arXiv subjects

Alex Levchenko

Publications and source records attributed to Alex Levchenko.

At least 127 records · Page 7Linked to original sources

arXiv2013

Tunneling Spectroscopy of a Spiral Luttinger Liquid in Contact with Superconductors

One-dimensional wires with Rashba spin-orbit coupling, magnetic field, and strong electron-electron interactions are described by a spiral Luttinger liquid model. We develop a theory to investigate the tunneling density of states into a spiral Luttinger liquid under the proximity effect with superconductors. This approach provides a way to disentangle the delicate interplay between superconducting correlations and strong electron interactions. If the wire-superconductor boundary is dominated by Andreev reflection, we find that in the vicinity of the interface the zero-bias tunneling anomaly reveals a power law enhancement with the unusual exponent. Far away from the interface strong correlations inherent to the Luttinger liquid prevail and restore conventional suppression of the tunneling density of states at the Fermi level, which acquire, however, a Friedel-like oscillatory envelope with the period renormalized by the strength of the interaction.

cond-mat.mes-hall↗

arXiv2012

Josephson current and density of states in proximity circuits with s+- superconductors

We study the emergent proximity effect in mesoscopic circuits that involve a conventional superconductor and an unconventional pnictide superconductor separated by a diffusive normal or ferromagnetic wire. The focus is placed on revealing signatures of the proposed s+- state of pnictides from the proximity-induced density of states and Josephson current. We find analytically a universal result for the density of states that exhibits both the Thouless gap at low energies, and peculiar features near the superconducting gap edges at higher energies. The latter may be used to discriminate between s+- and s++ symmetry scenarios in scanning tunneling spectroscopy experiments. We also calculate Josephson current-phase relationships for different junction configurations, which are found to display robust 0-πtransitions for a wide range of parameters.

cond-mat.supr-con↗

arXiv2012

Floquet Majorana Fermions for Topological Qubits

We introduce and develop an approach to realizing a topological phase transition and non-Abelian statistics with dynamically induced Floquet Majorana Fermions (FMFs). When the periodic driving potential does not break fermion parity conservation, FMFs can encode quantum information. Quasi-energy analysis shows that a stable FMF zero mode and two other satellite modes exist in a wide parameter space with large quasi-energy gaps, which prevents transitions to other Floquet states under adiabatic driving. We also show that in the asymptotic limit FMFs preserve non-Abelian statistics and, thus, behave like their equilibrium counterparts.

cond-mat.mes-hall↗

arXiv2012

Phase soliton and pairing symmetry of a two-band superconductor: Role of the proximity effect

We suggest a mechanism which promotes the existence of a phase soliton -- topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with s+- type of pairing. This mechanism exploits the proximity effect with a conventional s-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of s+- pairing, are pi-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between s+- and s++ types of pairing in the iron-based multiband superconductors.

cond-mat.supr-con↗

arXiv2012

Relaxation and edge reconstruction in integer quantum Hall systems

The interplay between the confinement potential and electron-electron interactions causes reconstructions of Quantum Hall edges. We study the consequences of this edge reconstruction for the relaxation of hot electrons injected into integer quantum Hall edge states. In translationally invariant edges, the relaxation of hot electrons is governed by three-body collisions which are sensitive to the electron dispersion and thus to reconstruction effects. We show that the relaxation rates are significantly altered in different reconstruction scenarios.

cond-mat.mes-hall↗

arXiv2011

Thermalization of nonequilibrium electrons in quantum wires

We study the problem of energy relaxation in a one-dimensional electron system. The leading thermalization mechanism is due to three-particle collisions. We show that for the case of spinless electrons in a single channel quantum wire the corresponding collision integral can be transformed into an exactly solvable problem. The latter is known as the Schrodinger equation for a quantum particle moving in a Poschl-Teller potential. The spectrum for the resulting eigenvalue problem allows for bound-state solutions, which can be identified with the zero modes of the collision integral, and a continuum of propagating modes, which are separated by a gap from the bound states. The inverse gap gives the time scale at which counterpropagating electrons thermalize.

