. We demonstrate that the combined effect of both interactions results in a conductivity that can be expressed as a dimensionless function of two temperature-dependent dimensionless constants, G[T] and G_e[T], which characterize the strength of electron-phonon and electron-electron interactions, respectively. We also discuss the behavior of conductivity away from the Dirac point as well as the role of the impurity potential and demonstrate and compare our predictions with available experimental data.

Coulomb drag in graphene near the Dirac point

5 September 2013 in 11:30

I.V. Gornyi (Karlsruhe Institute of Technology and Ioffe Physical Technical Institute)

We study Coulomb drag in graphene near the Dirac point, focusing on the regime of interaction-dominated transport. We establish a novel, graphene-specific mechanism of Coulomb drag based on fast interlayer thermalization, inaccessible by standard perturbative approaches. Using the quantum kinetic equation framework, we derive a hydrodynamic description of transport in double-layer graphene in terms of electric and energy currents. In the clean limit, the drag becomes temperature independent. In the presence of disorder the drag coefficient at the Dirac point remains nonzero due to higher-order scattering processes and interlayer disorder correlations. At low temperatures (diffusive regime) these contributions manifest themselves in the peak in the drag coefficient centered at the neutrality point with a magnitude that grows with lowering temperature. Motivated by recent experiments, further we propose a phenomenological model of the magnetodrag in graphene. Near charge neutrality, each of the layers contains two kinds of carriers (quasiparticles) with opposite electric charge — electrons and holes, such that the quasiparticle (or energy) and electric currents become noncollinear in the presence of external magnetic field. Precisely at the Dirac point, these currents are orthogonal to each other, giving rise to a giant magnetodrag. The sign of the drag resistance depends on the strength of the electron-phonon interaction, as well as on the geometry of the sample. Away from the Dirac point, the quasiparticle current acquires a component in the direction of the electric current, leading to non-zero Hall drag. The predictions of the model agree with the experimental data in weak magnetic fields measured in double-layer graphene/boron-nitride heterostructures.

Exciton many-body effects and Bose-Einstein condensation

23 May 2013 in 11:30

Monique Combescot (Institut des NanoSciences de Paris, Université Pierre et Marie Curie, CNRS)

In order to describe physical effects involving excitons in a convenient way, I will first introduce the «Shiva and Kali diagrams» which visualize many-body effects between composite quantum particles. I will also provide the theoretical grounds on which they are based. These diagrams will be used to present some non-linear optical effects involving virtual excitons coupled to unabsorbed photons such as spin precession and teleportation, Faraday rotation and oscillation, exciton trapping and so on... In a second part, I will turn to exciton BEC. I will explain (i) why excitons must condense into a linearly polarized dark state — which probably is the reason why this condensation has been missed for decades — (ii) how under a density increase, the dark condensate acquires a bright component, (iii) why a phase separation can take place between an exciton gas and an electron-hole plasma, the dense phase possibly being a BCS condensate of «electron-hole Cooper pairs». I will end by showing a few very recent experimental data, taken by François Dubin and his group, which evidence the dark exciton Bose-Einstein condensate. (References: [1] M. COMBESCOT, O. BETBEDER-MATIBET, F. DUBIN, The many-body physics of composite bosons, Physics Report 463, 215 (2008). [2] M. COMBESCOT, O. BETBEDER-MATIBET, General many-body formalism for composite quantum particles, Phys. Rev. Lett. 104, 206404, (2010). [3] M. COMBESCOT, S. SHIAU, Y. CHANG, Finite temperature formalism for composite quantum particles, Phys. Rev. Lett. 106, 206403 (2011). [4] M. COMBESCOT, O. BETBEDER-MATIBET, R. COMBESCOT, Bose-Einstein condensation in semiconductors: the key role of dark excitons, Phys. Rev. Lett. 99, 176403, (2007). [5] R. COMBESCOT, M. COMBESCOT, «Gray» BCS condensate of excitons and internal Josephson effect, Phys. Rev. Lett. 109, 026401 (2012). [6] M. ALLOING, M. BEIN, D. FUSTER, Y. GONZALES, L. GONZALES, R. COMBESCOT, M. COMBESCOT, F. DUBIN, Evidence for a Bose-Einstein condensate of excitons, arXiv:1304.4101.)

Coherent Back Scattering and Anderson Localization of Ultra Cold Atoms

25 April 2013 in 11:30

Alain Aspect (Institut d’Optique, Palaiseau, France)

We use ultra cold atoms in a disordered potential created with a laser speckle, to study Anderson Localization (AL) and Coherent Back Scattering (CBS). Localization has been observed in 1D and 3D, and 2D experiments are promising. Theory supports the conclusion that what we observe is AL, but a direct evidence of the role of coherence is desirable. Recently, we have observed CBS, an indisputable coherent effect in quantum transport, related to the first order manifestation of localization (weak localization).

Cantor sets and Chalker’s scaling: a simple picture of critical eigenfunction statistics

13 December 2012 in 11:30

Vladimir Kravtsov (ICTP, Trieste, Italy)

Multifractality of critical eigenstates in disordered conductors and superconductors is becoming an experimental issue. One of the key points is the critical enhancement of interaction matrix elements described by the Chalker’s scaling. In this work we show that the Chalker’s scaling is a simple consequence of the spectrum of local probe (like STM) which at criticality exhibits a Cantor set structure. We review the three-fold classification of spectra used by mathematicians and show their relation with the three types of local spectra.

Spin of superconducting quasiparticles

6 December 2012 in 11:30

Yu.V. Nazarov (Delft University of Technology, Netherlands)

We review the recent theoretical and experimental research that concerns the spin of superconducting quasiparticles and its possible use in the quantum manipulation context. We propose and theoretically investigate spin superconducting qubits. Spin superconducting qubit consists of a single spin confined in a Josephson junction. We show that owing to spin-orbit interaction, superconducting difference across the junction can polarize this spin. We demonstrate that this enables single qubit operations and more complicated quantum gates, where spins of different qubits interact via a mutual inductance of superconducting loop where the junctions are embedded. Recent experimental realizations of Josephson junctions made of semiconductor quantum dots in contact with superconducting leads have shown that the number of electrons in the quantum dot can be tuned by a gate voltage. Spin superconducting qubit is realized when the number of electrons is odd. We discuss the qubit properties at phenomenological level. We present a microscopic theory that enables us to make accurate estimations of the qubit parameters by evaluating the spin-dependent Josephson energy in the framework of fourth-order perturbation. We interpret a recent pioneering experiment [Zgirski M. et al., Phys. Rev. Lett., 106 (2011) 257003] on quasiparticle manipulation in a superconducting break junction in terms of spin blockade drawing analogy with spin qubits. We propose a novel qubit design that exploits the spin state of two trapped quasiparticles. We detail the coherent control of all four spin states by resonant quantum manipulation and compute the corresponding Rabi frequencies. The read-out technique is based on the spin-blockade that inhibits quasiparticle recombination in triplet states. We provide extensive microscopic estimations of the parameters of our model.

Superinductors: a novel type of Josephson ladders implementing quantum Ising model

20 September 2012 in 11:30

Lev Ioffe (Rutgers University & Universite Paris VI)

Implementation of superinductor, the element that is characterized by a purely inductive response and impedance much larger than quantum (6.5 kOhm), is a very long standing challenge. Recently, we have designed and implemented it in very special Josephson ladders with tunable frustration. The same ladders allows one an experimental realization of the one dimensional φ⁴ theory with the mass that changes sign with magnetic field. Close to the critical point the low energy excitations in this theory can be described by Ising model in transverse field. The solution of the latter shows that these excitations are Majorana fermions. Can these fermionic excitations be observed in Josephson ladders?

Surprises, Computational Methods, and Results for Metastable Phenomena and Homogeneous Nucleation and Growth

5 July 2012 in 11:30

Mark Novotny (Mississippi State University)

It is shown that even in the simplest models homogeneous nucleation and growth is complicated, leading to a “phase diagram” that depends on the various length scales in the problem. Algorithms that allow faster-than-real-time simulations to span the disparate timescales in nucleation will be presented. Specific results discussed will include the dependence of the hysteresis loop on the frequency, whether changing the dynamics can change the exponent in the nucleation rate, and a Dynamic Phase Transition (including experimental confirmation in magnetic multilayer systems).

Hanbury Brown and Twiss setup with anyons

24 May 2012 in 11:30

Gabriele Campagnano, Oded Zilberberg, Igor V. Gornyi, Dmitri E. Feldman, Andrew C. Potter, Yuval Gefen

We present a study of an Hanbury Brown and Twiss (HBT) interferometer realized with anyons. Such a device can directly probe entanglement and fractional statistics of initially uncorrelated particles. We calculate HBT cross-correlations of Abelian Laughlin anyons. The correlations we calculate exhibit partial bunching similar to bosons, indicating a substantial statistical transmutation from the underlying electronic degrees of freedom. We also find qualitative differences between the anyonic signal and the corresponding bosonic or fermionic signals, indicating that anyons cannot be simply thought as intermediate between bosons and fermions.

Double proximity effect in hybrid planar Superconductor-(Normal metal/Ferromagnet)-Superconductor structures

26 April 2012 in 11:30

V.V. Ryazanov (ISSP RAS)

We have investigated the differential resistance of hybrid planar Al-(Cu/Fe)-Al submicron bridges at low temperatures and in weak magnetic fields. The structure consists of Cu/Fe-bilayer forming a bridge between two superconducting Al-electrodes. In superconducting state of Al-electrodes, we have observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the minigap. We claim that this effect (the doubling of the minigap) is due to an electron spin polarization in the normal metal which is induced by the ferromagnet. We have demonstrated that the double-peak peculiarity is converted to a single peak at a coercive applied field corresponding to zero magnetization of the Fe-layer.

Vortex dynamics in Fermi superfluids at ultra low temperatures

12 April 2012 in 11:30

Mihail Silaev (Institute for Physics of Microstructures RAS, Nizhny Novgorod)

We show that the vortex dynamics in Fermi superfluids at ultra-low temperatures is governed by the local heating of the vortex cores creating the heat flux carried by non-equilibrium quasiparticles emitted by moving vortices. This mechanism provides a universal zero temperature limit of dissipation in Fermi superfluids. For the typical experimental conditions realized by the turbulent motion of ³He-B the temperature of vortex cores is estimated to be of the order 0.2 Т_с. The dispersion of Kelvin waves is derived and the heat flow generated by Kelvin cascade is shown to have the value close to the experimentally observed. The suggested mechanism can provide the low-temperature limit of dissipation for mechanical pendulum such as vibrating wire resonator in ³He-B.

Solution of the Dyson-Schwinger equation on de Sitter background in IR limit

22 March 2012 in 11:30

Emil Akhmedov (ITEP)

We propose an ansatz which solves the Dyson-Schwinger equation for the real scalar fields in Poincare patch of de Sitter space. In the IR limit the Dyson-Schwinger equation for this ansatz reduces to the kinetic equation, if one considers scalar fields from the principle series. Solving the latter equation we show that under the adiabatic switching on and then off the coupling constant the Bunch-Davies vacuum relaxes to the state with the flat Gibbons-Hawking density of out-Jost harmonics on top of the corresponding de Sitter invariant out-vacuum.

Counting free fermions on a line: a Fisher-Hartwig asymptotic expansion for the Toeplitz determinant in the double-scaling limit

19 January 2012 in 11:30

Dmitri Ivanov (ETH Zurich and Univeristy of Zurich), Alexander Abanov, Vadim Cheianov

We derive an asymptotic expansion for a Fredholm determinant arising in the problem of counting one-dimensional free fermions on a line segment at zero temperature. This asymptotic expansion was conjectured previously from numerical evidence. It is explicitly periodic in the «counting parameter» and describes the nonanalytic dependence of the asymptotic behavior of the determinant on this parameter. The derived expansion is an extension of the result in the theory of Toeplitz determinants known as the generalized Fisher-Hartwig conjecture. We present two ways to derive our result: the matrix Riemann-Hilbert problem and the Painleve V equation. We prove that the expansion coefficients are polynomials in the counting parameter, provide an algorithm for their calculation order by order and list explicitly first several coefficients. (arXiv:1112.2530)

Multifractality of wave functions: basic concepts and experiment

29 December 2011 in 11:30

Vladimir Kravtsov (ICTP, Trieste, Italy)

Wave functions in certain disordered systems exhibit a special scaling called «multifractality». Discovered in the 80-th as the property of critical states near the Anderson transition point, this phenomenon has been rapidly developing recently with new important results about the scaling relationships between the fractal dimensions and the origin of these relationships in CFT. At the same time, the critical states near the Anderson transition become accessible experimentally in cold atom systems, in disordered 2D electron gases in semiconductors and in sound wave propagation in strongly disordered media. I will review these new results and some recent theoretical predictions.

Heat, work and fluctuation relations in single-electron transport: experimental test of the Jarzynski equality

22 December 2011 in 11:30

Jukka Pekola (Aalto University, Finland )

The physical foundations of commonplace concepts such as information, work and heat can be studied in detail in small systems having just few degrees of freedom. Characteristic of such systems is that the magnitude of thermal fluctuations of energy and coordinate variables can be significant compared to their mean behavior. Fluctuation relations have been developed relatively recently to describe these phenomena. They have been successfully applied in the interpretation of, e.g., experiments performed on individual complex biomolecules. However, a «textbook» example of nontrivial nonequilibrium thermal fluctuations measured in a well-characterized system has been missing. Here we demonstrate experimental readout of the distribution of dissipated energy in a metallic single-electron box that we subject to an external nonadiabatic gate drive at sub-kelvin temperatures [1]. In contrast to previous experimental work, the total heat dissipated in an electron tunneling event can be directly determined from the timing of the tunneling event with respect to the external drive [2]. Also, being a lithographically defined electronic system, the experimental gate protocol can be repeated accurately and indefinitely without degrading the sample, allowing us to recover the heat distribution with a dynamic range of more than three orders of magnitude. The present work shows that nonequilibrium thermodynamics can be studied in solid state systems where realization of quantum coherence effects and an engineered non-Gaussian environment is experimentally feasible. [1] O.-P. Saira et al., in preparation. [2] D.V. Averin and J.P. Pekola, EPL 96, 67004 (2011).

Tunneling electro-conductance of Bose condensates

8 December 2011 in 11:30

Vladimir Akulin (Orsay, France)

Interaction of an electron with a Bose condensate of atoms having electron affinity is considered. States of an electron trapped by such atoms form a continuous band, although tunneling through this band is strongly suppressed by quantum fluctuations of the condensate density. Still the transport process turns to be possible when inelastic processes associated with quantum transitions in the condensate are taken into account. One can adapt the standard field theory methods that originally have been developed for description of a particle propagating trough a disordered potential and present an exactly soluble analytical model of the process.

