arXiv subjects

N. B. Kopnin

Publications and source records attributed to N. B. Kopnin.

At least 19 recordsLinked to original sources

arXiv2014

Information flow and optimal protocol for Maxwell's demon single electron pump

We study the entropy and information flow in a Maxwell demon device based on a single-electron transistor with controlled gate potentials. We construct the protocols for measuring the charge states and manipulating the gate voltages which minimizes irreversibility for (i) constant input power from the environment or (ii) given energy gain. Charge measurement is modeled by a series of detector readouts for time-dependent gate potentials, and the amount of information obtained is determined. The protocols optimize irreversibility that arises due to (i) enlargement of the configuration space on opening the barriers, and (ii) finite rate of operation. These optimal protocols are general and apply to all systems where barriers between different regions can be manipulated.

cond-mat.stat-mech↗

arXiv2013

Weak links in proximity superconducting two dimensional electron systems

We report a giant inverse proximity effect that arises in the low-dimensional devices and is crucially different from proximity in the standard superconductor-normal-superconductor (S/N/S) junctions. The normal conductors induce a giant back-action on the proximity superconducting regions even when the dimensions of the normal parts are smaller than the proximity coherence length. This essentially suppresses the superconducting characteristics of the entire system, including the critical current, well below those for the usual S/N/S structures.

cond-mat.mes-hall↗

arXiv2013

Vortex matter in low dimensional systems with proximity induced superconductivity

We study theoretically the vortex matter structure in low dimensional (LD) systems with superconducting order induced by proximity to a bulk superconductor. We analyze the effects of microscopic coupling mechanisms between the two systems and the effects of possible mismatch in the band structures of these materials on the energy spectrum of vortex-core electrons. The unusual structure of vortex cores is discussed in the context of recent tunneling microscopy/spectroscopy experiments.

cond-mat.supr-con↗

arXiv2012

Surface superconductivity in rhombohedral graphite

We show that rhombohedral graphite may support surface superconductivity with an unusual relation between the BCS coupling constant and the order parameter. This feature results from the properties of the states localized on the graphite surfaces. In a description including only the nearest neighbour coupling of the graphene layers, the surface states are topologically protected and have a flat band dispersion. We show that including higher order couplings destroys this flat band character and leads to a particle-hole symmetry breaking quadratic dispersion with a large effective mass. Employing this dispersion, we then show its effect on superconductivity and find two regimes of parameters, depending on the relation between the strength of the coupling constant and the details of the quadratic dispersion. For low coupling strengths, superconductivity is localized on the surfaces, but the order parameter is exponentially suppressed as in a conventional BCS superconductor, whereas for large coupling strengths we obtain surface superconductivity with a linear relation between the order parameter and the coupling constant. Our results may explain the recent findings of graphite superconductivity with a relatively high transition temperature.

cond-mat.supr-con↗

arXiv2012

High-temperature surface superconductivity in rhombohedral graphite

Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

cond-mat.supr-con↗

arXiv2012

Multiple Vortex Cores in 2D Electronic Systems with Proximity Induced Superconductivity

The structure of a proximity induced vortex core in a two-dimensional (2D) metallic layer covering a superconducting half-space is calculated. We predict formation of a multiple vortex core characterized by two-scale behavior of the local density of states (LDOS). For coherent tunnelling between the 2D layer and the bulk superconductor, the spectrum has two subgap branches while for incoherent tunnelling only one of them remains. The resulting splitting of the zero-bias anomaly and the multiple peak structure in the LDOS should be visible in the tunnelling spectroscopy experiments.

cond-mat.supr-con↗

arXiv2011

Magnetic-Field-Induced Stabilization of Non-Equilibrium Superconductivity

A small magnetic field is found to enhance relaxation processes in a superconductor thus stabilizing superconductivity in non-equilibrium conditions. In a normal-metal (N) - insulator - superconductor (S) tunnel junction, applying a field of the order of 100 μT leads to significantly improved cooling of the N island by quasiparticle (QP) tunneling. These findings are attributed to faster QP relaxation within the S electrodes as a result of enhanced QP drain through regions with locally suppressed energy gap due to magnetic vortices in the S leads at some distance from the junction.

cond-mat.supr-con↗

arXiv2011

Flat bands in topological media

Topological media are systems whose properties are protected by topology and thus are robust to deformations of the system. In topological insulators and superconductors the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac or Majorana fermions on the surface of the system and inside vortex cores. Here we show that in gapless topological media, the bulk-surface and bulk-vortex correspondence is more effective: it produces topologically protected gapless fermions without dispersion -- the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and we show that this property may give rise in particular to surface superconductivity which could exist even at room temperature.

cond-mat.str-el↗

arXiv2011

Induced superconductivity in 2D electronic systems

The approach applicable for spatially inhomogeneous and time-dependent problems associated with the induced superconductivity in low dimensional electronic systems is developed. This approach is based on the Fano--Anderson model which describes the decay of a resonance state coupled to a continuum. We consider two types of junctions made of a ballistic 2D electron gas placed in a tunnel finite-length contact with a bulk superconducting leads. We calculate the spectrum of the bound states, supercurrent, and the current-voltage curve which show a rich structure due to the presence of induced gap and dimensional quantization.

