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M. V. Feigel'man

Publications and source records attributed to M. V. Feigel'man.

At least 73 records · Page 4Linked to original sources
arXiv2000

Proximity Action theory of superconductive nanostructures

We review a novel approach to the superconductive proximity effect in disordered normal-superconducting (N-S) structures. The method is based on the multicharge Keldysh action and is suitable for the treatment of interaction and fluctuation effects. As an application of the formalism, we study the subgap conductance and noise in two-dimensional N-S systems in the presence of the electron-electron interaction in the Cooper channel. It is shown that singular nature of the interaction correction at large scales leads to a nonmonotonuos temperature, voltage and magnetic field dependence of the Andreev conductance.

cond-mat.supr-con↗
arXiv2000

Superconductive proximity effect in interacting disordered conductors

We present a general theory of the superconductive proximity effect in disordered normal--superconducting (N-S) structures, based on the recently developed Keldysh action approach. In the case of the absence of interaction in the normal conductor we reproduce known results for the Andreev conductance G_A at arbitrary relation between the interface resistance R_T and the diffusive resistance R_D. In two-dimensional N-S systems, electron-electron interaction in the Cooper channel of normal conductor is shown to strongly affect the value of G_A as well as its dependence on temperature, voltage and magnetic field. In particular, an unusual maximum of G_A as a function of temperature and/or magnetic field is predicted for some range of parameters R_D and R_T. The Keldysh action approach makes it possible to calculate the full statistics of charge transfer in such structures. As an application of this method, we calculate the noise power of an N-S contact as a function of voltage, temperature, magnetic field and frequency for arbitrary Cooper repulsion in the normal metal and arbitrary values of the ratio R_D/R_T.

cond-mat.supr-con↗
arXiv2000

Quantum superconductor-metal transition in a proximity array

A theory of the zero-temperature superconductor-metal transition is developed for an array of superconductive islands (of size d) coupled via a disordered two-dimensional conductor with the dimensionless conductance g>>1. At T=0 macroscopically superconductive state of the array with the lattice spacing b>>d is destroyed at g < g_c \approx 0.1 ln^2(b/d). At high temperatures the normal-state resistance between neighboring islands at b=b_c is much smaller than h/4e^2.

cond-mat.supr-con↗
arXiv2000

Self-consistent theory of the thermal softening and instability of simple crystals

We consider the thermal softening of crystals due to anharmonicity. Self-consistent methods find a maximum temperature for a stable crystal, which gives an upper bound to the melting temperature. Previous workers have shown that the self-consistent harmonic approximation (SCHA) gives misleading results for the thermal stability of crystals. The reason is that the most important diagrams in the perturbation expansion around harmonic theory are not included in the SCHA. An alternative approach is to solve a self-consistent Dyson equation (SCA) for a selection of diagrams, the simplest being a (3+4)-SCA. However, this gives an unsatisfactory comparison to numerical results on the thermal and quantum melting of two-dimensional (2D) Coulomb-interacting particles (equivalent to vortex-lattice melting in two and three dimensions). We derive an improved self-consistent method, the two-vertex-SCHA, which gives much better agreement to the simulations. Our method allows for accurate calculation of the thermal softening of the shear modulus for 2D crystals and for the lattice of vortex-lines in type-II superconductors.

cond-mat.stat-mech↗
arXiv2000

Defect-unbinding and the Bose-glass transition in layered superconductors

The low-field Bose-glass transition temperature in heavy-ion irradiated Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of irradiation-induced columnar defects, but saturates for densities in excess of 1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above which diffusion of two-dimensional pancake vortices between different vortex lines becomes possible, and above which the ``line-like'' character of vortices is lost. We develop a description of the Bose-glass line that is in excellent quantitative agreement with the experimental line obtained for widely different values of track density and material parameters.