Majorana fermions in pinned vortices

13 October 2011 in 11:30

Alexander Rozhkov (Institute for Theoretical and Applied Electromagnetics, RAS)

Exploiting the peculiar properties of proximity-induced superconductivity on the surface of a topological insulator, we propose a device which allows the creation of a Majorana fermion inside the core of a pinned Abrikosov vortex. The relevant Bogolyubov-de Gennes equations are studied analytically. We demonstrate that in this system the zero-energy Majorana fermion state is separated by a large energy gap, of the order of the zero-temperature superconducting gap Δ, from a band of single-particle non-topological excitations. In other words, the Majorana fermion remains robust against thermal fluctuations, as long as the temperature remains substantially lower than the critical superconducting temperature. Experimentally, the Majorana state may be detected by measuring the tunneling differential conductance at the center of the Abrikosov vortex. In such an experiment, the Majorana state manifests itself as a zero-bias anomaly separated by a gap, of the order of Δ, from the contributions of the nontopological excitations. [A.L. Rakhmanov, A.V. Rozhkov, Franco Nori, Phys. Rev. B 84, 075141 (2011)]

Universal manifestations of electron-electron interaction in conductivity of a 2D metal near a convex-concave Fermi surface transition

6 October 2011 in 11:30

Vladimir Yudson (Institute for Spectroscopy RAS, Troitsk)

In the absence of umklapp scattering, the existence of the Fermi-liquid (T²) term in conductivity of a 2D metal depends on the Fermi surface geometry, in particular, on whether it is convex or concave. Due to hexagonal warping, the Fermi surface of 2D metallic surface states of 3D topological insulators of the Bi₂Te₃ family changes its shape (from convex to concave) on doping, while still being too small to allow for umklapp scattering. We show that near a convex-concave transition the T² term in the conductivity obeys a universal scaling form. [H.K. Pal, V.I. Yudson, D.L. Maslov, arXiv:1108.2435]

Coulomb drag between quantum wires

23 June 2011 in 11:30

A.P. Dmitriev, I.V. Gornyi, D.G. Polyakov (Karlsruhe Institute of Technology, Germany)

We revisit the problem of Coulomb drag between ballistic one-dimensional electron systems. Within the Boltzmann kinetic approach, we demonstrate that the equilibration between the right- and left-movers in the wires is crucially important for establishing dc drag. This relaxation requires either interwire electron-electron backscattering or processes involving the scattering of electrons near the bottom of the spectrum. The forward pairwise scattering of electrons near the Fermi level is therefore not sufficient, in contrast to a number of works studying drag due to this mechanism of scattering, for nonzero dc drag resistivity. At sufficiently high temperatures, the «left-right» relaxation (and hence the drag effect) is dominated by the triple inelastic collisions involving a «cold» electron near the band bottom. This yields the activation temperature dependence of the transresistivity, which we calculate from the Fokker-Planck equation.

Theory of fractional quantum Hall interferometers

16 June 2011 in 11:30

E. Sukhorukov (University of Geneva)

Interference of fractionally charged quasi-particles is expected to lead to Aharonov-Bohm oscillations with periods larger than the flux quantum. However, according to the Byers-Yang theorem, observables of an electronic system are invariant under adiabatic insertion of a quantum of singular flux. We resolve this seeming paradox by considering a microscopic model of an electronic interferometer made from a quantum Hall liquid at filling factor 1/m in the shape of a Corbino disk. Quantum Hall edge states are utilized in place of optical beams, quantum point contacts play the role of beam splitters connecting different edge channels, and Ohmic contacts may be considered a source and drain of a quasi-particle current. Depending on the position of Ohmic contacts one distinguishes interferometers of Fabry-Perot (FP) and Mach-Zehnder (MZ) type. An approximate ground state of such interferometers is described by a Laughlin type wave function, and low-energy excitations are incompressible deformations of this state. We construct a low-energy effective theory by projecting the state space of the liquid onto the space of incompressible deformations and show that the theory of the quantum Hall edge so obtained is a generalization of a chiral conformal field theory. A quasi-particle tunneling operator in our theory is found to be a single-valued function of tunneling point coordinates, and its phase depends on the topology, determined by the positions of Ohmic contacts. We describe strong coupling of the edge states to Ohmic contacts and the resulting quasi-particle current through the interferometer with the help of a master equation. We find that the coherent contribution to the average quasi-particle current through MZ interferometers vanishes after the summation over quasi-particle degrees of freedom. Remaining contribution originates from electron tunneling and oscillates with the electronic period, in agreement with the Byers-Yang theorem. When a magnetic flux through FP interferometers is varied with a modulation gate, current oscillations have the quasi-particle periodicity, thus allowing for the spectroscopy of quantum Hall edge states. Importantly, in contrast to previous models our theory does not rely on any ad-hoc constructions, such as Klein factors.

Electronic transport through a contact of a correlated quantum wire with leads of higher dimension

19 May 2011 in 11:30

S.N. Artemenko, P.P. Aseev, D.S. Shapiro (IRE RAS)

We study theoretically electronic transport through a contact of a quantum wire with 2D or 3D leads. Usually, the boundary conditions derived by Egger and Grabert for ideal adiabatic contacts are used to describe electronic transport in such systems. But these conditions were derived only for expectation values and, therefore, they do not take into account fluctuations, which are very important in 1D systems, and do not describe relaxation processes in 1D conductor due to coupling to bulk electrodes playing a role of the heat bath. Further, the real contacts are not necessarily adiabatic, and this may result in formation of Friedel oscillations (FO) near the contacts. We derive boundary conditions for non-adiabatic contacts taking into account fluctuations and find that if the contact is not smooth and adiabatic then the FO are really formed and suppress conduction in 1D system with inter-electronic repulsion. We apply the derived boundary conditions to study dynamic regime of conduction related to formation of FO near the contacts.

Optical excitation of Electron-Glasses

21 April 2011 in 11:30

Z. Ovadyahu (Racah Institute of Physics, The Hebrew University, Jerusalem)

Electron-glasses can be readily driven far from equilibrium by a variety of means. Several mechanisms to excite the system and their relative merits are reviewed. In this talk we focus on the process of exciting electron-glasses by interaction with near infrared radiation. The efficiency of this protocol varies considerably among different electron-glasses, but it only weakly depends on their resistance at liquid helium temperatures. A dramatic enhancement of the excitation efficiency is observed upon doping crystalline indium-oxide with Au. Some enhancement is observed also in samples doped with Pb but this enhancement fades away with time unlike the situation in the Au-doped samples. Several structural and analytical tools are used to characterize the changes in the materials that may be responsible for these effects. Possible routes by which high-frequency electromagnetic fields take the system far from equilibrium are discussed.

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Three-way junctions of interacting quantum wires: Tunneling into a Luttinger liquid revisited

7 April 2011 in 11:30

Dmitry Polyakov (Institut fuer Nanotechnologie, Karlsruhe Institute of Technology, Germany)

I will first review what makes correlated electron systems in one dimension (for which the Luttinger liquid is a prototype model) special as far as the inelastic scattering processes and the elastic renormalizations are concerned. I will then focus on the question of how interactions renormalize tunneling into a Luttinger liquid without making the usual assumption that the tunneling amplitude is infinitesimally small. It will be shown that the conventional fixed point, which describes the zero-bias suppression of tunneling into a homogeneous liquid, has a finite basin of attraction only in the point-contact model. A finite size of the contact makes it generically unstable to the tunneling-induced breakup of the liquid into two semi-infinite parts. In the course of the renormalization, the tunneling conductance may behave nonmonotonically and the tunnel contact may become more transparent with decreasing temperature or bias voltage. In the point-contact model, the regions on the phase diagram which correspond to the conventional zero-bias anomaly and the breakup are separated by a quantum phase transition. I will put these results in the context of theory of «three-way junctions» of correlated quantum wires.

Fast vortices and superconducting fluctuations in the cuprates and amorphous superconductors

27 January 2011 in 11:30

Peter Armitage (John Hopkins University, Baltimore, USA)

The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be very significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity. In this work we use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above Tc in LSCO thin films over a doping range that spans almost the entire superconducting dome (x=0.06 to 0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, up to — at most — 16 K above Tc. We compare our results with the measurements of diamagnetism in a similarly doped crystals of LSCO and show through a vortex-plasma model that if the fluctuation diamagnetism solely originates in vortices, then they must necessarily exhibit an anomalously large vortex diffusion constant, which is more than two orders of magnitude larger than the Bardeen-Stephen estimate. This points to either an entirely different character to vortex dynamics in the underdoped d-wave cuprates or a contribution to the diamagnetic response that is not superconducting in origin. I will make a comparison to our related work using broadband microwave spectroscopy on amorphous superconducting InO thing films.

Statistics of energy flows, temperature fluctuations and underlying techniques

7 October 2010 in 11:30

Yu.V. Nazarov (Delft University of Technology, Netherlands)

I will review recent results on statistics of energy flows and temperature fluctuations in zero-dimensional systems with emphasis on underlying techniques. The motivation for this is a recent revival of interest to thermal transport and successes in micro-thermometry. The technique in question has been proposed in 2003 by Kindermann and Pilgram. I will start by putting it into general context. Further, I concentrate on simple model systems and give the details of technique application for concrete setups of double junction and single-electron transistor. I will also shortly address the flows of (quantum) information quantities, the problems arising in this context and possible relation to energy flows.

Nonequilibrium coherent dynamics in Charge Density Waves from femto-second optical experiments and their modeling

9 September 2010 in 11:30

S. Brazovskii

Fundamental problems in temporal evolution of symmetry breaking phase transitions span subjects from high energy and cosmology, to condensed matter. The reported new approach [1] is the optical study of a far-from-equilibrium evolution of the electronic charge ordering after a quench caused by an intense laser pulse. Technique of multi-pulse spectroscopy with a femto-second resolution allowed to simultaneously monitor both bosonic and fermionic components, coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode (akin to the Higgs boson), and evolution of the particle-hole gap which is due to the Peierls-BCS mechanism. The numerical modeling with no fitting parameters allowed reproducing the observations, particularly the spatio-temporal distortions («Higgs waves») arising from «earthquakes» — annihilation events of topological defects in depth of the sample. The results give a rare access to the in-situ dynamics of phase transitions. Other results apply to experiments by Latyshev et al. at IREE on nano-junctions of CDW materials. We model the ground state reconstruction with creating and propagation of dislocations in the electronic crystal. [1] In collaboration with D. Mihailovic group at the Jozef Stefan Institute, Ljubljana, Slovenia. R. Yusupov, et al, «Coherent dynamics of macroscopic electronic order through a symmetry breaking transition», Nature Physics, DOI:10.1038/NPHYS1738, on-line.

Kvantovaya zaputannost’ i osnovaniya statisticheskoi mekhaniki

13 May 2010 in 11:30

O.V. Lychkovskii (ITEF)

Состояние открытой квантовой системы, взаимодействующей с большим квантовым резервуаром, в большинстве случаев с течением времени приближается к некоторому равновесному состоянию. Это равновесное состояние может зависеть (вообще говоря) или почти не зависеть (как правило) от начального состояния открытой системы и резервуара, являться или не являться распределением Больцмана-Гиббса, и т.д. В последние два десятилетия был доказан ряд теорем о характере релаксации к равновесному состоянию, опирающихся исключительно на уравнение Шредингера для замкнутой системы. Важную роль в этих теоремах играет понятие квантовой запутанности состояний открытой системы и резервуара, генерируемой взаимодействием между ними. В докладе представлен обзор этих результатов.

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Anomalous scaling and solitary waves in systems with non-linear diffusion

29 April 2010 in 11:30

Alex Hansen (Norwegian University of Science and Technology, Trondheim, Norway)

With background in the dynamics of wetting films in porous media under two-phase flow conditions, we discuss a non-linear convective-diffusive equation, local in space and time, describing the thickness of a wetting film. The presence of a non-linear diffusion predicts the existence of fronts as well as shock fronts. Despite the absence of memory effects, solutions in the case of pure non-linear diffusion exhibit an anomalous sub-diffusive scaling. Due to a balance between non-linear diffusion and convection we, furthermore, show that solitary waves appear. For large times they merge into a single solitary wave exhibiting a topological stability. Even though our results concern a specific equation, numerical simulations supports the view that anomalous diffusion and the solitary waves disclosed will be general features in such non-linear convective-diffusive dynamics [A. Hansen, B.-S. Skagerstam and G. Tora, arxiv:1003.3638v1 (2010)].

Josephson effect in a long diffusive SNS junction in a magnetic field

8 April 2010 in 11:30

Dmitri Ivanov, Benoit Crouzy (EPFL)

We study a diffusive superconductor-normal metal-superconductor (SNS) junction in an external magnetic field. In the limit of a long junction, we find that the properties of such a system depend on the width of the junction relative to the length associated with the magnetic field. We compute the critical width separating the regime of pure decay (narrow junction) and the regime of damped oscillations (wide junction) of the critical current as a function of the magnetic flux through the junction. We find an exponential damping of the current, different from the well know Fraunhofer limit which corresponds to the limit of a tunnel junction. In the limit of a wide junction, the superconducting pair correlations and the critical current become localized near the border of the junction.

Ref: arXiv:0907.0632

Electronic transport in Mach-Zehnder and Fabry-Perot interferometers realized with quantum Hall edge states

1 April 2010 in 11:30

Dmitry Bagrets (INT, Karlsruhe Institute of Technology, Germany)

Motivated by a series of recent experiments I will discuss the influence of the long range Coulomb interaction on the quantum coherence in Mach-Zehnder and Fabry-Perot electronic interferometers realized with integer quantum Hall edge states. I will review the experimental status in this intriguing subfield of mesoscopics and further on will propose a simple physical model which enables to explain an experimentally observed unusual non-monotonic dependence of the visibility of the interference pattern. The origin of this effect is found to be a combination of the electrostatic Aharonov-Bohm effect, related to the charge imbalance on different arms of the interferometer, and the non-equilibrium dephasing, which stems from the emission of out-of-equilibrium plasmons in course of inelastic electron tunneling. The possible role of the edge-state reconstruction on the formation of the interference pattern is also discussed.

Vzaimodeistviya, korrelirovannye sostoyaniya i spin-dolinnaya simmetriya v dvukhsloinom grafene

18 February 2010 in 11:30

L. Levitov (MIT)

We analyze competition between different ordered states in bilayer graphene (BLG). Combining arguments based on SU(4) flavor symmetry with a mean field analysis, we identify the lowest energy state with the anomalous Hall insulator (AHI). This state is an SU(4) singlet excitonic insulator with broken time reversal symmetry, exhibiting quantized Hall effect in the absence of external magnetic field. Applied electric field drives an Ising-type phase transition, restoring time reversal symmetry. Applied magnetic field drives a transition from the AHI state to a quantum Hall ferromagnet state. We estimate energies of these ordered states, taking full account of screening, and predict the phase diagram.

