arXiv subjects

A. S. Mel'nikov

Publications and source records attributed to A. S. Mel'nikov.

At least 73 records · Page 4Linked to original sources

arXiv2009

Little-Parks Oscillations in Hybrid Ferromagnet-Superconductor Systems

On the basis of of linearized Usadel equations we consider superconductivity nucleation in multiply connected mesoscopic superconductor/ferromagnet hybrids such as thin-walled superconducting cylinders placed in electrical contact with a ferromagnetic metal. We study the interplay between the oscillations of $T_c$ due to the Little--Parks effect and the oscillations due to the exchange field. We demonstrate that the exchange field provokes the switching between the superconducting states with different vorticities and this may result in the increase the critical temperature of the superconducting transition in the magnetic field. Moreover we analyse the influence of the S/F transparency on the realisation of the states with higher vorticities.

cond-mat.supr-con↗

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.

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arXiv2009

Electronic Structure of Vortices Pinned by Columnar Defects

The electronic structure of a vortex line trapped by an insulating columnar defect in a type-II superconductor is analysed within the Bogolubov-de Gennes theory. For quasiparticle trajectories with small impact parameters defined with respect to the vortex axis the normal reflection of electrons and holes at the defect surface results in the formation of an additional subgap spectral branch. The increase in the impact parameter at this branch is accompanied by the decrease of the excitation energy. When the impact parameter exceeds the radius of the defect this branch transforms into the Caroli--de Gennes--Matricon one. As a result, the minigap in the quasiparticle spectrum increases with the increase in the defect radius. The scenario of the spectrum transformation is generalized for the case of arbitrary vorticity.

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arXiv2008

Electronic structure and heat transport in multivortex configurations in mesoscopic superconductors

On the basis of the Bogoliubov--de Gennes theory we study the transformation of the quasiparticle spectrum in the mixed state of a mesoscopic superconductor, governed by an external magnetic field. We analyze the low energy part of the excitation spectrum and investigate the field--dependent behavior of anomalous spectral branches crossing the Fermi level. Generalizing the Caroli--de Gennes--Matricon approach we present an analytical solution describing the anomalous branches in a vortex with an arbitrary winding number. We also study the spectrum transformation caused by the splitting of a multiquantum vortex into a set of well separated vortices focusing mainly on a generic example of a two--vortex system. For vortices positioned rather close to the sample surface we investigate the effect of the quasiparticle reflection at the boundary on the spectrum and the density of states at the Fermi level. Considering an arbitrary surface curvature we study the disappearance of an anomalous spectral branch for a vortex leaving the sample. The changes in the vortex configuration and resulting transformation of the anomalous branches are shown to affect strongly the density of states and the heat conductance along the magnetic field direction.

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arXiv2007

Vortex States Induced by Proximity Effect in Hybrid Ferromagnet-Superconductor Systems

We consider superconductivity nucleation in multiply connected mesoscopic samples such as thin-walled cylinders or rings placed in electrical contact with a ferromagnet. The superconducting critical temperature and order parameter structure are studied on the basis of linearized Usadel equations. We suggest a mechanism of switching between the superconducting states with different vorticities caused by the exchange field and associated with the oscillatory behavior of the Cooper pair wave function in a ferromagnet.

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arXiv2006

Enhanced vortex heat conductance in mesoscopic superconductors

Electronic heat transport along the flux lines in a long ballistic mesoscopic superconductor cylinder with a radius of the order of several coherence lengths is investigated theoretically using both semiclassical approach and the full quantum-mechanical analysis of the Bogoliubov--de Gennes equations. The semiclassical approach is constructed analogously to the Landauer transport theory in mesoscopic conductors employing the idea that heat is carried by the quasiparticle modes propagating along the vortex core. We show that the vortex heat conductance in a mesoscopic sample is strongly enhanced as compared to its value for a bulk superconductor; it grows as the cylinder radius decreases. This unusual behavior results from a strongly increased number of single-particle transport modes due to giant mesoscopic oscillations of energy levels, which originate from the interplay between the Andreev reflection at the vortex core boundary and the normal reflection at the sample edge. We derive the exact quantum-mechanical expression for the heat conductance and solve the Bogoliubov--de Gennes equations numerically. The results of numerical computations confirm the qualitative Landauer-type picture and allow us to take into account the partial reflections of excitations. We analyze the effect of surface imperfections on the spectrum of core excitations. We show that the giant oscillations of core levels and thus the essential features of the heat transport characteristic to ideal mesoscopic samples hold for a broad class of surface imperfections as well.

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arXiv2006

Intervortex quasiparticle tunneling and electronic structure of multi-vortex configurations in type-II superconductors

The electronic spectrum of multi-vortex configurations in type-II superconductors is studied taking account of the effect of quasiparticle tunneling between the vortex cores. The tunneling is responsible for the formation of strongly coupled quasiparticle states for intervortex distances $a<a_c$, where the critical distance $a_c$ is of the order of several coherence lengths $ξ$. Analyzing the resulting spectra of vortex clusters bonded by the quasiparticle tunneling we find a transition from a set of degenerate Caroli - de Gennes - Matricon branches to anomalous branches similar to the ones in multi-quanta giant vortices. This spectrum transformation results in the oscillatory behavior of the density of states at the Fermi level as a function of $a$ and could be observed in mesoscopic superconductors and disordered flux line arrays in the bulk systems.

