arXiv subjects

A. A. Shanenko

Publications and source records attributed to A. A. Shanenko.

At least 19 recordsLinked to original sources

arXiv2025

Superconducting properties of Fibonacci chains with enhanced superconducting pairing at the boundaries

Recently, the superconducting properties of Fibonacci quasicrystals have attracted considerable attention. By numerically solving the self-consistent Bogoliubov-de Gennes equations for an $s-$wave superconducting Fibonacci chain, we find that the system exhibits universal end superconductivity, where the pair potential at the chain ends can persist at higher temperatures compared to the bulk critical temperature ($T_{cb}$) of the condensate in the chain center. Furthermore, our study reveals two distinct critical temperatures at the left ($T_{cL}$) and right ($T_{cR}$) ends of the chain. This complex behavior arises from the competition between topological bound states and critical states, a characteristic of quasicrystals. With the chosen parameters, the maximal enhancement of $T_{cR}$ reaches up to $66\%$ relative to $T_{cb}$, while $T_{cL}$ can increase by up to $31\%$. Our study sheds light on the phenomenon of end superconductivity in Fibonacci quasicrystals, pointing to alternative pathways for increasing the superconducting critical temperature.

cond-mat.supr-con↗

arXiv2024

Universal crossover in surface superconductivity: Impact of varying Debye energy

Recently, interference-induced surface superconductivity (SC) has been predicted within an attractive Hubbard model with $s$-wave pairing, prompting intensive studies of its properties. The most notable finding is that the surface critical temperature $T_{cs}$ can be significantly enhanced relative to the bulk critical temperature $T_{cb}$. In this work, considering a $1D$ attractive Hubbard model for the half-filling level, we investigate how this enhancement is affected by variations in the Debye energy $\hbarω_D$, which controls the number of states contributing to the pair potential and, in turn, influences the critical temperature. Our study reveals a universal crossover of the surface SC from the weak- to strong-coupling regime, regardless of the specific value of the Debye energy. The location of this crossover is marked by the maximum of $τ= (T_{cs} - T_{cb})/T_{cb}$, which depends strongly on $\hbarω_D$. At its maximum, $τ$ can increase up to nearly $70\%$. Additionally, we examine the evolution of the ratio $Δ_{s0}/k_B T_{cs}$ along the crossover, where $Δ_{s0}$ is the zero-temperature pair potential near the surface (the chain ends), and demonstrate that this ratio can significantly deviate from $Δ_{b0}/k_B T_{cb}$, where $Δ_{b0}$ is the zero-temperature bulk pair potential (in the chain center). Our findings may offer valuable insights into the search for higher critical temperatures in narrow-band superconductors.

cond-mat.supr-con↗

arXiv2024

Yu-Shiba-Rusinov states in the s-wave superconducting kagome Hubbard model: Self-consistent Bogoliubov-de Gennes calculations

Significant research has recently been conducted into the Yu-Shiba-Rusinov (YSR) states in kagome superconductors through theoretical modeling and experimental investigations. However, additional efforts are still needed to further understand the local superconductivity near magnetic impurities in the kagome lattice and clarify how relevant quantities depend on the interaction strength $J$ between such impurities and electrons. In this study, we explore a self-consistent numerical solution of the Bogoliubov-de Gennes equations for an $s$-wave superconducting kagome model with a single classical magnetic impurity. Our study reveals that with increasing $J$, the local pair potential is systematically depressed in the vicinity of the impurity, similar to previous results obtained for the square and triangular lattices. Moreover, when further increasing $J$, the system undergoes a first-order phase transition with the appearance of stable and metastable states, reflecting the presence of the hysteresis loop in the pertinent quantities. As a consequence of this transition, the minimal energy of the stable YSR state is nonzero at any $J$, contrary to the expectations based on the assumption of a constant pair potential. A distinctive feature of the kagome lattice is that characteristics of the first-order transition are very sensitive to the position of the chemical potential within the kagome energy spectrum.

cond-mat.supr-con↗

arXiv2024

Emergence of Surface Superconductivity through Interference in Superconducting-proximity Topological Insulators