Superinsulating state in two-dimensional superconducting systems: experiment and theory

28 January 2010 in 11:30

T. Baturina (IFP SO RAN, Novosibirsk), N. Shchelkachyov (ITF), V. Vinokur (Argonne National Laboratory, USA)

We discuss the results of the experimental study of two-dimensional superconducting systems (TiN films and Josephson junction arrays) in the vicinity of the superconductor-insulator transition. We have found the transition from the activation-type conductivity characteristic to insulating state to the state with the practically zero conductivity. Experimental findings indicate that the observed superinsulating state is the low-temperature phase of the charge Berezinskii-Kosterlitz-Thouless (BKT) transition. Considering the charge transfer in two-dimensional Josephson junction arrays, we observe that the tunneling current in their insulating state is ensured by the relaxation mediated by the electron-hole environment. The energy gap, appearing in the electron-hole spectrum below the BKT transition, gives rise to suppression of the tunneling current offering the microscopic mechanism for the superinsulating behavior.

Tunntling into the nonequilibrium state of the Luttinger liquid

5 November 2009 in 11:30

D. Bagrets (Institut fuer Nanotechnologie, Forschungszentrum Karlsruhe, Germany)

will discuss the problem of tunneling into the voltage biased quantum wire containing weak backscattering defects. I will argue that one-dimensional interacting electrons in such a wire constitute a true non-equilibrium state of the Luttinger liquid (LL). This state is created as the result of inelastic electron backscattering leading to emission of non-equilibrium plasmons with typical frequency of the order of applied voltage eU. Tunneling rates of electron into/from such state are split into two edges. I show that a tunneling exponent of the main edge at the Fermi energy (E_F) is positive and is that of a bulk exponent in the equilibrium LL, while the power law exponent at the side edge at E_F − eU turns out to be negative if Coulomb interaction is not too strong.

Spontaneous symmetry breaking in general relativity. Brane world concept

9 April 2009 in 11:30

B.E. Meierovich (P.L. Kapitza Institute for Physical Problems)

Gravitational properties of a hedge-hog type topological defect in two extra dimensions are considered in General Relativity employing a vector as the order parameter. The developed macroscopic theory of phase transitions with spontaneous symmetry breaking is applied to the analysis of possible «thick» brane structures. The previous considerations were done using the order parameter in the form of a multiplet in a target space of scalar fields. The difference of these two approaches is analyzed and demonstrated in detail. There are two different symmetries of regular solutions of Einstein equations for a hedgehog type vector order parameter. Both solutions are analyzed in parallel analytically and numerically. Regular configurations in cases of vector order parameter have one more free parameter in comparison with the scalar multiplet solutions. It is shown that the existence of a negative cosmological constant is sufficient for the spontaneous symmetry breaking of the initially plain bulk. Regular configurations have a growing gravitational potential and are able to trap the matter to the brane. Among others there are solutions with the gravitational potential having several points of minimum. Identical in the uniform bulk spin-less particles, being trapped within separate points of minimum, acquire different masses and appear to an observer within the brane as different particles with integer spins.

Kramers degeneracy in a magnetic field and zeeman spin-orbit coupling in antiferromagnets

12 February 2009 in 11:30

Revaz Ramazashvili (Orsay University)

Essential dependence of the electron g-tensor on the quasiparticle momentum is a fundamental and, so far, largely overlooked property of antiferromagnetic conductors. I will discuss some of its symmetry underpinnings and experimental consequences. The predictions may be relevant to antiferromagnetic conductors from chromium to electron- and hole-doped cuprates, borocarbides, pnictides, organic and heavy fermion antiferromagnets.

Nonequilibrium kinetics of a disordered Luttinger liquid

20 November 2008 in 11:30

Dmitry Bagrets (Institut fur Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe, Germany)

We develop a kinetic theory for strongly correlated one-dimensional electron systems out of equilibrium, within the framework of the Luttinger liquid model. In the absence of disorder (backscattering off impurities), the model is completely integrable and as such exhibits no relaxation to equilibrium. We formulate a theoretical framework to study relaxation processes due to the interplay of disorder and electron-electron interactions, based on kinetic equations for electron and plasmon distribution functions. We calculate the rate of energy relaxation (equilibration) in a disordered Luttinger liquid, relevant to the transport properties of quantum wires at finite bias voltage. Remarkably, for not too low temperature and bias, the energy-relaxation rate is found to be given by the rate of elastic scattering off disorder, independent of the strength of electron-electron interaction and temperature.

f(R) models of dark energy in the Universe

10 April 2008 in 11:30

A. AlexeiStarobinsky

Recent progress in construction of viable dark energy models in f(R) gravity (where R is the scalar curvature) satisfying laboratory, Solar system and cosmological tests is discussed. This class of models represents an interesting alternative to the standard cosmological model with a cosmological constant. Further problems and most critical tests for this approach are outlined.

On statistical mechanics of a single particle in random logarithmically-correlated potentials

20 March 2008 in 11:30

Yan Fyodorov (Nottingham)

I am going to discuss recent results (obtained in collaboration with J.-P. Bouchaud) of the replica approach to statistical mechanics of a single classical particle placed in a random Gaussian landscape. Particular attention will be paid to landscapes with logarithmically growing correlations which give rise to a multifractal structure of the Boltzmann-Gibbs weights. In the limit of infinite spatial dimension we will show that the model is precisely equivalent to the Derrida’s GREM. If time allows, I will also briefly discuss a one-dimensional variant of the model which can be analyzed by a mapping to the Dyson’s Coulomb gas. In particular, in the latter case we conjecture the explicit form of the distribution of the lowest minimum in such a potential.

Computational investigation of short pulse laser irradiation with metals

14 February 2008 in 11:30

Zhibin Lin (University of Virginia, USA)

Short pulse laser irradiation with metals has been investigated using a computational method combining the Two-Temperature-Model (TTM) with Molecular Dynamic (MD). In this talk, short pulse laser-induced thermoelastic deformation, generation of crystal defects and ultrafast melting in several metal targets obtained in the computer simulations will be presented. Transient changes in the thermophysical properties, namely the electron-phonon coupling and the electron heat capacity, due to strong laser excitation will be discussed within the framework of Density Functional Theory.

Munchhausen effect: tunneling in an asymmetric SQUID

10 January 2008 in 11:30

V. Geshkenbein

A classical system cannot escape out of a metastable state at zero temperature. However, a composite system made from both classical and quantum degrees of freedom may drag itself out of the metastable state by a sequential process. The tunneling of the quantum degree of freedom entails a distortion in the trapping potential of the classical junction, which might be sufficiently large to transform the metastable state into an unstable one. The classical component then escapes. Such a situation can be conveniently studied and implemented in a dynamically asymmetric dc SQUID with two Josephson junctions of equal critical current Ic but strongly different shunt capacities C and/or shunt resistances R. We determine the dynamical phase diagram of this SQUID for various choices of junction parameters.

Interplay of coulomb and electron-phonon interactions in graphene

27 December 2007 in 11:30

Denis Basko (SISSA, Trieste)

Electron-phonon coupling in graphene is currently a subject of intense research. Theoretically, the coupling constants are usually calculated using density-functional theory (DFT), where the exchange (Fock) term is treated in the local density approximation, or the generalized gradient approximation.
In this work we consider mutual effect of weak electron-phonon and strong Coulomb interactions on each other by summing up leading logarithmic corrections via the renormalization group approach. We find that coupling constants to different phonon modes are renormalized differently, which explains the experimentally observed intensities of Raman peaks.

Non-Abelian strings and monopoles in supersymmetric gauge theories

27 September 2007 in 11:30

Alexei Yung (PIYaF)

We review progress made in understanding the mechanism of non-Abelian confinement in supersymmetric gauge theories. In particular, we discuss construction of non-Abelian flux tubes (strings) in N=2 supersymmetric QCD. In certain regimes Z_N Abelian strings acquire orientational zero modes associated with rotation of their color flux inside a non-Abelian subgroup of the gauge group. This make these strings genuinely non-Abelian. For bulk theories with U(N) gauge group the internal dynamics of non-Abelian strings is described in terms of two dimensional CP(N-1) model. Next we focus on the notion of non-Abelian monopole. In particular, we discuss non-Abelian monopoles confined by non-Abelian strings. In the world sheet theory on the string they appear as CP(N-1) model kinks.

Investigation of larkin-imry-ma effect: random anisotropy of aerogel destroys the long-range orientational order in superfluid ³He-A

22 March 2007 in 11:30

G.E. Volovik

We discuss recent NMR experiments on superfluid ³He-A confined in aerogel. Silicon strands of aerogel play the role of impurities with quenched random anisotropy. We present experimental evidences of Larkin-Imry-Ma effect: randomly oriented silicon strands destroy the long-range orientational order in ³He-A. The long-range orientational order is restored when a small regular anisotropy is applied either by uniaxial deformation of aerogel or by external superfluid current. There are many open problems which require further experimental and theoretical efforts, such as the role of the topological defects and whether the superfluidity is suppressed by the Larkin-Imry-Ma effect.

How to measure a fractional statistical phase

22 February 2007 in 11:30

Yuval Gefen (Weizmann Institute of Sciences)

One of the pillars of quantum mechanics is the (quantum) statistics of identical particles. In two-dimensions particles (anyons) whose statistics (hence statistical phase) is intermediate between bosons and fermions do exist. I will discuss proposals to measure their statistical phase.

Carbon nanotube superconducting quantum interference device

21 December 2006 in 11:30

V. Bouchiat (CNRS Grenoble)

Recent progress in improving the contact of single-walled carbon nanotubes (SWNTs) to superconducting nanoelectrodes showed that such molecules can accommodate a superconducting current and behave as gate-controlled Josephson junctions. Such carbon nanotube (CNT) junctions allow to implement a new type of weak link that can be coupled quantum dots reminiscent to the well studied superconducting single-electron transistor but with the added presence of strong quantum confinement. We report on the study of a superconducting quantum interference device (SQUID) with SWNTs Josephson junctions. Quantum confinement in each junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with two lateral electrostatic gates. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting to change the hybridization of the QD states with the superconducting contacts. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of the switching current with magnetic flux is achieved when both QD junctions are in the “on” or “off” state. In particular, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled π-junctions, that is, the sign of the current-phase relation across the CNT junctions can be tuned with a gate voltage. The CNT-SQUIDs provide a new generation of ultra-sensitive magnetometers of nanometer-sized samples, which are very promising to study the magnetisation reversal of an individual magnetic particle or molecule placed on one of the two carbon nanotube Josephson junctions. Such devices offer also the opportunity to test interesting physical phenomena ranging from Kondo physics to π-junctions and pave the way for non-locality experiments by generating pairs of entangled electrons in a nanotube.

Universal and non-universal tails of free energy distribution function in the random directed polymer problem

7 December 2006 in 11:30

I.V. Kolokolov, S.E. Korshunov

A modification of the optimal fluctuation approach is applied to study the tails of the free energy distribution function P_L(F) for an elastic string in random media both in the regions of the universal behavior of P_L(F) and in the regions of large fluctuations, where the behavior of P_L(F) deviates from universal. The difference between the two regimes is shown to consist in whether it is necessary or not to take into account the renormalization of parameters by fluctuations of disorder in the vicinity of the optimal fluctuation.

Recent discoveries in quasi one dimentional conductors as an access to physics of solitons

22 June 2006 in 11:30

Serguei Brazovskii

This talk will firstly review two recent discoveries done by scientists of the IRE RAN (Latyshev, Nad, et al) in collaboration at Grenoble (Monceau et al), Orsay (S.B.) and ITP RAN (Matveenko, S.B.). The first one (F.Ya. Nad et al) is the observation of the Ferroelectric transition coupled with the charge disproportionation in organic conductors. The state gives rise to three types of solitons: pi- solitons (holons) are observed in conductivity and optics; fractionally charged solitons are seen as ferroelectric domain walls via the frequency dispersion of the electric response; topologically coupled combined spin-charge solitons appear below occasional subsequent structural transitions. The second discovery (Yu.I. Latyshev et al) is based on the new techniques of the internal coherent tunneling within nanoscale mesa-junctions, which was applied to compounds with Charge Density Waves. The experiments give a direct access to observations of two types of solitons in dynamics, as well as of their aggregated states in statics. Based on these experimental proves, we shall discuss possible generalizations for the role of microscopic topological objects. We shall construct a scheme of combined topological objects which may work in general types of strongly correlated systems.

Osobennosti mikrovolnovoi provodimosti kristallov vtsp s raznymi urovnyami dopirovaniya

8 June 2006 in 11:30

M.R. Trunin (IFTT RAN)

Results of recent investigations of the temperature dependences of the surface impedance Z(T) = R(T) + iX(T) and conductivity \sigma(T) in the ab-plane and along c-axis of high-Tc superconductors (HTSC) are discussed. The main attention is focused on the peculiarities of these dependences in single crystals of YBa2Cu3O7-x with the oxygen deficiency varied in the range 0.07 <= x <= 0.47 (92 >= Tc >= 41 K) and of Ba1-xKxBiO3 with different potassium content 0.6 >= x >= 0.35 (5 <= Tc <= 31 K).
An analysis of the resistivity \hat\rho(T) = 1 / \hat\sigma(T) tensor in the normal state indicates that the optimally doped YBa2Cu3O6.93 is a three-dimensional anisotropic metal. As the oxygen content is decreased, the Drude c-axis conductivity changes to the hopping one, the crossover occurring at \rho_c \rho_ab \approx 10^-6 (Ohm∙cm)2. In the superconducting state of YBa2Cu3O7-x with x > 0.35 the behavior of the superfluid density n_s(T,x) \propto \sigma''_ab(T,x) can be treated in the framework of d-density wave (DDW) scenario of pseudogap in underdoped HTSC. The observed peculiarities of the imaginary part \sigma''_c(T,x) of the c-axis conductivity at T << Tc are determined by a strong decrease of the interlayer coupling integral with an increase of x.
Measurements of the surface impedance Z(T) in Ba1-xKxBiO3 crystals with Tc = 11 K (x \approx 0.5) and Tc = 30 K (x \approx 0.4) allow one to establish that the former is a BCS-superconductor (Z(T) saturates exponentially with lowering temperature at T << Tc) and the latter is not (Z(T) is linear at T << Tc). In addition, it is found that the temperature dependences of the upper critical field Hc2(T) of the crystals with Tc > 20 K are similar to those in HTSCs, both exhibiting strong positive curvature. On the contrary, Hc2(T) curves of the crystals with Tc < 15 K are in complete agreement with the BCS theory. The transition from BCS-type to HTSC-like superconductivity in Ba1-xKxBiO3 is interpreted within Abrikosov's extended saddle-point model.
The report is based on recent review articles [1,2] of the author and latest experimental results. [1] M.R. Trunin and A.A. Golubov. In-plane microwave response of high-Tc single crystals: Experiment and theory. in 'HTSC Spectroscopy', eds. N.M.Plakida, chapter 3, p.p.159-233 (Taylor and Francis, London and New York, 2003). [2] M.R. Trunin. Conductivity anisotropy and pseudogap in the microwave response of high-Tc superconductors. Physics-Uspekhi 48, 979 (2005).

