arXiv subjects

Mihail Silaev

Publications and source records attributed to Mihail Silaev.

At least 19 recordsLinked to original sources

arXiv2019

Non-London electrodynamics in a multiband London model: anisotropy-induced non-localities and multiple magnetic field penetration lengths

The London model describes strongly type-2 superconductors as massive vector field theories, where the magnetic field decays exponentially at the length scale of the London penetration length. This also holds for isotropic multi-band extensions, where the presence of multiple bands merely renormalises the London penetration length. We show that, by contrast, the magnetic properties of anisotropic multi-band London models are not this simple, and the anisotropy leads to the inter-band phase differences becoming coupled to the magnetic field. This results in the magnetic field in such systems having N+1 penetration lengths, where N is the number of field components or bands. That is, in a given direction, the magnetic field decay is described by N+1 modes with different amplitudes and different decay length scales. For certain anisotropies we obtain magnetic modes with complex masses. That means that magnetic field decay is not described by a monotonic exponential increment set by a real penetration length but instead is oscillating. Some of the penetration lengths are shown to diverge away from the superconducting phase transition when the mass of the phase-difference mode vanishes. Finally the anisotropy-driven hybridization of the London mode with the Leggett modes can provide an effectively non-local magnetic response in the nominally local London model. Focusing on the two-component model, we discuss the magnetic field inversion that results from the effective non-locality, both near the surface of the superconductor and around vortices. In the regime where the magnetic field decay becomes non-monotonic, the multiband London superconductor is shown to form weakly-bound states of vortices.

cond-mat.supr-con↗

arXiv2019

Skyrmions formation due to unconventional magnetic modes in anisotropic multi-band superconductors

Multiband superconductors have a sufficient number of degrees of freedom to allow topological excitations characterized by Skyrmionic topological invariants. In the most common, clean s-wave multiband, systems the interband magnetic coupling favours composite vortex solutions, without a Skyrmionic topological charge. It was discussed recently that certain kinds of anisotropies lead to hybridisation of the interband phase difference (Leggett) mode with magnetic modes, dramatically changing the hydromagnetostatics of the system. Here we report this effect for a range of parameters that substantially alter the nature of the topological excitations, leading to solutions characterized by a nontrivial topological invariant, rather than the standard composite vortex solutions. This invariant is not truly topologically conserved but is energetically conserved, leading to a texture formed of bound excitations in each band, namely fractional vortices, each carrying a fraction of the flux quantum. We demonstrate that in this regime there is a rich spectrum of Skyrmion solutions, with various topological charges, that are robust with respect to changes of parameters of the system and present for a wide range of anisotropies.

cond-mat.supr-con↗

arXiv2019

Hierarchies of Length Scale Based Typology in Anisotropic Multiband Superconductor

Since Ginzburg and Landau's seminal work in 1950 superconducting states have been classified by the hierarchy of the fundamental length scales of the theory; the magnetic field penetration lengths and coherence lengths. In the simplest single-component case they form a dimensionless ratio κ. As pointed out by Ginzburg in 1952, in general the ratio depends on the direction of the applied magnetic field due to material anisotropies. Single component materials can therefore display type-1 superconductivity when the field is applied in one direction and type-2 when the field is applied in a different direction. In this paper we expand the above length scale analysis to anisotropic multi-component superconductors, that can have multiple coherence lengths as well as multiple magnetic field penetration lengths, leading to unconvential length scale hierarchies. We demonstrate that the anisotropies in multi-band superconductors cannot in general be rescaled, leading to new regimes with various mixed hierarchies in different directions. For example, a regime is possible, where for a field applied in a certain direction coherence lengths are smaller than the magnetic field penetration lengths in one of the perpendicular directions, where as the penetration lengths are lager in the another direction. We demonstrate a new regime, where vortices form stripes in the direction where coherence lengths exceed the magnetic field penetration length and vortex cores overlap, while the vortex stripes repel each other in the orthogonal direction where the magnetic field penetration length exceeds the coherence lengths.

cond-mat.supr-con↗

arXiv2019

Comment on the paper by D. Efremov and Yu.N. Ovchinnikov "Singular ground state of multiband inhomogeneous superconductors", Phys. Rev. B 99, 224508 (2019)

We show that the conclusion reported in Ref. 1, that there are no spontaneous magnetic fields in multiband superconductors that break time reversal symmetry, is incorrect. We demonstrate that the state proposed in Ref. 1 is not a solution of the Ginzburg-Landau equations for the considered model. The reason is that in Ref. 1 one of the Ginzburg-Landau equations is neglected and substituted by the spurious zero current restriction. This restriction together with all of the Ginzburg-Landau equations leads to an overdetermined system which does not have a solution. This inconsistency invalidates all the results of the paper.