cond-mat.supr-con↗
arXiv1999

Phase Diagram of Heavy-Ion Irradiated Bi_2Sr_2CaCu_2O_8

Using dynamic and thermodynamic magnetization measurements, we analyze the phase diagram of Bi_2Sr_2CaCu_2O_8delta single crystals containing amorphous columnar defects created by heavy-ion irradiation. Reversible magnetization experiments yield the respective magnitudes of the pinning energy and entropy contributions to the free energy of the vortex lattice. It appears that the entropy contribution in the London regime is relatively minor in both unirradiated and irradiated crystals, except in the case of high density of columns and inductions B that are smaller than the interaction field H_int ~ B_phi / 6. The dependence of the entropy contribution on vortex- and defect density correlates well with measurements of the irreversibility line H_irr, which shows a sharp increase at H_int.

cond-mat.supr-con↗
arXiv1999

Proximity Effect in Presence of Quantum Fluctuations

The effect of Coulomb interaction upon superconductive proximity effect in disordered metals is studied, employing newly developed Keldysh functional approach (cond-mat/9907358). We have calculated subgap Andreev conductance between superconductor and 2D dirty film, as well as Josephson coupling via such a film. Both two qualitatively different Coulomb effects - suppression of the tunneling density of states and disorder-enchanced repulsion in the Cooper channel - are shown to be important at sufficiently low temperatures.

cond-mat↗
arXiv1999

Keldysh action for disordered superconductors

Keldysh representation of the functional integral for the interacting electron system with disorder is used to derive microscopically an effective action for dirty superconductors. In the most general case this action is a functional of the 8 x 8 matrix Q(t,t') which depends on two time variables, and on the fluctuating order parameter field and electric potential. We show that this approach reproduces, without the use of the replica trick, the well-known result for the Coulomb-induced renormalization of the electron-electron coupling constant in the Cooper channel. Turning to the new results, we calculate the effects of the Coulomb interaction upon: i) the subgap Andreev conductance between superconductor and 2D dirty normal metal, and ii) the Josephson proximity coupling between superconductive islands via such a metal. These quantities are shown to be strongly suppressed by the Coulomb interaction at sufficiently low temperatures due to both zero-bias anomaly in the density of states and disorder-enhanced repulsion in the Cooper channel.

cond-mat.supr-con↗
arXiv1999

Andreev Spectroscopy for Superconductive Phase Qubit

New method to probe quantum dynamics of the order parameter phase of a small superconductive island is proposed. The idea is to measure Andreev subgap conductance between the island and a dirty normal-metal wire connected with the island by very high-resistance tunnel barrier. The phase on the island is supposed to stay primarly in one of the two nearly degenerate classical minima i.e. the island is an active element of the Superconductive Phase Qubit. Voltage-dependence of the differential subgap conductance measured at low temperatures are predicted to demonstrate crucially different behavior depending on the type of the internal dynamics of phase (coherent tunnelling of phase, non-coherent tunnelling, or no tunneling). Thus the measurements of nonliner Andreev conductance is a tool to probe the type of quantum behavior of the artificial two-level system.

cond-mat↗
arXiv1999

Quantum Glass Transition in a Periodic Long-Range Josephson Array

We show that the ground state of the periodic long range Josephson array frustrated by magnetic field is a glass for a sufficiently large Josephson energies despite the absence of a quenched disorder. Like superconductors, this glass state has non-zero phase stiffness and Meissner response; for smaller Josephson energies the glass "melts" and the ground state loses the phase stiffness and becomes insulating. We find the critical scaling behavior near this quantum phase transition: the excitation gap vanishes as (J-J_c)^2, the frequency-dependent magnetic susceptibility behaves as χ(ω) ~ \sqrtω\lnω.

cond-mat.dis-nn↗
arXiv1998

Phonon relaxation of subgap levels in superconducting quantum point contacts

Superconducting quantum point contacts are known to possess two subgap states per each propagating mode. In this note we compute the low-temperature relaxation rate of the upper subgap state into the lower one with the emission of an acoustic phonon. If the reflection in the contact is small, the relaxation time may become much longer than the characteristic lifetime of a bulk quasiparticle.