Evolution of the head-tail patterning system in the fly

18 May 2006 in 11:30

Eric Siggia (Rockefeller University)

A central tenant of the emerging field of evolution and development is that most novelity arises from the redeployment of old genes in new contexts rather than the creation of entirely new genes. This view makes gene regulation an important force in evolution. The seminar will discuss the earliest patterning in the fly embryo and how it differs among related species of flies that diverged ~15-20 million years ago (a distance measured in molecular terms of twice the distance between human and mouse). I will show how campuational methods can predict regulation from genomic sequence and the molecular events that lead to step wise evolution of the genome.

Time-dependent random matrices: quantum interference effects

12 January 2006 in 11:30

M.A. Skvortsov

It is well established that the energy level statistics in disordered mesoscopic samples is universal and can be described by random matrices of an appropriate symmetry. The random matrix theory studied in great detail provides complete information about the spectral statistics. However, much less is known about time-dependent random matrices, which appear in studying, e.g., a quantum dot subject to a time-dependent gate voltage.
We review recent progress in time-dependent random matrices. On a semiclassical limit, the energy absorption rate can be calculated with the help of the Kubo formula. Quantum phenomena modify this result. We show that there are two types of interference effects. The first effect is controlled by the velocity of the perturbation and is responsible for the transition between Kubo and Landau-Zener regimes of dissipation. The second effect is operative for time-reentrant perturbations when dynamic localization in the energy space may take place. We demonstrate that these effects can be described on the same footing within the Keldysh sigma-model formalism.

Quantum phase transitions from topology in momentum space

3 November 2005 in 11:30

G. Volovik

Many quantum condensed-matter systems, and, probably, the quantum vacuum of Standard Model, are strongly correlated and strongly interacting fermionic systems, which cannot be treated perturbatively. However, physics which emerges in the low-energy corner does not depend on the complicated details of the system and is relatively simple. It is determined by the nodes in the fermionic spectrum, which are protected by topology in momentum space (in some cases, in combination with the vacuum symmetry). We illustrate this universality on some examples of quantum phase transitions, which can occur between the vacua with the same symmetry but with different topology of nodes in momentum space, such as Lifshitz transition in the case of the Fermi surfaces. The quantum phase transitions between the fully gapped states with the same symmetry but with different momentum-space topology (such as plateau-plateau transition in intrinsic QHE and quantum transition in 1D Ising model) are also discussed.

Femtosecond laser ablation

13 October 2005 in 11:30

S.I. Anisimov, N.A. Inogamov, V.A. Khokhlov, K. Nishihara, Yu.V. Petrov, V.V. Zhakhovskii

Theoretical, numerical, and experimental description of ultrashort (τl sim 30 - 300 fs) laser pulse action on metals and semiconductors is discussed. Duration of laser pulse τl is so short that during the pulse only electron subsystem absorbs laser energy. This separates physics of femtosecond pulses from cases with longer durations (several ps and more) when electron and ion subsystems are in mutual thermal equilibrium. Variation of matter reply to laser action is described in continuous wide interval of absorbed energy.

The primordial perturbation spectrum

29 September 2005 in 11:30

A.A. Starobinsky

Primordial scalar (adiabatic) perturbations and tensor ones (gravitational waves) are the main observable quantities which remained from very early stages of evolution of our Universe. Observational data show that the Fourier power spectrum of density perturbations is very close to the flat (i.e., the Harrison-Zeldovich) one and their statistics is Gaussian, in agreement with predictions of the simplest versions of the inflationary scenario of the early Universe. However, some small deviations from the flat spectrum and the Gaussian statistics are expected generically. I review recent theoretical and observational results on the primordial perturbation spectrum including a new general expression for the spectrum of adiabatic perturbations unifying different ways of their generation both during and after inflation, as well as two exact solution for inflaton field potentials producing a) the exactly flat adiabatic spectrum, b) the constant ratio of tensor/scalar perturbation power spectra.

Wave turbulence theory (WTT) for discrete systems

22 September 2005 in 11:30

Elena Kartashova

Classical WTT allows to approximate original nonlinear evolution equations by corresponding kinetic equations which are easier to study, to model numerically, to compute Kolmogorov's spectra for them, etc. It is well known that kinetic equations do not work in large scale wave systems (so-called finite-size effects in resonators), i.e. in systems with discrete spectra. We are going to present self-consistent theory of wave turbulence for these systems and illustrate it with examples for waves in water, ocean, atmosphere, plasma. Mathematical part of this theory is based on the general results and methods of number theory which are used to construct reductions of the original evolution nonlinear PDE to a few small systems of ODEs describing resonances of the wave system.

Novyi malyi parametr i stepennoi ryad dlya 3-mernoi sistemy Nav’e-Stoksa vo vsem prostranstve

16 June 2005 in 11:30

Ya.G. Sinai

We consider the Fourier transform of the usual Navier-Stokes system on R³. Surprisingly enough, some simplification arises if one allows solutions with infinite energy and enstrophy. A new small parameter for such solutions can be introduces and a power series in powers of this parameter can be written. The solution can be represented as a sum of new diagrams which can be effectively estimated.

Analytical realization of finite-size scaling for Anderson localization: is there transition in the 2D case?

19 May 2005 in 11:30

I.M. Suslov (P.L. Kapitza Institute for Physical Problems)

Roughly half of numerical investigations of the Anderson transition are based on consideration of an associated quasi-1D system and postulation of one-parameter scaling for the minimal Lyapunov exponent. If this algorithm is taken seriously, it leads to unumbiguous prediction of the 2D phase transition. The transition is of the Kosterlitz-Thouless type and occurs between exponential and power law localization (Pichard and Sarma, 1981). This conclusion does not contradict numerical results if the raw data are considered. As for interpretation of these data in terms of one-parameter scaling, such interpretation is inadmissible: the minimal Lyapunov exponent does not obey any scaling. A scaling relation is valid not for minimal, but for some effective Lyapunov exponent, whose dependence on parameters is determined by scaling itself. If finite-size scaling is based on the effective Lyapunov exponent, existence of the 2D transition becomes not definite, but still rather probable. Interpretation of the results in terms of the Gell-Mann - Low equation is also given.

Soliton creation in quantum field theory: induced tunnelling in particle collisions

28 April 2005 in 11:30

D. Levkov, S. Sibiryakov (Nuclear Research Institute, Moscow State Univ)

We consider tunneling transitions between states separated by an energy barrier in a simple field theoretical model. We analyse the case of boundary soliton creation induced by collisions of a few highly energetic particles. We present semiclassical, but otherwise first principle, study of this process at all energies of colliding particles. We find that in the leading semiclassical approximation the probability of this process remains constant at high energies, and calculate the value of this constant.

Charge transfer between a superconductor and a hopping insulator

3 March 2005 in 11:30

V.I. Kozub, A.A. Zuzin, Y.M. Galperin, V. Vinokur

A theory of the low-temperature charge transfer between a superconductor and a hopping insulator is analyzed, and the corresponding interface resistance is calculated. This resistance is dominated by proposed electron-hole processes similar to Andreev reflection, but involving localized states in the insulator. The possibility of a new type of qubit where one of the quantum states is split between two spatially separated centers is discussed.

Anyons and haldane statistics: a dimensional reduction

24 February 2005 in 11:30

S. Ouvry (Orsay University)

An introduction to anyon statistics (particles in two dimensions beying nor Bose neither Fermi statistics) and to Haldane statistics (Hilbert space counting arguments) is made. The equation of state is given for a gas of anyons in the lowest Landau level of a strong magnetic field (Quantum Hall situation). By regularizing the system at long distance with an harmonic well, a link is established with the 1d Calogero-Moser model (Calogero in an harmonic well) and Haldane statistics. Recent tentatives to map the anyon model on a sphere to the Calogero-Sutherland model (Calogero on a circle) are finally discussed.

Field theory for the global density of states distribution function in disordered conductors

9 December 2004 in 11:30

V.I. Yudson (Institut spektroskopii RAN, Troitsk)

A field-theoretical representation is suggested for distribution functions of global quantities (like conductance and electron global density of states) in extended disordered conductors. This opens a way to study the complete statistics of fluctuations. The approach is based on a functional integration over bi-local functions \Psi(r1, r2) instead of the integration over local functions in the usual functional representation for moments of physical quantities. The formalism allows one to perform the disorder averaging and to derive an analog of the usual nonlinear sigma-model - a "slow" functional of a supermatrix field Q(r; r1,r2) \sim \Psi(r, r1)\bar{\Psi}(r2, r). As an application of the formalism, the long-tail asymptotics of the global density of states distribution function P(\nu) is derived.

Sound propagation in rouse polymer melts via generalized dynamic random phase approximation

11 November 2004 in 11:30

Igor Erukhimovich (Moscow State University)

An extended generalization of the dynamic random phase approximation (DRPA) for L-component polymer systems is presented. Unlike the original version of the DRPA, which relates the (L x L) matrices of the collective density-density time correlation functions and the corresponding susceptibilities of concentrated polymer systems to those of the tracer macromolecules and so-called broken-links system (BLS), our generalized DRPA solves this problem for the (5 x L)x(5 x L) matrices of the coupled susceptibilities and time correlation functions of the component number, kinetic energy and flux densities. The presented technique is used to study propagation of sound and dynamic form-factor in disentangled (Rouse) monodisperse homopolymer melt. The calculated ultrasonic velocity and absorption coefficient reveal substantial frequency dispersion. The relaxation time T is proportional to the degree of polymerization N, which is N times less than the time of relaxation of a single chain, which evidences strong dynamic screening due to interchain interaction. We discuss also some peculiarities of the Brillouin scattering in polymer and copolymer melts.

Field theory description of the polymer-induced many-body interaction and aggregation instability in colloid solutions

11 November 2004 in 11:30

Igor Erukhimovich (Moscow State University)

Starting from microscopic consideration we present a field theory descriptions of the polymer-colloid systems - the composite systems formed by both long polymer chains and relatively small (but large as compared to the monomers) particles. The peculiarity of such systems is that the ultraviolet divergencies in the corresponding field theory describe the polymer-colloid interactions and could be rigorously calculated. The infrared divergencies allow for the critical behavior of the system as the whole and in the present talk we take them into account within the mean-field approximation. The equation of state, phase diagram and the polymer-polymer and colloid-colloid correlation functions are calculated.

Monte-Carlo study of some orientational transitions in 2D

17 June 2004 in 11:30

Bertrand Berche (Universite Henri Poincare, Nancy, France)

In the numerical study of nematic-isotropic transition of liquid crystals, one usually considers lattice models with n-component spins interacting within nearest neighbours through Lengendre polynomials. In 2D,it is interesting to compare the phase transitions exhibited by such models with the special cases of XY and Heisenberg models. We present the results of Monte Carlo simulations analysed using Finite-Size scaling or conformal mapping. The results are not fully conclusive, but seem to confirm the possible occurrence of a first-order transition for a P4 interaction.

The quantum Hall effect: an experimental realization of the instanton vacuum

17 June 2004 in 11:30

A. Pruisken (University of Amsterdam, The Netherlands)

No annotation present.

Heisenberg constraints on mesoscopic and molecular amplifiers

10 June 2004 in 11:30

U. Gavish (LKB, Ecole Normale Superieure, Paris)

The Heisenberg principle puts constraints on the performances of linear amplifiers. We derive these constraints for the case of molecular or mesoscopic amplifiers with a narrow-band input. We then explain what physical processes create the noise which is necessarily added to the signal in order to satisfy these constraints. Finally we specify ways to minimize this noise.

Nanoscale inhomogeneities and spectroscopies on BSCCO-2212

27 May 2004 in 11:30

Peter Hirschfeld (University of Florida)

In almost all high-temperature superconducting cuprate materials, doping naturally introduces disorder. Recent STM experiments have provided us with a remarkable window on the real space electronic structures which reflect this disorder, and found localized atomic scale resonances, nanoscale gap inhomogeneity, and long-range spatial modulations with well-defined wave vectors. I discuss to what extent these results can be understood in terms of BCS d-wave quasiparticle states interfering in the presence of many potential scatterers, and present both analytical results as well as numerical solutions of the Bogoliubov-de Gennes equations on 120x120 lattices. I further propose a ``realistic" model for the disorder, involving in-plane unitary scatterers and a smooth weak disorder component, which seems to work well for BSCCO-2212 but should be quite nonuniversal. Remaining discrepancies may point to novel physics such as correlation-induced magnetic moments, or coexistence with anomalous subdominant order. Implications for transport properties and angle-resolved photoemission using this model are also discussed.

Determining properties of dark energy in the Universe

20 May 2004 in 11:30

A.A. Starobinsky

Recent numerous observational data obtained from such independent sources as angular anisotropies of the cosmic microwave background radiation, large-scale gravitational clustering of galaxies and their clusters and observations of supernovae explosions at high redshifts prove convincingly that about 70% of the total energy density of matter in the present Universe is due to a new kind of matter in the Universe ("dark energy") which is non-baryonic, has negative pressure which modulus is very close to dark energy density (if the Einsteinian form of gravity field equations is assumed) and remains unclustered at all scales where the clustering of baryons and dust-like cold dark matter is seen. I discuss different forms of phenomenological description of dark energy properties, present limits on variation of dark energy density with redshift which follow from the most recent supernovae data and make a brief review of different theoretical models of dark energy including those in which it has a purely geometrical origin. The simplest possibility of dark energy being a cosmological constant and nothing more still remains a good fit to all existing observational data. However, more complicated behaviour including breaking of the weak energy condition for dark energy for redshifts z < 0.5 combined with some increase of its energy density with redshift for larger z is possible, too.

Critical currents and current-phase relation of SFS Josephson junctions

13 May 2004 in 11:30

V.V. Ryazanov (ISSP)

We have investigated experimentally the pi-state of Josephson SFS (superconductor-ferromagnet-superconductor) junctions. The origin of the pi-state is an oscillating and sign-reversal superconducting order parameter induced in the ferromagnet close to the SF-interface. Transition to the pi-state occurs for ferromagnetic interlayer thickness close to a half-period of the order parameter spatial oscillations. We have also observed back transition to conventional "0-state" at the F-layer thickness about of the period of the oscillations. Weak ferromagnetism of the F-layer (Cu/Ni alloy) allows us to fabricate thin-film Josephson SFS sandwiches with continuous homogeneous interlayers whose thickness (10-30 nm) is comparable with the pair-decay length in the ferromagnet and to observe a temperature 0-pi-transitions. We present also measurements of the current-phase relation (CPR) of SFS Josephson junctions as a function of temperature. The CPR is determined by incorporating the junction in a superconducting loop coupled to a dc SQUID, allowing measurement of the junction phase difference.