cond-mat.mes-hall↗

arXiv2011

Interaction effects on thermal transport in quantum wires

We develop a theory of thermal transport of weakly interacting electrons in quantum wires. Unlike higher-dimensional systems, a one-dimensional electron gas requires three-particle collisions for energy relaxation. The fastest relaxation is provided by the intrabranch scattering of comoving electrons which establishes a partially equilibrated form of the distribution function. The thermal conductance is governed by the slower interbranch processes which enable energy exchange between counterpropagating particles. We derive an analytic expression for the thermal conductance of interacting electrons valid for arbitrary relation between the wire length and electron thermalization length. We find that in sufficiently long wires the interaction-induced correction to the thermal conductance saturates to an interaction-independent value.

cond-mat.mes-hall↗

arXiv2011

Probing the pseudogap phase of cuprates by a Giaever transformer

We develop a theory of the rectification effect in a double-layer system where both layers are superconductors, or one of the layers is a normal metal. The Coulomb interaction is assumed to provide the dominant coupling between the layers. We find that superconducting fluctuations strongly enhance the drag conductivity, with rectification most pronounced when both layers are superconductors. In view of their distinct dependence on temperature near Tc and layer separation, drag measurements based on a Giaever transformer could distinguish whether rectification occurs due to fluctuating pairs or inductively coupled fluctuating vortices.

cond-mat.supr-con↗

arXiv2010

Interaction-induced corrections to conductance and thermopower in quantum wires

We study transport properties of weakly interacting spinless electrons in one-dimensional single channel quantum wires. The effects of interaction manifest as three-particle collisions due to the severe constraints imposed by the conservation laws on the two-body processes. We focus on short wires where the effects of equilibration on the distribution function can be neglected and collision integral can be treated in perturbation theory. We find that interaction-induced corrections to conductance and thermopower rely on the scattering processes that change number of right- and left-moving electrons. The latter requires transition at the bottom of the band which is exponentially suppressed at low temperatures. Our theory is based on the scattering approach that is beyond the Luttinger-liquid limit. We emphasize the crucial role of the exchange terms in the three-particle scattering amplitude that was not discussed in the previous studies.

cond-mat.mes-hall↗

arXiv2010

Transport in partially equilibrated inhomogeneous quantum wires

We study transport properties of weakly interacting one-dimensional electron systems including on an equal footing thermal equilibration due to three-particle collisions and the effects of large-scale inhomogeneities. We show that equilibration in an inhomogeneous quantum wire is characterized by the competition of interaction processes which reduce the electrons total momentum and such which change the number of right- and left-moving electrons. We find that the combined effect of interactions and inhomogeneities can dramatically increase the resistance of the wire. In addition, we find that the interactions strongly affect the thermoelectric properties of inhomogeneous wires and calculate their thermal conductance, thermopower, and Peltier coefficient.

cond-mat.mes-hall↗

arXiv2010

Transport theory of superconductors with singular interaction corrections

We study effects of strong fluctuations on the transport properties of superconductors near the classical critical point. In this regime conductivity is set by the delicate interplay of two competing effects. The first is that strong electron-electron interactions in the Cooper channel increase the life-time of fluctuation Cooper pairs and thus enhance conductivity. On the other hand, quantum pair-breaking effects tend to suppress superconductivity. An interplay between these processes defines new regime where fluctuation induced transport becomes more singular. The most singular contributions to the conductivity stem from the dynamic Aslamazov-Larkin term, and novel Maki-Thompson and interference corrections. The crossover temperature from weakly to strongly fluctuating regime is generated self-consistently as the result of scattering on dynamic variations of the order parameter. We suggest that the way to probe nonlinear-fluctuations in superconductors is by magnetoconductivity measurements in the perpendicular field.

cond-mat.supr-con↗

arXiv2010

Interaction corrections to tunneling conductance in ballistic superconductors

It is known that in the two-dimensional disordered superconductors electron-electron interactions in the Cooper channel lead to the negative logarithmic in temperature correction to the tunneling conductance above the critical temperature. Physically this result appears due to the density of states suppression by superconductive fluctuations near the Fermi level. It is interesting that the other correction, which accounts for the Maki-Thompson-type interaction of fluctuations, is positive and exhibits strong power law, which dominates the logarithmic term in the immediate vicinity of the critical temperature. An interplay between these two contributions determines the zero-bias anomaly in fluctuating superconductors. This paper is devoted to the fate of such interaction corrections in the ballistic superconductors. It turns out that ballistic dynamic fluctuations perturb single-particle density of states near the Fermi level at the energy scale which is different from that in the diffusive case. As the consequence, fluctuation region becomes much broader. In this regime we confirm that correction to the tunneling conductance remains negative and logarithmic not too close to the critical temperature while in the immediate vicinity of the transition we find novel power law for the Maki-Thompson contribution. It is suggested that peculiar non-monotonous temperature dependence of the tunneling conductance may be probed via magneto-tunnel experiments.