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arXiv2005

Resonance energy and charge pumping through quantum SINIS contacts

We propose a mechanism of quantum pumping mediated by the spectral flow in a voltage-biased SINIS quantum junction and realized via the sequential closing of the minigaps in the energy spectrum in resonance with the Josephson frequency. We show that the dc current exhibits giant peaks at rational voltages.

cond-mat.supr-con↗

arXiv2005

Giant oscillations of energy levels in mesoscopic superconductors

The interplay of geometrical and Andreev quantization in mesoscopic superconductors leads to giant mesoscopic oscillations of energy levels as functions of the Fermi momentum and/or sample size. Quantization rules are formulated for closed quasiparticle trajectories in the presence of normal scattering at the sample boundaries. Two generic examples of mesoscopic systems are studied: (i) one dimensional Andreev states in a quantum box, (ii) a single vortex in a mesoscopic cylinder.

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arXiv2004

Loss of Andreev Backscattering in Superconducting Quantum Point Contacts

We study effects of magnetic field on the energy spectrum in a superconducting quantum point contact. The supercurrent induced by the magnetic field leads to intermode transitions between the electron waves that pass and do not pass through the constriction. The latter experience normal reflections which couple the states with opposite momenta inside the quantum channel and create a minigap in the energy spectrum that depends on the magnetic field.

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arXiv2003

Re-entrant localization of single particle transport in disordered Andreev wires

We study effects of disorder on the low energy single particle transport in a normal wire surrounded by a superconductor. We show that the heat conductance includes the Andreev diffusion decreasing with increase in the mean free path $\ell $ and the diffusive drift produced by a small particle-hole asymmetry, which increases with increasing $\ell$. The conductance thus has a minimum as a function of $\ell$ which leads to a peculiar re-entrant localization as a function of the mean free path.

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arXiv2003

Little-Parks effect and multiquanta vortices in a hybrid superconductor--ferromagnet system

Within the phenomenological Ginzburg-Landau theory we investigate the phase diagram of a thin superconducting film with ferromagnetic nanoparticles. We study the oscillatory dependence of the critical temperature on an external magnetic field similar to the Little-Parks effect and formation of multiquantum vortex structures. The structure of a superconducting state is studied both analytically and numerically.

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arXiv2003

Domain wall superconductivity in hybrid superconductor -- ferromagnetic structures

On the basis of phenomenological Ginzburg-Landau approach we investigate the problem of order parameter nucleation in hybrid superconductor/ferromagnetic systems with a domain structure in applied external magnetic field. Both the isolated domain boundaries and periodic domain structures in ferromagnetic layers are considered. We study the interplay between the superconductivity localized at the domain walls and far from the walls and show that such interplay determines a peculiar field dependence of the critical temperature Tc. For a periodic domain structure the behavior of the upper critical field of superconductivity nucleation near Tc is strongly influenced by the overlapping of the superconducting nuclei localized over different domains.

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arXiv2003

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

We develop a microscopic theory of single-electron transport in N-S-N hybrid structures in the presence of applied magnetic field introducing vortex lines in a superconductor layer. We show that vortex cores in a thick and clean superconducting layer are similar to mesoscopic conducting channels where the bound core states play the role of transverse modes. The transport through not very thick layers is governed by another mechanism, namely by resonance tunneling via vortex core levels. We apply our method to calculation of the thermal conductance along the magnetic field.

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arXiv2000

Influence of surface irregularities on barriers for vortex entry in type-II superconductors

A theoretical description of the influence of surface irregularities (such as wedge-like cracks) on the Bean-Livingston energy barrier is presented. A careful quantitative estimate of the field of the first vortex entry H* into a homogeneous bulk superconductor with crack-like defects is obtained. This estimate is based on an exact analytical solution for the Meissner current distribution in a bulk superconductor with a single two-dimensional infinitely thin crack. This thin crack is characterized by the lowest H* value, and, thus, appears to be the best gate for vortex entry.

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arXiv2000

Aharonov-Bohm Effect for Quasiparticles around a Vortex Line in a D-wave Superconductor

On the basis of the Bogolubov-de Gennes theory we develop an analytical description of low-energy extended quasiparticle states around an isolated flux line in a superconductor with gap nodes. The wavefunctions of these excitations and the corresponding density of states are shown to be strongly influenced by the interaction with a pure gauge potential due to the Aharonov-Bohm scenario.

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arXiv2000

Nonsingular vortices in (s+d)-wave superconductors

The structure of a single flux line in (s+d)-wave superconductors has been analyzed within the Ginzburg-Landau (GL) model generalized for two order parameter components. The fourfold symmetric singular vortex solution is shown to be unstable in a certain range of the GL parameters with respect to the mutual shift of s- and d- wave unit vortices. The resulting nonsingular vortex structure is studied both analytically and numerically.

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arXiv1999

Theory of vortex lattice effects on STM spectra in d-wave superconductors

Theory of scanning tunneling spectroscopy of low energy quasiparticle (QP) states in vortex lattices of d-wave superconductors is developed taking account of the effects caused by an extremely large extension of QP wavefunctions in the nodal directions and the band structure in the QP spectrum. The oscillatory structures in STM spectra, which correspond to van Hove singularities are analysed. Theoretical calculations carried out for finite temperatures and scattering rates are compared with recent experimental data for high temperature cuprates.

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