Quantum percolation in granular metals

22 April 2004 in 11:30

M.V. Feigel’man, A.S. Ioselevich, M.A. Skvortsov

Theory of quantum corrections to conductivity of granular metal films is developed for the realistic case of large randomly distributed tunnel conductances. Quantum fluctuations of intergrain voltages suppress mean conductance much stronger than its variance. At sufficiently low energies any distribution becomes broad, leading to strong local fluctuations of tunneling density of states. Percolative nature of metal-insulator transition is established by combination of analytic and numerical analysis of matrix renormalization group equations.

Andreev states near surfaces and in thin films of high-temperature superconductors

15 April 2004 in 11:30

Yu.S. Barash (ISSP)

Investigations of Andreev bound states in high-temperature superconductors will be shortly described in the Introduction. In the second and the third parts of the talk, new results of two papers will be represented. Effects of impurities, situated on (110) surface of d-wave superconductor, will be considered regarding the low-bias conductance of tunnel NIS junctions. Impurity-induced quasiparticle bound states on a pair-breaking surface of a d-wave superconductor are theoretically described, taking into account hybridization of impurity- and surface-induced Andreev states. Further, a theory for effects of surface disorder (of thin impurity surface layer) on the low-bias conductance of tunnel junctions is developed. In the third part of the talk we present analytical and numerical results for the electronic spectra of thin films (quantum wires) of a d-wave superconductor on a square lattice. The spectra of Andreev and other quasiparticle states, as well as the spatial and particle-hole structures of their wave functions, depend on interference effects caused by the presence of the surfaces and are qualitatively different for half-filled wires with even or odd number of chains. Effects of deviations from half-filling and results of the self-consistent calculations are also presented.

Puzzles of ultra-high -- energy cosmic rays (UHECRS)

1 April 2004 in 11:30

Dmitry Gorbunov (INR RAS)

Traveling in intergalactic medium protons of E>5*10¹⁹ eV start to lose energy rapidly due to pion production on cosmic microwave background (CMB) photons. Thus at higher energies only protons from local sources can reach the Earth. At lower energies protons cover a very large distance almost without attenuation. Based on this simple considerations the cut off in UHECRs spectrum has been predicted, that conflicts with experimental data. Although the statistic of anomalous events is low, one can try to provide an explanation for the observed behavior of the UHECR spectra. All attempts within the Standard Model of particle physics are failed. Therefore, possibly, UHECRs provide one of the few direct experimental evidences for the physics beyond the Standard Model. We are about to review the current status of the problems related to the absence of the cut off in UHECRs spectrum.

Coherent phenomena in multiply connected sns systems

25 March 2004 in 11:30

T.I. Baturina (Insitut fiziki poluprovodnikov, Novosibirsk)

Mesoscopic systems, consisting of a normal metal (N) or heavily doped semiconductor being in contact with a superconductor (S), have lately received much attention mainly because of a big variety of associated quantum effects. The key mechanism governing the carrier transport through the NS contact is the Andreev reflection. When a normal metal is placed between two superconducting electrodes another mechanism is involved in the charge transfer. It is the multiple Andreev reflection process (MAR). These phenomena result in nonlinear current-voltage characteristics, which exhibit an anomalous resistance dip at zero bias, the subharmonic energy gap structure, etc. Although at present the properties of single SNS junctions are well studied both theoretically and experimentally, the effect of the Andreev reflection and MAR process on the properties of a system consisting of a large number of normal metal regions connected by superconducting islands is practically not investigated. I will present the results of low-temperature transport measurements on two-dimensional arrays, on chains of SNS junctions and on single SNS junctions fabricated on the basis of superconducting PtSi film and perform a comparative analysis of their properties. Some unexpected coherence effects are found in multiply connected SNS systems, namely: (i) the gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases, (ii) the strengthening of subharmonic energy gap structure in two-dimensional arrays of SNS junctions, (iii) a rich fine structure in the dV/dI-V dependences at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2Δ/e in chains of SNS junctions. All these results show that coherent phenomena governed by the Andreev reflection are not only maintained over the macroscopic scale but manifest novel pronounced effects as well.
I will discuss some possible approaches to explain the observed phenomena.

Relativistic doppler effect: universal spectra and zeptosecond pulses

18 March 2004 in 11:30

A. Pukhov, S. Gordienko

For the first time we report on a numerical observation of the train of zeptosecond pulses produced by reflection of a relativistically intense femtosecond laser pulse from the oscillating boundary of an overdense plasma because of the Doppler effect. These pulses promise to become a unique experimental and technological tool since their length is of order of the Bohr radius and the intensity is extremely high ~10¹⁹ W/cm2. We present the physical mechanism, analytical theory, and direct particle-in-cell simulations. We show that the harmonic spectrum is universal: the intensity of the n-th harmonic scales as 1/n³ for n < 4γ², where γ is the largest γ-factor of the electron fluid boundary. The subattosecond pulses originate from this universal spectrum.

Border ledge fluctuations of crystal facets

4 March 2004 in 11:30

Michael Praehofer

Crystals in equilibrium consist of facets connected by rounded surfaces. On atomic scale the facet is surrounded by step lines or ledges whose density vanishes. There fluctuations are entropically reduced with respect to a single meandering ledge. We develop a scaling theory for the fluctuations and connect the statistics of the last ledge to the eigenvalue statistics of random matrices.

Semiclassical statistics of noise in mesoscopic systems

26 February 2004 in 11:30

K. Nagaev

A brief overview of the traditional Full Counting Statistics in mesoscopic systems will be given and principles of recently proposed semiclassical statistics of noise will be formulated. Special emphasis will be put on the frequency dependence of higher cumulants of noise.

Cluster model of local structure and bond orientational order in Lennard-Jones liquid

19 February 2004 in 11:30

V.N. Ryzhov (HPPI)

While the experimental and phenomenological knowledge of non-ergodic amorphous phases has been considerably improved in the last time, progress in the first-principle statistical mechanical studies of physical properties of supercooled liquids and glasses is much more slow. In this talk we discuss the microscopic approach to the study of the local structure and possible phase transitions in supercooled liquids based on the generalization of the density functional theory in classical statistical mechanics. A concept of the bond orientational order in simple liquids in two and three dimensions is reviewed, and the model of interacting cubic and icosahedral clusters in the Lennard-Jones liquid is proposed. In the framework of the model the analog of spin glass quenched disorder appears in a natural way in the Lennard-Jones system. The model is solved in the mean-field replica symmetric approximation. It is shown that the bond orientational order grows smoothly upon cooling, the symmetry of the ordered state being mainly cubic. The temperature of the possible glass transition is identified with the temperature at which the replica symmetry is broken.

Ordered states and phase transitions in two-dimensional frustrated XY-models

12 February 2004 in 11:30

S.E. Korshunov

Mean-field replica approach for liquid-glass phase transition

5 February 2004 in 11:30

V. Dotsenko

In this talk we discuss recently developed statistical mechanical ideas for structural glasses. In particular we focus onto the first principle and mean-field computations for simple models of glasses with the two-body interparticle potentials. It is argued that on a qualitative level the equilibrium thermodynamics of the low-temperature glassy phase, as well as the liquid-glass phase transitions, can be described using the methods developed in the replica theory of spin-glasses and others disordered systems.

Bubble motion in inclined pipes

22 January 2004 in 11:30

A.Yu. Dem’yanov, N.A. Inogamov, A.M. Oparin

We analyze strongly nonlinear fluid motion with free surface in vertical, inclined, and horizontal pipes. The problem concerning rise of buoyant bubbles in vertical pipes is closely connected to the problem of Rayleigh-Taylor instability (instability of hydrostatic equilibrium when heavy fluid is imposed above light one). Inclined pipes are intensively investigated in connection with problems of transportation of gas-liquid or liquid-liquid flows. We develop a new approach to the problem of motion of large bubbles in wide pipes (large and wide mean that capillary scale is small). As against the previous approaches based on semiempirical methods, in the given work the analytical methods concerning the theory of potential are used. We do careful comparison of obtained solutions for two and three-dimensional spaces. It is shown that not always increase of dimension leads to an increase in velocity of rise of bubbles (as it is usually supposed). For the first time direct numerical simulations (DNS) are applied for studies of flows with free boundary in inclined pipes. We also use them in our work. They allow us, first, to check up accuracy of our analytical models and, second, to obtain the general picture of motion.

Weak crystallization in polymer systems

8 January 2004 in 11:30

Igor Erukhimovich (Condensed matter theory group, Institute of Physics, Johannes-Gutenberg-University of Mainz, Germany and A.N. Nesmeyanov Institute of Organoelement Compounds RAS, Moscow, Russia)

In this talk we review the current state of the theoretical and experimental results on polymer systems capable to undergo weak crystallization phase transition and present some of our new results in this field. The following issues will be addressed: 1. What are the block copolymers, polyelectrolytes, random and randomly correlated copolymers and why are they expected to undergo weak crystallization. 2. The basic idea of the Leibler microscopic theory of weak crystallization in block copolymers: relationship between the vertices of the phenomenological weak crystallization Hamiltonian and the structural correlators of the ideal macromolecules. 3. The phenomenological and microscopic theories of the conventional (body-centered cubic, hexagonal and lamellar) and non-conventional (gyroid, bcc2, face-centered cubic, simple cubic etc.) phases in block copolymers. 4. The SAXS data and other experimental methods to locate the phase diagrams of the weak crystallized polymer systems. Fluctuation (Brazovskii-Fredrickson-Helfand) effects and their experimental confirmation. 5. Non-centrosymmetric lamellar phase in block copolymer blends: experiment. 6. Phenomenological theory of non-centrosymmetric lamellar phase in the degenerate many-component weak crystallized systems.

Classical and quantum regimes of the superfluid turbulence

25 December 2003 in 11:30

G.E. Volovik

We discuss the turbulence of quantized vortex lines in connection with recent experiments in superfluid ³He [1]. This turbulence is governed by two dimensionless parameters. One of them is the intrinsic parameter q which characterizes the friction forces acting on a vortex moving with respect to the heat bath, with 1/q playing the same role as the Reynolds number Re = UR/ ν in classical hydrodynamics. It marks the transition between the "laminar" and "turbulent" regimes of vortex dynamics as suggested by recent experiments in Helsinki. The developed turbulence described by Kolmogorov cascade occurs when Re ›› 1 in classical hydrodynamics, and it must occur at q ‹‹ 1 in the superfluid hydrodynamics. Another parameter of the superfluid turbulence is the superfluid Reynolds number Res = UR/κ, which contains the circulation quantum κ characterizing quantized vorticity in superfluids. This parameter may regulate the crossover or transition between two classes of superfluid turbulence: (i) the classical regime of Kolmogorov cascade where vortices are locally polarized forming fat vortex tubes, so that the quantization is not important; (ii) the quantum turbulence whose properties are determined by the quantization of vorticity. The phase diagram of the dynamical vortex states is suggested [2]. [1] A.P. Finne, T. Araki, R. Blaauwgeers, V.B. Eltsov, N.B. Kopnin, M. Krusius, L. Skrbek, M. Tsubota, and G.E. Volovik, "An intrinsic velocity-independent criterion for superfluid turbulence", Nature 424, 1022-1025 (2003). [2] G.E. Volovik, "Classical and quantum regimes of the superfluid turbulence", Pis'ma ZhETF 78, 1021-1025 (2003).

Instanton-like baryon number violation in high-energy electroweak collisions

18 December 2003 in 11:30

F. Bezrukov, D. Levkov, V. Rubakov (INR, Moscow), C. Rebbi (U. Boston), P. Tinyakov (EPFL, Lausanne and INR, Moscow)

We make use of a semiclassical method for calculating the suppression exponent for topology changing transitions in high-energy collisions. In the Standard Model these processes are accompanied by violation of baryon and lepton number. By using a suitable computational technique we obtain results for s-wave scattering in a large region of initial data. Our results show that baryon and lepton number violation remains exponentially suppressed up to very high energies of at least 30 sphaleron masses (250 TeV). We also conclude that the known analytic approaches inferred from low energy expansion provide reasonably good approximations up to the sphaleron energy (8 TeV) only.

Duality in (2+1)D quantum elasticity: superconductivity and quantum nematic order

4 December 2003 in 11:30

J. Zaanen, Z. Nussinov, and S.I. Mukhin

Superfluidity and superconductivity are traditionally understood in terms of an adiabatic continuation from the Bose-gas limit. We do not use this approach. Taking the theory of quantum elasticity (describing phonons) in a 2+1 D Bose system as a literal quantum field theory, we show that superfluidity and superconductivity (in the EM charged case) emerge automatically when the shear rigidity of the elastic state is destroyed by the proliferation of topological defects (quantum dislocations). We consider the nematic states, corresponding with condensates of dislocations, with Burgers vectors as topological charges, under condition that disclinations remain massive. Due to glide principle for dislocations in 2+1 D the compression rigidity decouples from the dislocation condensate and stays massless. The shear rigidity does not decouple, and as a result the shear modes acquire a Higgs mass in the dual (dislocation) condensate. Hence, the fluids are characterized by an isolated massless compression mode and are therefore superfluids (Landau criterium reconfirmed!). We also consider different ordered/disordered states of Burgers vectors calling them "Coulomb nematic" and "quantum smectic" of a novel kind. Finally, we find that the Higgs mass of the shear gauge fields, becoming finite in the nematic quantum fluids, automatically causes a Higgs mass in the electromagnetic sector by a novel mechanism. Hence, a new hydrodynamical way of understanding the conventional electromagnetic Meissner state (superconducting state) is proposed.

Specific features of the insulator formed under the superconductor-insulator transition

20 November 2003 in 11:30

V.F. Gantmakher (Institute of Solid State Physics)

Various experimental observations of the magnetic-field-induced superconductor-insulator transition are described and compared with different theoretical models: one based on boson-vortex duality (Girvin, M.P.A. Fisher, et al.), next exploring the properties of granular superconductors (Beloborodov and Efetov) and the third analyzing effect of the superconducting fluctuations in the magnetic field at low temperature (Galitski and Larkin). All the models point to the existence of pairwise electron correlations at the Fermi-level of the insulator (so-called localized pairs) which should vanish in high magnetic fields. The localized pairs apparently come from the parity effect in ultra small quasigrains - local minima of the random potential which can admit only small limited number of electrons.