Superconducting-proximity topological insulators (STIs) have garnered significant research attention over the past two decades. In this Letter, we demonstrate that a low-dimensional STI in the topological-nontrivial phase (TP) exhibits an interference-induced surface (boundary) superconductivity with the surface critical temperature $T_{cs}$ significantly higher than the bulk one $T_{cb}$. Such a surface superconductivity is built due to the interference of the scattering quasiparticle states, rather than due to the presence of the topological bound states (TBSs). As the system goes deeper into the TP, the surface exhibits a crossover from the interference- to TBS-induced phase, where the surface enhancement of superconductivity is governed by the TBSs. Our study unveils a substantial variation in the maximal $T_{cs}$ along this crossover, attaining values being twice the maximal bulk critical temperature of the STI. Beyond shedding light on the nature of surface superconductivity in STIs, our study introduces a tangible method for experimentally manipulating their critical superconducting temperatures.

cond-mat.supr-con↗

arXiv2023

Surface superconductor-insulator transition: Reduction of the critical electric field by Hartree-Fock potential

Recently, a surface superconductor-insulator transition has been predicted for a bulk superconductor in an electric field applied perpendicular to its surface. The related calculations were performed within a one-dimensional Hubbard model by numerically solving the Bogoliubov-de Gennes (BdG) equations without the Hartree-Fock (HF) interaction potential. The phase diagram of the surface superconducting, metallic, and insulating states was obtained as dependent on the electric field and temperature. This diagram was found to be in agreement with experimental results reported previously for (Li,Fe)OHFeSe thin flakes. In the present work, by taking into account the HF potential, we find that the latter acts as a kind of an extra electrostatic potential that enhances the electric-field effects on the surface states. The qualitative features of the phase diagram remain the same but the surface superconductor-insulator transition occurs at significantly lower electric fields, which supports prospects of its experimental observation in bulk samples.

cond-mat.supr-con↗

arXiv2023

Intertype superconductivity evoked by the interplay of disorder and multiple bands

Nonmagnetic impurity scattering is known to shift up the Ginzburg-Landau parameter $κ$ of a superconductor. In this case, when the system is initially in type I, it can change its magnetic response, crossing the intertype domain with $κ\sim 1$ between the two standard superconductivity types and arriving at type II. In the present work we demonstrate that the impact of disorder can be much more profound in the presence of the multiband structure of the charge carrier states. In particular, when the band diffusivities differ from each other, the intertype domain tends to expand significantly, including points with $κ\gg 1$ that belong to deep type-II in conventional single-band superconductors. Our finding sheds light on the nontrivial disorder effect and significantly complements earlier results on the enlargement of the intertype domain in clean multiband superconductors.

cond-mat.supr-con↗

arXiv2023

Tailoring of the interference-induced surface superconductivity by an applied electric field

Nucleation of the pair condensate near surfaces above the upper critical magnetic field and the pair-condensate enhancement/suppression induced by changes in the electron-phonon interaction at interfaces are the most known examples of the surface superconductivity. Recently, another example has been reported, when the surface enhancement of the critical superconducting temperature occurs due to quantum interference. In this case the pair states spread over the entire volume of the system while exhibiting the constructive interference near the surface. In the present work we investigate how an applied electric field impacts the interference-induced surface superconductivity. The study is based on a numerical solution of the self-consistent Bogoliubov-de Gennes equations for a one-dimensional attractive Hubbard model. Our results demonstrate that the surface superconducting characteristics, especially the surface critical temperature, are sensitive to the applied electric field and can be tailored by changing its magnitude.

cond-mat.supr-con↗

arXiv2023

Surface superconductor-insulator transition induced by an electric field

It is well-known that the electric field can induce phase transitions between superconducting, metallic and insulating states in thin-film materials due to its control of the charge carrier density. Since a similar effect on the charge carriers can also be expected for surfaces of bulk samples, here we investigate the transformation of the surface states in a superconductor under an applied screened electric field. Our study is performed by numerically solving the self-consistent Bogoliubov-de Gennes equations for the one-dimensional attractive Hubbard model. It is found that the surface insulating regime occurs at sufficiently large (but still experimentally accessible) electric fields. Our calculations yield the phase diagram of the surface superconducting, metallic, and insulating states for a wide range of temperatures and applied fields. Our results are in qualitative agreement with the phase diagram obtained by the transport measurements for (Li, Fe)OHFeSe thin flakes [Sci. Bull. 64, 653 (2019); ACS Nano 14, 7513 (2020)].