cond-mat.supr-con↗

arXiv2011

Surface superconductivity in multilayered rhombohedral graphene: Supercurrent

The supercurrent for the surface superconductivity of a flat-band multilayered rhombohedral graphene is calculated. Despite the absence of dispersion of the excitation spectrum, the supercurrent is finite. The critical current is proportional to the zero-temperature superconducting gap, i.e., to the superconducting critical temperature and to the size of the flat band in the momentum space.

cond-mat.supr-con↗

arXiv2011

High-temperature surface superconductivity in topological flat-band systems

We show that the topologically protected flat band emerging on a surface of a nodal fermionic system promotes the surface superconductivity due to an infinitely large density of states associated with the flat band. The critical temperature depends linearly on the pairing interaction and can be thus considerably higher than the exponentially small bulk critical temperature. We discuss an example of surface superconductivity in multilayered graphene with rhombohedral stacking.

cond-mat.supr-con↗

arXiv2010

Coulomb-enhanced resonance transmission of quantum SINIS junctions

Coherent charge transfer through a ballistic gated SINIS junction is mediated by the resonant tunneling via the Andreev states. Extra charge accommodated on the Andreev levels partially compensates the charge induced by the gate voltage preserving the electron wavelength and maintaining the resonance conditions in a broad range of gate voltages. As a result, the transparency of the junction as well as the supercurrent trough it can be substantially increased as compared to the zero-Coulomb case.

cond-mat.supr-con↗

arXiv2010

Supercurrent in superconducting graphene

The problem of supercurrent in superconducting graphene is revisited and the supercurrent is calculated within the mean-field model employing the two-component wave functions on a honeycomb lattice with pairing between different valleys in the Brillouin zone. We show that the supercurrent within the linear approximation in the order-parameter-phase gradient is always finite even if the doping level is exactly zero.

cond-mat.supr-con↗

arXiv2009

Nonequilibrium electrons in tunnel structures under high-voltage injection

We investigate electronic distributions in nonequilibrium tunnel junctions subject to a high voltage bias $V$ under competing electron-electron and electron-phonon relaxation processes. We derive conditions for reaching quasi-equilibrium and show that, though the distribution can still be thermal for low energies where the rate of the electron-electron relaxation exceeds significantly the electron-phonon relaxation rate, it develops a power-law tail at energies of order of $eV$. In a general case of comparable electron-electron and electron-phonon relaxation rates, this tail leads to emission of high-energy phonons which carry away most of the energy pumped in by the injected current.

cond-mat.mes-hall↗

arXiv2009

Vortex Core States in Superconducting Graphene

The distinctive features of the electronic structure of vortex states in superconducting graphene are studied within the Bogolubov-de Gennes theory applied to excitations near the Dirac point. We suggest a scenario describing the subgap spectrum transformation which occurs with a change in the doping level. For an arbitrary vorticity and doping level we investigate the problem of existence of zero energy modes. The crossover to a Caroli - de Gennes - Matricon type of spectrum is studied.

cond-mat.supr-con↗

arXiv2008

Recombination limited energy relaxation in a BCS superconductor

We study quasiparticle energy relaxation at sub-kelvin temperatures by injecting hot electrons into an aluminium island and measuring the energy flux from electrons into phonons both in the superconducting and in the normal state. The data show strong reduction of the flux at low temperatures in the superconducting state, in qualitative agreement with the presented quasiclassical theory for clean superconductors. Quantitatively, the energy flux exceeds that from the theory both in the superconducting and in the normal state, possibly suggesting an enhanced or additional relaxation process.

cond-mat.mes-hall↗

arXiv2008

Charge transport through weakly open one dimensional quantum wires

We consider resonant transmission through a finite-length quantum wire connected to leads via finite transparency junctions. The coherent electron transport is strongly modified by the Coulomb interaction. The low-temperature current-voltage ($IV$) curves show step-like dependence on the bias voltage determined by the distance between the quantum levels inside the conductor, the pattern being dependent on the ratio between the charging energy and level spacing. If the system is tuned close to the resonance condition by the gate voltage, the low-voltage $IV$ curve is Ohmic. At large Coulomb energy and low temperatures, the conductance is temperature-independent for any relationship between temperature, level spacing, and coupling between the wire and the leads.

cond-mat.mes-hall↗

arXiv2008

BCS superconductivity of Dirac electrons in graphene layers

Possible superconductivity of electrons with the Dirac spectrum is analyzed using the BCS model. We calculate the critical temperature, the superconducting energy gap, and supercurrent as functions of the doping level and of the pairing interaction strength. Zero doping is characterized by existence of the quantum critical point such that the critical temperature vanishes below some finite value of the interaction strength. However, the critical temperature remains finite for any nonzero electron or hole doping level when the Fermi energy is shifted away from the Dirac point of the normal-state electron spectrum. We analyze the behavior of the characteristic length scales, i.e., the London penetration depth and the coherence length, which determine the critical magnetic fields.

cond-mat.supr-con↗