A short introduction to conformal field theory and its integrable perturbations, part II

13 November 2003 in 11:30

A.A. Belavin

No annnotation presented.

Superfluid and crystal phases in bilayer system

6 November 2003 in 11:30

E. YuriiLozovik (Institute of Spectroscopy, Troitsk)

Bilayer electron-hole and electron-electron systems are considered. Phase diagram, transport, drag effect, optical properties and Josephson phenomena will be reviewed. Liquid excitonic phase will be analyzed. Light backscattering in excitonic condensate will be reported. The electron-hole system in strong magnetic field will be considered.
Pairing in composite fermion bilayer is analyzed. The problem of BCS instability of compressible unpaired quantum Hall bilayer state at nu = 2 x 1/(2m) total filling fraction in large interlayer separation, d, limit is discussed. Microscopic analysis is carried out within the framework of composite fermion formalism. Gauge field fluctuations both diagonal and off-diagonal on layer indexes is taken into account. The first defines singular renormalization in one layer and leads to marginality of composite fermion liquid; the nondiagonal contribution defines interlayer interaction. Interlayer composite fermion attraction governed by antisymmetric density fluctuations is taken into account. The role of marginality on BCS pairing is analyzed. The quantum phase transition governed by interlayer separation is discussed.

Vortex phase diagram in clean and disordered layered superconductors

30 October 2003 in 11:30

Kees van der Beek (Ecole Polytechnique, France), S. Colson, M. Konczykowski, Y. Matsuda, M. Gaifullin, P. Gierlowski, I. Abalosheva, M. Li, P. Kes

The (B,T) phase diagram in layered superconductors in a magnetic field can be characterized by two transitions of the vortex lattice: a first order phase transition, commonly called vortex lattice melting, from a "vortex solid" to a "vortex liquid" without long range phase coherence, and a "depinning" transition above which no critical current can be measured. The two transitions follow quite different B(T) dependences, and can be studied independently by tuning material parameters, strength of disorder, or magnetic field. In the layered superconductor Bi2Sr2CaCu2O8, vortex fluctuations leading to either transition can be studied using the Josephson Plasma Resonance. In the first part of the talk, I shall present measurements in the "vortex solid" phase, at fields close to the first order transition field. The results show the predominant role of the vortex line tension, and the near-irrelevance of the vortex lattice shear modulus. This result is underscored by the observation of the first order transition in Bi2Sr2CaCu2O8 containing strong columnar pins. Finally, I shall present results obtained at higher fields, near the "depinning" or "delocalisation" transition. The results are put into perspective by a conjecture about the nature of the two transitions in magnetic field.

Magnetization reversal and two level fluctuations by spin-injection in a ferromagnetic metallic layer

30 October 2003 in 11:30

Jean-Eric Wegrowe (Ecole Polytechnique, France)

Slow magnetic relaxation and two level fluctuations measurements under high current injection is performed in single-contacted ferromagnetic nanostructures. The magnetic configurations of the samples are described by two metastable states of the uniform magnetization. The current-dependent effective energy barrier due to spin-transfer from the current to the magnetic layer is measured. The comparison between the results obtained with Ni nanowires of 6 μm length and 60 nm diameter, and Co (10 nm) / Cu (10 nm) / Co (30 nm) nanometric pillars of about 40 nm in diameter refined the characterization of this effect. It is shown that all observed features cannot be reduced to the action of a current dependent effective field. Instead, all measurements can be described in terms of an effective temperature, which depends on the current amplitude and direction, and on the magnetization state. The effective temperature is measured to be about 2000 K for 1 mA in nano-pillar structures, and 30 000 K for 1mA in Ni nanowires (far beyond the Curie temperature). The system is then analogous to an unstable open system. The effect of current induced magnetization reversal is interpreted as the balance of spin injection between both interfaces of the ferromagnetic layer.

A short intorduction to conformal fiel theory (CFT) and its integrable pertrubations

23 October 2003 in 11:30

A.A. Belavin

Part I. Critical behaviour and conformal symmetry.
Conformal symmetry in two dimensions.
Operator products expansion. Conformal bootstrap.
Classification of conformal field thories.
Minimal models of CFT. Spectrum of dimensions.
Correlation functions in CFT.

Growth in systems of vesicles and membranes

19 June 2003 in 11:30

E. Kats

A simple phenomenological model of nucleation and growth in systems of vesicles and membranes is presented and analyzed. It is shown that the process of vesicle growth can be understood as a certain "reaction" and the driving force for this reaction is determined by anharmonic contributions to the curvature elasticity. Assuming that the aggregation is controlled by diffusion a simple kinetic approach predicts that the average radius increases in time $t$ as $t^{1/6}$ in close agreement with experimental data.

Surface tension of liquid helium

19 June 2003 in 11:30

A. Dyugaev

The temperature dependence σ (T) of liquid He³ and He⁴ is determined in the intermediate temperature region when He³ and He⁴ are quantum but not degenerate liquids. This dependence is given by the universal law σ (T) - σ (0) ∝ T² that is related to the contribution of the surface energy levels of helium vapor. These discrete levels also alter the mobility of electrons on the surface of liquid helium or of solid hydrogen. The work is joint with P. Grigoriev.

Collision group and renormalization of the Boltzmann collision integral

29 May 2003 in 11:30

V.L. Saveliev (Institute of Ionosphere)

On the basis of a recently discovered collision group, the Boltzmann collision integral is exactly rewritten in two parts. The first part describes the scattering of particles with small angles. In this part the infinity due to the infinite cross sections is extracted from the Boltzmann collision integral. Moreover, the Boltzmann collision integral is represented as a divergence of the flow in velocity space. Owing to this, the role of collisions in the kinetic equation can be interpreted in terms of the nonlocal friction force that depends on the distribution function.

Elastic turbulence: experiment and theory

15 April 2003 in 11:30

V. Lebedev

We are going to discuss a new phenomenon recently discovered experimentally by Groisman and Steinberg. It is the so-called elastic turbulence which is a random flow developed in weak polymer solutions at small Reynolds numbers. The reason for the chaotic state is in elastic instabilities which are relevant when the characteristic time of the flow fluctuations is of the order of the polymer relaxation time. The most interesting experimental data concern mixing, since the elastic turbulence is in some sense the ideal mixer. A research experimental information is obtained concerning statistical properties of the passive scalar advected by the chaotic flow in the elastic turbulence regime. The statistics admits a detailed theoretical derivation. We give a comparison of the theory and the experimental data.

On infinite symmetries in integrable lattice models and the corner transfer matrix approach

4 April 2003 in 11:30

Y. Pugai

During last ten years the algebraic approach based on vertex operators has been applied for finding exact integral representations for correlation functions and form-factors in many exactly solvable two-dimensional models of statistical mechanics. This approach can be treated as a natural generalization of the Baxter Corner Transfer Matrix method. Using off-critical RSOS models as an example I would like to briefly discuss some properties of corner transfer matrices and give their algebraic interpretation in the spirit of the conformal field theory Virasoro algebra approach. Finally I would like to report on some recent results for correlation functions of Z(N) symmetric models based on the deformed parafermionic algebra.

Boundary value problems for integrable PDE's

20 March 2003 in 11:30

A. Degaperis, S.V. Manakov, P. Santini (Dipartamento di Fisica, Universita di Roma la Sapienza)

We review recent developments in the study of the Dirichlet and Newmann boundary value problems for linear and soliton PDE's.

Kelvin-Helmholtz instability in superfluids and balck hole horizon at the brane separating two quantum vacua

13 March 2003 in 11:30

G.E. Volovik

An analog of black hole can be realized in the future experiments in Helsinki. The horizon can be constructed for the 'relativistic' ripplons (surface capillary-gravity waves) living on the 'brane' represented by the interface between two superfluid vacua, ³He-A and ³He-B, sliding along each other without friction. Similar experimental arrangement has been already used in ROTA experiments for the observation and investigation of the Kelvin-Helmholtz type of instability in superfluids.
The shear-flow instability in superfluids is characterized by two critical velocities. The lowest threshold which has been measured in recent experiments corresponds to appearance of the ergoregion for ripplons (the region where ripplons have negative energy in the frame of the environment). In the shallow-water geometry this will give rise to the black-hole event horizon in the effective metric experienced by ripplons. In the region beyond the ergosurface or horizon, the brane quantum vacuum is unstable due to interaction of brane matter (ripplons) with bulk matter (quasiparticles living in the higher-dimensional world of bulk superfluids). The development of this instability results in nucleation of vortices in ³He-B and shrinking of the black hole horizon. This mechanism of the black hole decay can be faster than due to the traditional Hawking radiation.
The second critical velocity, the proper Kelvin-Helmholtz instability threshold, corresponds to the 'physical' singularity inside the black hole, where the determinant of the effective metric becomes infinite.

Cosmology near singularity

6 March 2003 in 11:30

D. Podolsky

We discuss classical and quantum properties of several string-inspired cosmological models. The subject of main interest for us is their behaviour near the cosmological singularity where one should expect strong discrepancies between Einstein-Hilbert general relativity and gravitation described by string low-energy effective action.

Light-cone integral approach to the Landau-Pomeranchuk-Migdal effect in QED and QCD

20 February 2003 in 11:30

B.G. Zakharov

In 1953 Landau and Pomeranchuk predicted within classical electrodynamics that multiple scattering can suppress considerably bremsstrahlung of high energy charged particles in medium. Later, in 1956 Migdal developed a quantum theory of this effect (usually called the Landau-Pomeranchuk-Migdal effect).
In this talk I discuss a new approach to the LPM effect based on the path integral treatment of multiple scattering. The approach is also applicable to gluon emission from a fast quark in a hot QCD matter. The rate of photon (gluon) radiation by an electron (quark) in a medium is expressed in terms of the Green function of a two-dimensional Schrodinger equation with an maginary potential. In QED this potential is proportional to the dipole cross section for scattering of an e e- pair off an atom, while in QCD it is proportional to the cross section of interaction of the color singlet quark-antiquark-gluon system with a color center.
In the case of QED we compare theoretical predictions with the first accurate data on the LPM effect obtained at SLAC. For most of the targets our predictions are in excellent agreement with the experimental data.

Waves and particles in layered media

23 January 2003 in 11:30

V. Ignatovich

We deduce and demonstrate simple analytical formulas for calculation of scattering of waves and particles in multilayered systems. In particular, reflection and transmission in quantum mechanics for arbitrary periodic potentials with finite number of periods. Algorithm for preparation of supermirror, or filters. Calculation of bound levels splitting in periodic potentials. Diffraction in three dimensional periodic systems. New form of dynamical diffraction in single crystals.

Spin transport in superconducting hybrid structures

16 January 2003 in 11:30

N.M. Chtchelkatchev

Not long ago emerged new direction in mesoscopic physics dealing with spin transport in nanostructures (traditionally mesoscopic physics deals with (coherent) electron transport in nanostructures). One of the important questions is description of spin transport in superconducting hybrid mesoscopic structures. For example, consider a beam of spin-polarized electrons scattering from thin superconducting region. It turns out that the superconductor at certain conditions plays the role of spin-filter separating the spin-current from the charge current. Consider now a beam of Cooper pairs (supercurrent) going through a constriction from one superconductor to another one. The constriction plays the role of quantum dot for Bogoliubov quasiparticles (Andreev quantum dot (AQD) is a superconducting junction where Bogolyubov quasiparticles can be trapped in discrete Andreev levels). Andreev levels in superconducting junctions are in general spin-degenerate. However in the presence of magnetic/exchange field or spin-orbit interaction spin degeneracy is lifted. Sometimes it is feasible to manipulate individual spin-state in an AQD. It can be shown that AQD can be brought into spin-1/2 state. The coupling between spin and superconducting current facilitate manipulation and measurement of this state in comparison with common semiconductor quantum dots. AQD's coupled inductively can serve as a solid-state base for universal quantum computing.

Single-electron transport through the vortex core levels in clean superconductors

26 December 2002 in 11:30

N.B. Kopnin, A.S. Mel’nikov, V.M. Vinokur

We investigate the low temperature electron transport in N-S-N structures in the presence of vortex lines perpendicular to the interfaces. It is shown that, in the absence of elastic scattering at the interface, the single-electron transport along the vortices in clean superconductors is not determined by the density of states in vortex cores. Within the quasiclassical approach the vortex contribution to the transport is determined by resonance tunneling of electrons via vortex core levels. We calculate the thermal conductance in the direction along the magnetic field and show that it decays with an increase in the superconducting slab thickness.

Self-organization of the critical state in granular superconductor

19 December 2002 in 11:30

S.L. Ginzburg and N.E. Savitskaya (Petersburg Nuclear Physics Institute)

We study the critical state of a one-dimensional multijunction SQUID with a random arrangement of junctions in an increasing magnetic field. Using two mathematical models (system of differential equations for gauge-invariant phase differences and a simplified algorithm), we show that the system demonstrates a self- organized behavior. An intrinsic spatial randomness introduced into the model allows us to obtain self-organization in one-dimensional case under fully deterministic perturbation. We also show that our simplified algorithm represents a new model of a self-organized criticality.

Coulomb correlation effects in the tunelling spectroscopy of individual impurities

5 December 2002 in 11:30

P.I. Arseyev (Lebedev Physical Institute)

Some non-equilibrium Coulomb effects in resonant tunnelling through deep impurity states are analyzed. It is shown that corrections to the tunnelling vertex caused by the Coulomb interaction can result in nontrivial behavior of the tunnelling characteristics and should be taken into account. One encounters with effects similar to the Mahan edge singularities in the problem of X - ray absorption spectra in metals. One might expect in this situation a smeared power-law singularity in current-voltage characteristics near the threshold voltage.

Shedding and interaction of solitons in weakly disordered optical fibers

31 October 2002 in 11:30

M. Chertkov, Y. Chung, A. Dyachenko, I. Gabitov, I. Kolokolov, and V. Lebedev

Propagation of a soliton pattern through an optical fiber with weakly disordered dispersion is considered. Solitons, perturbed by this disorder, radiate, and, as a consequence, decay. The average radiation profile and the degradation law of a single soliton is found. The emergence of a long-range intra-channel interaction between the solitons, mediated by the radiation, is reported. We show that soliton in a multi-soliton pattern experience a random jitter: average force acting on a soliton is negligible and fluctuations of the soliton velocity are Gaussian, with a typical fluctuation proportional to square of the distance passed by the soliton in the fiber and to square root of the information rate (number of solitons per unit length of the fiber). We also present results of direct numerical simulation of the soliton decay and two-soliton interaction, confirming our theoretical analysis.

N=1 supersymmetric gauge theories and geometry

17 October 2002 in 11:30

K. Saraikin

We will review recent progress in gauge theories with minimal (N=1) supersymmetry: confinement, instanton corrections, superpotentials, relation with Seiberg-Witten theory etc. In particular, we will discuss how low-energy effective action of such theories can be reproduced in pure geometrical terms using certain (Calabi-Yau) three dimensional complex manifolds.