cond-mat.supr-con↗

arXiv2023

Interference-induced surface superconductivity:Enhancement by tuning the Debye energy

In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to 20-25% as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interferenceinduced enhancement can be much more pronounced, up to nearly 70%. Furthermore, here it is shown that such an interference enhancement persists over a wide range of microscopic parameters.

cond-mat.supr-con↗

arXiv2022

Robust Superconductivity in Quasi-One-Dimensional Multiband Materials

Recently it has been demonstrated that the pair-exchange coupling of quasi-one-dimensional (Q1D) bands with conventional higher-dimensional bands in one multiband superconducting material can result in the formation of robust aggregate pair condensate. In particular, it has been found that the Q1D thermal pair fluctuations are suppressed in the presence of deep conventional band(s), where the Fermi level is much larger than the characteristic cut-off energy. Here we report that impact of the Q1D fluctuations is significantly weakened even in the presence of nearly shallow higher-dimensional band(s), which shed new light on robust superconducting state observed in emerging chain-like-structured superconducting materials A_2Cr_3As_3 (A = K, Rb, Cs).

cond-mat.supr-con↗

arXiv2022

Multiband Superconductors: Two Characteristic Lengths for Each Contributing Condensate

Traditionally, the characteristic length of a superconducting condensate is associated with the spatial distribution of the corresponding gap function. However, the superconducting condensate is the quantum condensate of Cooper pairs and thus, the broader readership is more familiar with the concept of the Cooper-pair wave function. For conventional single-band superconductors, the gap function coincides with the center-of-mass wave function of a Cooper pair up to the coupling constant, and the corresponding gap- and wave-function characteristic lengths are the same. Surprisingly, we find that in two-band superconductors, these lengths are the same only near the critical temperature. At lower temperatures they can significantly deviate from each other, and the question arises as to which of these lengths should be given the preference when specifying the spatial scale of the band-dependent condensate in multiband superconducting materials.

cond-mat.supr-con↗

arXiv2022

Suppression of fluctuations in a two-band superconductor with a quasi-1D band

Chain-like structured superconductive materials (such as A_2Cr_3As_3, with A = K, Rb, Cs) exhibit the multiband electronic structure of single-particle states, where coexisting quasi-one-dimensional (Q1D) and conventional higher-dimensional energy bands take part in the creation of the aggregate superconducting condensate. When the chemical potential approaches the edge of a Q1D band in a single-band superconductor, the corresponding mean-field critical temperature increases significantly but the superconductivity is quenched by fluctuations. However, recent investigation has revealed that when a Q1D band is coupled to a higher dimensional one by the interband Cooper-pair transfer, the thermal superconductive fluctuations can be suppressed so that the resulting critical temperature can be close to its mean-field value. In the present work, we calculate the mean-field T_c0 and fluctuation-shifted T_c critical temperatures for a two-band superconductor where a Q1D band coexists with a higher-dimensional band, and investigate how the thermal fluctuations are sensitive to the system parameters. We find that T_c is close to T_c0 in a wide range of microscopic parameters, and even the dimensionality of the higher-dimensional band does not play an essential role. Thus, the screening mechanism for suppressing fluctuations via the pair-exchange coupling between the bands is indeed relevant for a large class of Q1D multiband superconducting materials, encouraging further experiments aimed at reaching larger critical temperatures in such multiband superconductors.

cond-mat.supr-con↗

arXiv2021

Current-induced self-organisation of mixed superconducting states

Small-angle neutron scattering is used in combination with transport measurements to investigate the current-induced effects on the morphology of the intermediate mixed state domains in the intertype superconductor niobium. We report the robust self-organisation of the vortex lattice domains to elongated parallel stripes perpendicular to the applied current in a steady-state. The experimental results for the formation of the superstructure are supported by theoretical calculations, which highlight important details of the vortex matter evolution. The investigation demonstrates a mechanism of a spontaneous pattern formation that is closely related to the universal physics governing the intermediate mixed state in low-$κ$ superconductors.