cond-mat.supr-con↗

arXiv2009

Magnetoconductivity of low-dimensional disordered conductors at the onset of the superconducting transition

Magnetoconductivity of the disordered two- and three-dimensional superconductors is addressed at the onset of superconducting transition. In this regime transport is dominated by the fluctuation effects and we account for the interaction corrections coming from the Cooper channel. In contrast to many previous studies we consider strong magnetic fields and various temperature regimes, which allow to resolve the existing discrepancies with the experiments. Specifically, we find saturation of the fluctuations induced magneto-conductivity for both two- and three-dimensional superconductors at already moderate magnetic fields and discuss possible dimensional crossover at the immediate vicinity of the critical temperature. The surprising observation is that closer to the transition temperature weaker magnetic field provides the saturation. It is remarkable also that interaction correction to magnetoconductivity coming from the Cooper channel, and specifically the so called Maki-Thompson contribution, remains to be important even away from the critical region.

cond-mat.supr-con↗

arXiv2009

Keldysh technique and non-linear sigma-model: basic principles and applications

The purpose of this review is to provide a comprehensive pedagogical introduction into Keldysh technique for interacting out-of-equilibrium fermionic and bosonic systems. The emphasis is placed on a functional integral representation of underlying microscopic models. A large part of the review is devoted to derivation and applications of the non-linear sigma-model for disordered metals and superconductors. We discuss such topics as transport properties, mesoscopic effects, counting statistics, interaction corrections, kinetic equation, etc. The sections devoted to disordered superconductors include Usadel equation, fluctuation corrections, time-dependent Ginzburg-Landau theory, proximity and Josephson effects, etc. (This review is a substantial extension of arXiv:cond-mat/0412296.)

cond-mat.other↗

arXiv2009

Voltage-driven quantum oscillations of conductance in graphene

Locally-gated single-layer graphene sheets have unusual discrete energy states inside the potential barrier induced by a finite-width gate. These states are localized outside the Dirac cone of continuum states and are responsible for novel quantum transport phenomena. Specifically, the longitudinal (along the barrier) conductance exhibits oscillations as a function of barrier height and/or width, which are both controlled by a nearby gate. The origin of these oscillations can be traced back to singularities in the density of localized states. These graphene conductance-oscillations resemble the Shubnikov-de-Haas (SdH) magneto-oscillations; however, here these are driven by an electric field instead of a magnetic field.

cond-mat.mes-hall↗

arXiv2008

Coulomb drag in quantum circuits

We study drag effect in a system of two electrically isolated quantum point contacts (QPC), coupled by Coulomb interactions. Drag current exhibits maxima as a function of QPC gate voltages when the latter are tuned to the transitions between quantized conductance plateaus. In the linear regime this behavior is due to enhanced electron-hole asymmetry near an opening of a new conductance channel. In the non-linear regime the drag current is proportional to the shot noise of the driving circuit, suggesting that the Coulomb drag experiments may be a convenient way to measure the quantum shot noise. Remarkably, the transition to the non-linear regime may occur at driving voltages substantially smaller than the temperature.

cond-mat.mes-hall↗

arXiv2008

Josephson current noise above Tc in superconducting tunnel junctions

Tunnel junction between two superconductors is considered in the vicinity of the critical temperature. Superconductive fluctuations above Tc give rise to the noise of the ac Josephson current although the current itself is zero in average. As a result of fluctuations, current noise spectrum is peaked at the Josephson frequency, which may be considered as precursor of superconductivity in the normal state. Temperature dependence and shape of the Josephson current noise resonance line is calculated for various junction configurations.

cond-mat.supr-con↗

arXiv2008

Crossover in the local density of states of mesoscopic SNS junctions

Andreev levels deplete energy states above the superconductive gap, which leads to the peculiar nonmonotonous crossover in the local density of states of mesoscopic superconductor/normal-metal/superconductor junctions. This effect is especially pronounced in the case when the normal metal bridge length is small compared to the superconductive coherence length. Remarkable property of the crossover function is that it vanishes not only at the proximity induced gap but also at the superconductive gap. Analytical expressions for the density of states at the both gap edges, as well as general structure of the crossover are discussed.

cond-mat.supr-con↗