Trans-planckian particle creation in cosmology

26 September 2002 in 11:30

A.A. Starobinsky

If the Lorentz invariance is broken (or "deformed") at some large energy close to the Planckian one, so that the dispersion law for elementary particles ω(k) differs from the standard one, the expansion of the Universe may result in gravitational creation of pairs of particles and antiparticles with a very high energy. The expansion of the Universe (both at present time and in the early Universe) gradually redshifts momenta of all Fourier modes of a quantum field and transports them from the trans-Planckian region of very high momenta to the sub-Planckian region where the standard particle interpretation is valid. Then, if the WKB condition is violated somewhere in the trans-Planckian region, the field modes enter the sub-Planckian region in a non-vacuum state containing equal number of particles and antiparticles.
This effect, if exists at all, can be found or limited by cosmological observations. The most restrictive upper limit follows from the number of ultra-high energy cosmic rays created now. In turn, their total amount can be shown to be bounded by the cosmic diffuse gamma-ray background. This limit rules out the possibility to detect signatures of such short distance effects by studying the temperature anisotropy of the cosmic microwave background. On the other hand, a remarkable possibility that some part of observed ultra-high energy cosmic rays originates from new trans-Planckian physics remains open.

Statistical mechanics of magnetic-field sensitive structural glass

12 September 2002 in 11:30

Giancarlo Jug

Recent experiments on the thermal, dielectric and acoustic properties of multicomponent glasses at low temperatures have revealed an unusual response in some window-glasses to a weak applied magnetic field. A statistical-mechanics theory will be presented to explain quantitatively, at least in part, these puzzling experimental findings. The theory is based on an extension of the standard tunneling model for structural low-temperature glasses and on the mean-field theory of spin-glasses, suitably adapted to the problem.

Acceleration of chemical reactions by chaotic mixing

20 June 2002 in 11:30

V.V. Lebedev

A comprehensive theory of the binary chemical reaction A+B->C in chaotic flows at large Schmidt (Sc) and Damköhler (Da) numbers, and with initially injected equal (or close to equal) amounts of both chemicals, is developed. The combined effects of advection and diffusion are important and result in essential enhancement (in comparison with a situation without advection) of the overall reaction rate. Diffusion controls the fluxes of chemicals towards the interface separating the chemicals, while advection increases area of the interface. We identify four different stages of the spatio-temporal evolution. The chemicals are well separated during all but the final (and the only spatially uniform) stage. The relatively complex division of the evolution into the stages is due to finite-size effects which appear when the chemical reaction rate is controlled by the bulk, peripheral, and boundary domains of the flow.

Internal waves and synchronized precession in a cold vapor

13 June 2002 in 11:30

L. Levitov

It was shown by E. Bashkin in 1981 that exchange in a Boltzmann gas of bosons with several internal states leads to collective transport of internal polarization. The internal dynamics can be understood as Larmor precession in the presence of a torque induced by atoms on each other via exchange coupling. In a recent experiment carried out in JILA, collective waves of internal state polarization in a cold magnetically trapped Rb vapor were observed by a new spatially resolved technique. A generalized Bloch equation that includes interatomic exchange effects as well as orbital motion in the gas is derived and used to interpret the JILA experiment. It is predicted that exchange leads to formation of domains in which precession frequencies are synchronized.
(M. Oktel and L.L., PRL 88, 230403, 2002)

On the motion of muplicharge bubbles in helium.(short communication)

13 June 2002 in 11:30

R. Arhipov

No anntotation.

On integrable systems connected with quantum groups in root of unity

6 June 2002 in 11:30

A.A. Belavin

Centre of Quantum Group (QG) in root of unity is extended. It makes possible to perform Quantum Group reduction of Integrable model connected with the QG. It can be shown that after this Quantum Group reduction the transfer-matrices of the model satisfy the closed system of the truncated functional relations.

Quantization of some cubic Poisson brackets

6 June 2002 in 11:30

A. Odesskii

We discuss the problem of quantization of certain class of Poisson structures, for example {x,y}=z³, {y,z}=x³, {z,x}=y³. Our main tool is invariant pseudodifferential operators on the complex half-plane.

Simple interfacial model of surface-induced smectic layering in liquid crystals

30 May 2002 in 11:30

E.S. Pikina and V.E. Podnek

We propose simple interfacial model of surface-induced smectic layering (quantized wetting layer growth) observed just above the bulk isotropic-smectic A phase transition in liquid crystals [1]. The model generalizes known interfacial models of critical wetting and roughening transition. It is shown that smectic wetting in the layering regime is always incomplete. The comparison with experimental data is made.
[1] B.M.Ocko, A.Braslau, P.S.Pershan, J.Als-Nielsen, and M.Deutsch, Phys.Rev.Lett., 1986, vol. 57, p. 94

Possible realization of an ideal quantum computer in Josephson junction array with topologically protected degenerate ground states

23 May 2002 in 11:30

L.B. Ioffe and M.V. Feigelman

We introduce a new class of Josephson arrays which have non-trivial topology and exhibit a novel state at low temperatures. This state is characterized by long range order in a two Cooper pair condensate and by a discrete topological order parameter. These arrays have degenerate ground states with this degeneracy 'protected' from the external perturbations (and noise) by the topological order parameter. We show that in ideal conditions the low order effect of the external perturbations on this degeneracy is exactly zero and that deviations from ideality lead to only exponentially small effects of perturbations. We argue that this system provides a physical implementation of an ideal quantum computer with a built in error correction and show that even a small array exhibits interesting physical properties such as superconductivity with double charge, 4e, and extremely long decoherence times.

The ground state and excitations in 2deg in magnetic field at large Coulomb interaction

16 May 2002 in 11:30

S.V. Iordanski, A. Kashuba

Some general properties of possible classification of the ferromagnetic ground state are investigated. The spectrum of the low energy collective excitations consists of Kohn exciton and gapless spin exciton. It is shown that the energy of topological excitations (skyrmions) is lower than the cyclotron energy (at large Coulomb interaction) and does not contain the large exchange energy typical for opposite case of extremely large magnetic field.

Isotropisation of Bianchi I brane cosmological models

4 April 2002 in 11:30

A.V. Toporensky (Sternberg Astronomical Institute)

The problem of isotropisation of Bianchi I brane cosmological model is described. It is shown that unlike a standard scenario the presence of an ordinary matter with a positive pressure enlarges anisotropy of a brane at early stages of cosmological expansion (when brane corrections to Einstein equations are significant). A possible influence of an anisotropic stress on the brane dynamics is also discussed.

Hidden long range order in Heisenberg Kagome antiferromagnets

21 March 2002 in 11:30

A.V. Syromyatnikov (St. Petersburg)

We give a physical picture of the low-energy sector of the spin 1/2 Heisenberg Kagome antiferromagnet (KAF). It is shown that Kagome lattice can be presented as a set of stars which are arranged in a triangular lattice and contain 12 spins. Each of these stars has two degenerate singlet ground states which can be considered in terms of pseudospin. As a result of interaction between stars we get Hamiltonian of the Ising ferromagnet in magnetic field. So in contrast to the common view there is a long range order in KAF consisting of definite singlet states of the stars.

The threshold processes in external electromagnetic fields

21 March 2002 in 11:30

V.N. Rodionov, G.A. Kravtsova, and A.M. Mandel (Moscow)

The threshold processes in external electromagnetic fields of some configurations are considered. The decay of quasistationary states is studied by means of the probability equations. Starting with these equations, we obtain the expressions for complex energy as a function of the field. Moreover, asymptotics for Re and Im parts of the energy are studied. In particular, we prove the absence of the stable action of magnetic field for some configurations and discuss consequences of this result.

Bosonisation techniques in the theory of exactly solvable models

21 February 2002 in 11:30

M. Lashkevich

It is a review of bosonization techniques, their advantages and problems. These techniques take their origin in Baxter's corner transfer matrix approach and bosonization of conformal field theory. Bosonization makes it possible to calculate exactly short-range correlation functions and form factors in a class of lattice models. In the case of models of quantum field theory similar techniques are related to quantization in the Rindler coordinates and provide exact integral expressions for form factors.

Nuclear ferromagnetism induced Fulde-Ferrel-Larkin-Ovchinnikov state

24 January 2002 in 11:30

A.M. Dyugaev

We present a theoretical study of the influence of the nuclear ferromagnetism on superconductivity in the presence of the electron-nuclear spin interaction. It is demonstrated that in some metals, e.g. Rh, W, the BCS condensate imbedded in a matrix of ferromagnetically ordered nuclear spins should manifest the FFLO (Fulde-Ferrel-Larkin-Ovchinnikov) state. We outline that the optimal experimental conditions for observation of FFLO could be achieved by creation, via adiabatic nuclear demagnetization, of the negative nuclear spin temperatures. In this case the nuclear polarization points in the opposite to the external magnetic field direction and the electromagnetic part of the nuclear spin magnetization compensates the external magnetic field, while the exchange part creates the nonhomogeneous superconducting order parameter.

Charge-density wave (CDW) and spin-density-wave (SDW) phases induced by a magnetic field in low-dimensional solids

10 January 2002 in 11:30

A.G. Lebed

A cascade of the metal-CDW phase transitions (recently discovered by D.Anders et al.) and the cascades of the metal-SDW transitions (which are observed in some low-dimensional organic conductors) are theoretically analyzed. We calculate the transition temperatures and free energies of the CDW and SDW subphases induced by a magnetic field. For CDW, we predict novel kind of an angular resonance and a novel type of a metal-DW phase diagram. We discuss the connections of our results with the existing experimental data including the so-called "three-dimensional quantum Hall effect".

Effects of doping and disorder on the spin-Peierls state (experiment)

20 December 2001 in 11:30

S.V. Demishev (General Physics Institute of RAS)

1. Introduction. Organic and inorganic spin-Peierls materials. Universal properties of spin-Peierls materials: magnetic phase diagram and concentrational phase diagram. Specific case of CuGeO3.
2. Universality vs. experiment (cases of CuGeO3 and MEM(TCNQ)2). Discussion of the Mostovoi and Khomskii calculation of the concentrational phase diagram.
3. Outside the universal scenario: the case of CuGeO3 doped with magnetic impurities.
4. Specific problems of ESR in the 1D s=1/2 antifferomagnetic chains. Latest developments.

Josephson current between chiral superconductors

6 December 2001 in 11:30

Yu.S. Barash, A.M. Bobkov and M. Fogelstrom

We study chiral interface Andreev bound states and their influence on the Josephson current between clean superconductors. Possible examples are superconducting Sr2RuO_4 and the B-phase of the heavy-fermion superconductor UPt3. We show that, under certain conditions, the low-energy chiral surface states enhance the critical current of symmetric tunnel junctions at low temperatures. The enhancement is substantially more pronounced in quantum point contacts. In classical junctions dispersive chiral states result in a logarithmic dependence of the critical current. This logarithmic behavior contains the temperature, the barrier transparency and the broadening of the bound states, and depends on the detailed relation between these parameters. The Josephson current through the domain wall doesn't acquire this logarithmic enhancement, although the contribution from the bound states is important in this case as well.

Towards the theory of quantum black holes

22 November 2001 in 11:30

Victor Berezin (Institute for Nuclear Research RAS)

Everybody knows what the classical black holes are.
The main feature that characterizes black holes and distinguishes them from other physical objects is their universality. Such an universality is widely known as the "no hair conjecture". It means that everything that can be radiated away does radiate during the gravitational collapse, and the resulting black hole is described by only few parameters, namely, by its mass, angular momentum and gauge charges. The universal character of black holes allowed J.Bekenstein to put forward an analogy between black hole physics and thermodynamics. According to Bekenstein, the black hole is endowed by some temperature and entropy, the value of entropy being proportional to the area of the event horizon. The rigorous proof of four laws of thermodynamics for the most general stationary black holes was given by Bardeen, Carter and Hawking. And at the top of these is Hawking's discovery that a black hole does radiate as a black body with this very temperature. S. Hawking considered a quantized scalar field on the given Schwarzschild background and showed that it is nontrivial causal structure of the black hole metric that is responsible for the black body radiation. Such an effect is purely quantum, and the Hawking temperature is a generalization of the Unruh temperature seen by an uniformly accelerated observer in the flat spacetime. The analogy between these two effects illustrates the famous equivalence principle.
The discovery of the black hole evaporation opened, in a sense, the quantum era in the black hole physics. The matter is that the very definition of the black hole involves the notion of the so called event horizon (which is the boundary between geodesics that can escape to infinity and those that cannot). The event horizon can be determined only globally, and the procedure requires knowledge of the whole history. Since, due to evaporation, the black holes disappear (at least, if do not take into account the back reaction of the radiation on the spacetime metric) the very notion of the black hole becomes only approximate.
Why should we study quantum black holes? First, this problem is interesting by itself and it could provide additional links between General Relativity and Quantum Theory. Second, at the final stage of evaporation black holes are so small that quantum effects can no more be ignored. Third, small black holes can be formed by large enough fluctuations of both matter fields or spacetime metric in the Very Early Universe (the so called Primordial Black Holes) or in the course of vacuum phase transitions. But how small should be a black hole in order to be considered as a quantum object? On purely dimensional grounds using Newton's constant G, Planck's constant $hbar$ and velocity of light c, we are able to construct two different quantities with the dimension of length for some object of mass m, namely, the Compton length $lambdabar/mc$ and the gravitational (Schwarzschild) radius rg = 2 G m/c². For masses much smaller than the Planckian mass mPl = $sqrt{hbar c/G}$ the gravitational radius is much smaller than the Compton length and the object is purely quantum (this also indicates that there may be no black holes with so small masses). If the mass is much higher than the Planckian mass, the Compton length is well inside the black hole horizon and in this case we are already have purely classical black holes (if it is possible, of course, to ignore the Hawking radiation). Thus, the range of quantum black hole masses is somewhere in-between. The first attempt to obtain the quantum black hole mass spectrum was due to J. Bekenstein. He noticed that an event horizon area for slowly evolving black holes is an adiabatic invariant. So, the usual quasiclassical quantization leads to an equidistant spectrum for a black hole surface area. In the case of the Schwarzschild black hole this results in the now famous square-root mass spectrum mBH∼$sqrt{n}$, where n is an integer quantum number. The same type of spectrum was then advocated by J.Bekenstein and S.Mukhanov later on by many others. It was shown also that such a spectrum is compatible with the Hawking radiation.
In what follows we confine ourselves with consideration of the Schwarzschild (neutral, nonrotating) black holes only. The very fact that the mass (total energy) of black holes depends on only one quantum number n can be viewed as a generalization of the "no hair" conjecture and confirms the universal character of black holes also on the quantum level. But we should pay some price for such a universality. And this price is that the every energy level is highly degenerate. Indeed, since the black hole entropy is proportional to the area of the horizon, and the latter has (approximately) an equidistant spectrum, the number of quantum states with the same total energy (mass) grows exponentially with the quantum number nThe real physical explanation of this phenomenon is still an open problem. In our opinion, this is because we do not yet know what is an object that could be called a quantum black hole. In other words, we need a definition. It seems it is the universality that could become the crucial feature which would distinguish quantum black holes from any other quantum object. It is worth noting that in the recent paper by G.Gour the equidistant spectrum for the black hole area and the exponential degeneracy of the energy levels have been taken as postulates. The author constructed the Hamiltonian operator and the algebra of observables for quantum Schwarzschild black holes and, thus, showed the selfconsistency of these two postulates. The abovementioned attempts to construct a theory of quantum black holes can be called phenomenological.
The equidistant (or any other) black hole area spectrum which leads to a discrete mass spectrum poses one serious problem. The classical, in our case - Schwarzschild, black hole state is described by only one parameter, its mass, irrespective of how this black hole has been formed. Let us suppose that the black hole is formed by the collapse of gravitating particles. The motion of these particles can be either bound or unbound, such qualitative difference in the history of the constituents in no way reflected in the mass of the resulting black hole. This is not surprising in classical theory. But in quantum theory it does makes a difference. The bound motions give rise to discrete mass spectra while the mass spectra for unbound motions are continuous. And if the black hole mass spectrum is just the mass (energy) spectrum of this system of particles (what is the case in classical theory) we can easily distinguish the black holes formed due to bound motion from that formed due to unbound motion. But this contradicts the principle of universality (quantum "no-hair" conjecture). Then, what is it that gives the discrete mass spectrum for quantum black holes? Clearly, we need a deeper insight into the nature of these objects.