cond-mat.supr-con↗

arXiv2021

Multiband mechanism of the pair fluctuation screening

Recent chain-like structured materials have shown a robust superconducting phase. These materials exhibit the presence of quasi-one-dimensional bands (q1D) coupled to conventional higher-dimensional bands. On the mean-field level such systems have a high critical temperature when the chemical potential is close to the edge of a q1D band and the related Lifshitz transition is approached. However, the impact of the pair fluctuations compromises the mean-field results. Recently it has been demonstrated that these fluctuations can be suppressed (screened) by a specific multiband mechanism based on the pair-exchange coupling of the q1D condensate to a stable higher-dimensional one. In the present work we demonstrate that strikingly enough, this mechanism is not very sensitive to the basic parameters of the stable condensate such as its strength and dimensionality. For example, even the presence of a passive higher-dimensional band, which does not exhibit any superconducting correlations when taken as a separate superconductor, results in suppression of the pair fluctuations.

cond-mat.supr-con↗

arXiv2020

Interplay of Fermi velocities and healing lengths in two-band superconductors

By numerically solving the Bogoliubov-de Gennes equations for the single vortex state in a two-band superconductor, we demonstrate that the disparity between the healing lengths of two contributing condensates is strongly affected by the band Fermi velocities, even in the presence of the magnetic field and far beyond the regime of nearly zero Josephson-like coupling between bands. Changing the ratio of the band Fermi velocities alters the temperature dependence of the condensate lengths and significantly shifts parameters of the ``length-scales locking" regime at which the two characteristic lengths approach one another.

cond-mat.supr-con↗

arXiv2020

Quasi-one-dimensional system as a high-temperature superconductor

It is well-known that quasi-one-dimensional superconductors suffer from the pairing fluctuations that significantly reduce the superconducting temperature or even completely suppress any coherent behavior. Here we demonstrate that a coupling to a robust pair condensate changes the situation dramatically. In this case the quasi-one-dimensional system can be a high temperature superconductor governed by the proximity to the Lifshitz transition at which the Fermi level approaches the lower edge of the single-particle spectrum.

cond-mat.supr-con↗

arXiv2020

Multiband superconductors with degenerate excitation gaps

There is a tacit assumption that multiband superconductors are essentially the same as multigap superconductors. More precisely, it is usually assumed that the number of excitation gaps in the single-particle energy spectrum of a uniform superconductor determines the number of contributing bands in the corresponding superconducting model. Here we demonstrate that contrary to this widely accepted viewpoint, the superconducting magnetic properties are sensitive to the number of contributing bands even when the corresponding excitation gaps are degenerate and cannot be distinguished. In particular, we find that the crossover between superconductivity types I and II - the intertype regime - is strongly affected by difference between characteristic lengths of multiple contributing condensates. The reason for this is that condensates with diverse characteristic lengths coexisting in one system interfere constructively or destructively, which results in multi-condensate magnetic phenomena regardless of the presence/absence of the multigap structure in the single-particle excitation spectrum.

cond-mat.supr-con↗

arXiv2019

Vortex interaction in thin films -- a crossover from type I to type II superconductivity

Interactions between vortices in thin superconducting films are investigated in the crossover (intertype) regime between superconductivity types I and II. We consider two main factors responsible for this crossover: a) changes in the material characteristics of the film and b) variations of the film thickness controlling the effect of the stray magnetic fields outside superconducting sample. The analysis is done within the formalism that combines the perturbation expansion of the microscopic equations to one order beyond the Ginzburg-Landau theory with the leading contribution of the stray fields. It is shown that the latter gives rise to qualitatively different spatial profile and temperature dependence of the vortex interaction potential, as compared to bulk vortex interactions. The resulting interaction is long-range repulsive while exhibiting complex competition of attraction and repulsion at small and intermediate separations of vortices. This explains the appearance of vortex chains reported earlier for superconducting films.

cond-mat.supr-con↗