cond-mat.supr-con↗
arXiv1998

Level statistics inside the core of a superconductive vortex

Microscopic theory of the type of Efetov's supermatrix sigma-model is constructed for the low-lying electron states in a mixed superconductive-normal system with disorder. The developed technique is used for the study of the localized states in the core of a vortex in a moderately clean superconductor (1/Δ<< τ<< 1/ω_0 = E_F/Δ^2). At sufficiently low energies E << ω_{Th}, the energy level statistics is described by the "zero-dimensional" limit of this supermatrix theory, with the effective "Thouless energy" ω_{Th} \sim (ω_0/τ)^{1/2}. Within this energy range the result for the density of states is equivalent to that obtained within Altland-Zirnbauer random matrix model of class C. Nonzero modes of the sigma-model increase the mean interlevel distance ω_0 by the relative amount of the order of [2\ln(1/ω_0τ)]^{-1}.

cond-mat.mes-hall↗
arXiv1998

Quantum Superconductor-Metal Transition in a 2D Proximity-Coupled Array

We construct a theory of quantum fluctuatons in a regular array of small superconductive islands connected via low-resistance tunnel contacts to a dirty thin metal film. Electron-electron interaction in the film is assumed to be repulsive. The system is macroscopically superconductive when the distance between neighbouring islands is short enough. The zero-temperature phase transition from superconductive to normal-conductive state is shown to occur with the increase of distance between superconductive islands; the logarithm of the critical distance is proportional to the low-frequency zero-voltage Andreev conductance between the SC island and the film. This critical distance is always much less the than the two-dimensional localization length, so the considered effect develops when weak-localization corrections are still small. The dependence of the critical temperature on the film conductance and inter-island distance is found.

cond-mat.supr-con↗
arXiv1997

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy $E_C \ll Δ$. We consider a small superconducting grain of finite capacity connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is supposed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system may be described as quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima which become degenerate at the phase difference on the leads equal to $π$, the tunneling amplitude between them being controlled by the gate voltage at the grain. We find the Josephson current and its low-frequency fluctuations and predict their periodic dependence on the induced charge $Q_x=C V_g$ with period $2e$.

cond-mat↗
arXiv1997

Low Field Magnetic Response of the Granular Superconductor LaSrCuO

The properties of the low excitation field magnetic response of the granular high temperature (HTSC) superconductor LaSrCuO have been analyzed at low temperatures. The response of the Josephson currents has been extracted from the data. It is shown that intergrain current response is fully irreversible, producing shielding response, but do not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absense of external magnetic field, which is argued to be a consequence of the d-wave nature of superconductivity in LaSrCuO. The macroscopic diamagnetic response to very weak variations of the magnetic field is shown to be strongly irreversible but still qualitatively different from any previously known kind of the critical-state behaviour in superconductors. A phenomenological description of these data is given in terms of a newly proposed ``fractal'' model of irreversibility in superconductors.

cond-mat.supr-con↗
arXiv1997

Supersymmetric Model of a 2D Long-Range Bose Liquid

The model Hamiltonian of a two-dimensional Bose liquid (proposed earlier by Kane, Kivelson, Lee and Zhang as the Hamiltonian which has Jastrow-type wavefunctions as the ground-state solution), is shown to possess nonrelativistic supersymmetry. For the special value of the coupling constant $α=1/2$ the quantum mechanics described by this Hamiltonian is shown to be equivalent to the dynamics of (complex) eigenvalues of random Gaussian ensemble of normal complex matrices. For general $α$, an exact relation between the equal-time current-current and density-density correlation functions is obtained, and used to derive an asymptotically exact (at low wavevectors q) spectrum of single-particle excitations beyond the superfluid ground-state (realized at low $α$'s). The ground-state at very large $α$ is shown to be of ``Quantum Hexatic" type, possessing long-range orientational order and quasi-long-range translational order but with zero shear modulus. Possible scenaria of the ground-state phase transitions as function of $α$ are discussed.

cond-mat.mes-hall↗