Inhomogenious charged states and phase stratification for manganites

25 October 2001 in 11:30

M.Yu. Kagan, K.I. Kugel, D.I. Khomskii

Analiziruyutsya mekhanizmy elektronnogo fazovogo rassloeniya v oksidnykh materialakh tipa manganitov. Rassmotrenie provoditsya v ramkakh prostoy modeli kondo-reshetki s mezhuzel'nym kulonovskim ottalkivaniem elektronov. Eta model' pozvolyaet vyyavit' neustoychivost' odnorodnogo magnitnogo ili zaryadovogo uporyadocheniya otnositel'no obrazovaniya kapel'nykh struktur (magnitnykh polyaronov) v shirokoy oblasti parametrov fazovoy diagrammy. Issleduyutsya razlichnye tipy i formy magnitnykh polyaronov. Obsuzhdayutsya transportnye kharakteristiki i spektr shumov v fazovo-rassloennom sostoyanii.

Uniqueness of electrostatic solutions in the bulk of perfect crystals

18 October 2001 in 11:30

E.V. Kholopov (Institute of Inorganic Chemistry)

Although the investigation of lattice sums is a classical subject for more than a century, some problems concerned with their uniqueness in dependence on the mode of summation, as well as with the rate of convergence exist heretofore. In the present work a rigorous electrostatic theorem has been proved about the definite convergence of Coulomb sums over translation invariant lattices. As a result, any uniform potential contribution to bulk potentials is absent in perfect crystals. The topological nature of this issue is associated with the periodic boundary conditions invariant to the definition of the unit cell. The close relation between absolute local potentials and the bulk Coulomb energy is substantiated. It is principal that the analysis may be performed in terms of absolutely convergent series by means of the incorporation of fictitious charges appropriate to the case. As a result, the general criterion on admissible orders of the lattice summation is proposed. The shell-by-shell order of summation accounting for crystal fields is verified as universal. The numerical efficiency of the direct summation modified properly is exhibited in a set of various model cases, with including depolar sums. The topological nature of the Lorentz field becomes evident therefrom.

Topologically protected quantum bits from Josephson junction arrays

11 October 2001 in 11:30

L.B. Ioffe, M.V. Feigelman, A.S. Ioselevich, D.A. Ivanov, M. Troyer, G. Blatter

All physical implementations of quantum bits (qubits), carrying the information and computation in a putative quantum computer, have to meet the conflicting requirements of environmental decoupling while remaining manipulable through designed external signals. Protecting qubits from decoherence by exploiting topological stability, a qualitatively new proposal due to Kitaev, holds the promise for long decoherence times, but its practical physical implementation has remained unclear so far. Here, we show how strongly correlated systems developing an isolated two-fold degenerate quantum dimer-liquid groundstate can be used in the construction of topologically stable qubits. We propose two implementations based on Josephson junction arrays and show how to construct the gates shifting the amplitude and phase of individual qubits and enforcing the entanglement of two qubits.

This seminar was cancelled

4 October 2001 in 11:30

A.M. Gurin. Geometric attractor of an electron beam which has passed through a crystal. The Lemmlein algorithm assigns a cyclic interaction of a mathematical point with other (n+1) points of the n-dimensional Euclidean space. In this paper generalization A of the Lemmlein algorithm for an arbitrary number of points m situated in the n-dimensional Riemann space is proposed. Algorithm A generates a Markovian chain consisting of the finite number of combinatorially different strongly convergent attractors.

Brane-world effective action and origin of inflation

20 September 2001 in 11:30

A. Barvinsky (Lebedev Physical Institute)

We construct braneworld effective action in two brane Randall-Sundrum model and show that the radion mode plays the role of a scalar field localizing essentially nonlocal part of this action. Non-minimal curvature coupling of this field reflects the violation of AdS/CFT-correspondence for finite values of brane separation. Under small detuning of the brane tension from the Randall-Sundrum flat brane value, the radion mode can play the role of inflaton. Inflationary dynamics corresponds to branes moving apart in the field of repelling interbrane inflaton-radion potential and implies the existence acceleration stage caused by remnant cosmological constant at late (large brane separation) stages of evolution. We discuss the possibility of fixing initial conditions in this model within the concept of braneworld creation from the tunneling or no-boundary cosmological state, which formally replaces the conventional moduli stabilization mechanism.

The elasticity of single DNA molecules (a review)

28 November 2000 in 11:30

V. Golo (MSU)

The mechanical properties of the DNA molecule have been a subject of intense study for the last few years. The current approach is to consider DNA as a worm-like chain. The model allows for explaining major experimental results, but still there are questions to the effect, especially concerning the elastic behaviour as a function of the ionic strength of the solvent. In particular it contradicts the familiar model of elastic rod often used for describing DNA.

Instanton approach to quantum chemical dynamics

28 November 2000 in 11:30

V.A. Benderskii (ICP)

Quantum chemical dynamics is based on the eigenvalue problem for multidimensional vibration Hamiltonians with potential energy surfaces (PES) having at least two minima, which are associated with the initial and final quasi-stationary states, reactants and products of chemical reaction. Although a semiclassical approximation is to be valid, commonly used WKB methods appear to be ineffective for multidimensional problems due to singularities of WKB solutions on caustics. Instanton approach (IA) differs from WKB method by replacement of energy as the quantity of h-order from Gamilton-Jacobi equation into transport equation. IA allows us to solve the Landau-Zener problem in the configuration space representation and derive the quantization rules for intersecting diabatic potentials at an arbitrary value of adiabatic coupling. In multidimensional IA, the minimum action path (MAP) is found from the classical equation of motion and then the Euclidean action is calculated in the vicinity of MAP as the expansion over the powers of transverse displacements with tunneling coordinate dependent coefficients. Coherent-incoherent transitions in the asymmetric double-well potentials and vibration stimulated tunneling are discussed as examples of IA applications.

Gravitational fields described by integrable reductions of Einstein's field equations

14 November 2000 in 11:30

G.A. Alekseev (Steklov Institute)

Einstein's equations for gravitational fields in vacuum, the Einstein - Maxwell and Einstein - Maxwell - Weyl equations for gravitational, electromagnetic and massless Weyl spinor fields, some string theory induced gravity models with electromagnetic, axion and dilaton fields are known to become integrable, if the space-time admits the Abelian two-dimensional isometry group and all field components and their potentials are dependent on some two of the four space-time coordinates only. In the talk we consider a number of specific features of very rich and universal (i.e. common for all these integrable cases) internal structure of reduced Einstein's equations which gave rise to a development of some general and simple approach to solution of these equations which can be called as the "monodromy transform" approach. This approach has provided a convenient base for further development of some effective methods for construction of solutions with wanted physical and geometrical interpretation, superposition of some known field configurations, generation of infinite hierarchies of solutions with infinitely increasing number of parameters and gave rise to the linear algorithms for solutions of the Cauchy and characteristic initial value problems for the reduced Einstein's equations. We also discuss particular examples of the solutions corresponding to physically different types of field configurations, such as the black holes immersed into the external gravitational and electromagnetic fields, various types of waves with smooth profiles or distinct wavefronts colliding on the Minkowski background or propagating in some other space-times, the waves emitted by accelerated sources, dynamics of some inhomogeneous cosmological models, and describe their physical and geometrical properties.

Landau Institute for Theoretical Physics

Previous seminars in Kapitza Institute for Physical Problems

A Josephson relation for e/3 and e/5 fractionally charged anyons

On energy landscape of elastic manifolds pinned by random potentials

Unconventional magnons and their impact on spin pumping transport

Josephson and non-Josephson emission from Bi2Sr2CaCu2O8+δ mesa structures

Velocity distribution functions and intermittency in one-dimensional randomly forced Burgers turbulence

Post-Ehrenfest many-body quantum interferences in ultracold atoms far-out-of-equilibrium

Spontaneous Time-Reversal Breaking and Edge Reconstruction in Quantum Hall and Topological Insulators

Spin liquids from Majorana Zero Modes in a network of Cooper Boxes

Large deviations of surface height in the Kardar-Parisi-Zhang equation

Duality in Power-Law Localization in Disordered One-Dimensional Systems

Quantum simulation of Luttinger liquid physics in Josephson transmission lines

Interaction induced topological phases in one dimension

Together we can: transport properties of electron viscous flows

Replica approach with one step replica symmetry breaking in the problem of Anderson localization on the Bethe lattice

Finite-temperature conductance of strongly interacting quantum wire with a helical nuclear spin order

Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi2Se3

Dissipation-induced topological insulators: A recipe

Spiraling energy dispersion of arc states in Weyl semimetals

Random matrix approach to the jamming transition

Majorana fermion from Landau quantization in 2D topological superconductors

Electro-Convective Instability in Concentration Polarization

Electronic phase transitions induced by electric or optical impacts

Recent identifications of microscopic solitons in quasi 1D electronic systems and generalisations to higher dimensions

Thermal transport in the disordered electron liquid

The Essentially Entangled Component of Multipartite Mixed Quantum States, its Properties and an Efficient Algorithm for its Extraction

Occurrence of flat bands in strongly correlated Fermi systems and high-Tc superconductivity of electron-doped compounds

Topological Valley Currents in Gapped Dirac Materials

Dissipation of turbulence in superfluid 4He in the limit of zero temperature

Electron-electron interaction effects in monotonic and oscillatory magnetotransport in 2D electron systems

Entropy measurements in 2D systems

Can liquid water unmix?

New results on dark energy in the early and present Universe and the observational status of the inflationary scenario

Spinful fermionic ladders at incommensurate filling: Phase diagram, local perturbations, and ionic potentials

Nonequilibrium transport in high Landau levels of 2D systems: recent developments

Probing interactions with thermal transport

Composite fermion state of spin-orbit coupled bosons

Electrodynamics of strongly disordered superconductors

Anderson localization at the edge of 2D topological insulator

Hydrodynamics of active polar fluids

Conductivity of suspended graphene at the Dirac point

Coulomb drag in graphene near the Dirac point

Exciton many-body effects and Bose-Einstein condensation

Coherent Back Scattering and Anderson Localization of Ultra Cold Atoms

Cantor sets and Chalker’s scaling: a simple picture of critical eigenfunction statistics

Spin of superconducting quasiparticles

Superinductors: a novel type of Josephson ladders implementing quantum Ising model

Surprises, Computational Methods, and Results for Metastable Phenomena and Homogeneous Nucleation and Growth

Hanbury Brown and Twiss setup with anyons

Double proximity effect in hybrid planar Superconductor-(Normal metal/Ferromagnet)-Superconductor structures

Vortex dynamics in Fermi superfluids at ultra low temperatures

Solution of the Dyson-Schwinger equation on de Sitter background in IR limit

Counting free fermions on a line: a Fisher-Hartwig asymptotic expansion for the Toeplitz determinant in the double-scaling limit

Multifractality of wave functions: basic concepts and experiment

Heat, work and fluctuation relations in single-electron transport: experimental test of the Jarzynski equality

Tunneling electro-conductance of Bose condensates

Majorana fermions in pinned vortices

Universal manifestations of electron-electron interaction in conductivity of a 2D metal near a convex-concave Fermi surface transition

Coulomb drag between quantum wires

Theory of fractional quantum Hall interferometers

Electronic transport through a contact of a correlated quantum wire with leads of higher dimension

Optical excitation of Electron-Glasses

Three-way junctions of interacting quantum wires: Tunneling into a Luttinger liquid revisited

Fast vortices and superconducting fluctuations in the cuprates and amorphous superconductors

Statistics of energy flows, temperature fluctuations and underlying techniques

Nonequilibrium coherent dynamics in Charge Density Waves from femto-second optical experiments and their modeling

Kvantovaya zaputannost’ i osnovaniya statisticheskoi mekhaniki

Anomalous scaling and solitary waves in systems with non-linear diffusion

Josephson effect in a long diffusive SNS junction in a magnetic field

Electronic transport in Mach-Zehnder and Fabry-Perot interferometers realized with quantum Hall edge states

Vzaimodeistviya, korrelirovannye sostoyaniya i spin-dolinnaya simmetriya v dvukhsloinom grafene

Superinsulating state in two-dimensional superconducting systems: experiment and theory

Tunntling into the nonequilibrium state of the Luttinger liquid

Spontaneous symmetry breaking in general relativity. Brane world concept

Kramers degeneracy in a magnetic field and zeeman spin-orbit coupling in antiferromagnets

Nonequilibrium kinetics of a disordered Luttinger liquid

f(R) models of dark energy in the Universe

On statistical mechanics of a single particle in random logarithmically-correlated potentials

Computational investigation of short pulse laser irradiation with metals

Josephson and non-Josephson emission from Bi₂Sr₂CaCu₂O_8+δ mesa structures

Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi₂Se₃

Occurrence of flat bands in strongly correlated Fermi systems and high-T_c superconductivity of electron-doped compounds

Investigation of larkin-imry-ma effect: random anisotropy of aerogel destroys the long-range orientational order in superfluid ³He-A