cond-mat.supr-con↗

arXiv2018

Properties of dirty two-bands superconductors with repulsive interband interaction: normal modes, length scales, vortices and magnetic response

Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an $s+is$ superconducting state, that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal-symmetry breakdown. We find that two-bands superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second order phase transition from $s_{\pm}/s_{++}$ states to the $s+is$ state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.

cond-mat.supr-con↗

arXiv2018

Field-induced coexistence of $s_{++}$ and $s_{\pm}$ superconducting states in dirty multiband superconductors

In multiband systems, such as iron-based superconductors, the superconducting states with locking and anti-locking of the interband phase differences, are usually considered as mutually exclusive. For example, a dirty two-band system with interband impurity scattering undergoes a sharp crossover between the $s_{\pm}$ state (which favors phase anti locking) and the $s_{++}$ state (which favors phase locking). We discuss here that the situation can be much more complex in the presence of an external field or superconducting currents. In an external applied magnetic field, dirty two-band superconductors do not feature a sharp $s_{\pm}\to s_{++}$ crossover but rather a washed-out crossover to a finite region in the parameter space where both $s_{\pm}$ and $s_{++}$ states can coexist for example as a lattice or a microemulsion of inclusions of different states. The current-carrying regions such as the regions near vortex cores can exhibit an $s_\pm$ state while it is the $s_{++}$ state that is favored in the bulk. This coexistence of both states can even be realized in the Meissner state at the domain's boundaries featuring Meissner currents. We demonstrate that there is a magnetic-field-driven crossover between the pure $s_{\pm}$ and the $s_{++}$ states.

cond-mat.supr-con↗

arXiv2017

Change of the vortex core structure in two-band superconductors at impurity-scattering-driven $s_\pm/s_{++}$ crossover

We report a nontrivial transition in the core structure of vortices in two-band superconductors as a function of interband impurity scattering. We demonstrate that, in addition to singular zeros of the order parameter, the vortices there can acquire a circular nodal line around the singular point in one of the superconducting components. It results in the formation of the peculiar "moat"-like profile in one of the superconducting gaps. The moat-core vortices occur generically in the vicinity of the impurity-induced crossover between $s_{\pm}$ and $s_{++}$ states.

cond-mat.supr-con↗

arXiv2017

Type-1.5 superconductivity in multicomponent systems

In general a superconducting state breaks multiple symmetries and, therefore, is characterized by several different coherence lengths $ξ_i$, $i=1,...,N$. Moreover in multiband material even superconducting states that break only a single symmetry are nonetheless described, under certain conditions by multi-component theories with multiple coherence lengths. As a result of that there can appear a state where some coherence lengths are larger and some are smaller than the magnetic field penetration length $λ$: $ξ_1\leq ξ_2... < \sqrt{2}λ<ξ_M\leq...ξ_N$. That state was recently termed "type-1.5" superconductivity. This breakdown of type-1/type-2 dichotomy is rather generic near a phase transition between superconducting states with different symmetries. The examples include the transitions between $U(1)$ and $U(1)\times U(1)$ states or between $U(1)$ and $U(1)\times Z_2$ states. The later example is realized in systems that feature transition between s-wave and $s+is$ states. The extra fundamental length scales have many physical consequences. In particular in these regimes vortices can attract one another at long range but repel at shorter ranges. Such a system can form vortex clusters in low magnetic fields. The vortex clustering in the type-1.5 regime gives rise to many physical effects, ranging from macroscopic phase separation in domains of different broken symmetries, to unusual transport properties.

cond-mat.supr-con↗

arXiv2017

Phase diagram of dirty two-band superconductors and observability of impurity-induced $s+is$ state

We investigate the phase diagram of dirty two-band superconductors. This paper primarily focuses on the properties and observability of the time-reversal symmetry-breaking $s+is$ superconducting states, which can be generated in two-band superconductors by interband impurity scattering. We show that such states can appear in two distinct ways. First, according to a previously discussed scenario, the $s+is$ state can form as an intermediate phase at the impurity-driven crossover between $s_{\pm}$ and $s_{++}$ states. We show that there is a second scenario where domains of the $s+is$ state exists in the form of an isolated dome inside the $s_{\pm}$ domain, completely detached from the transition between $s_{\pm}$ and $s_{++}$ states. We demonstrate that in both cases the $s+is$ state, generated by impurity scattering exists in an extremely small interval of impurity concentrations. Although this likely precludes direct experimental observation of the $s+is$ state formation due to this mechanism, this physics leads to the appearance of a region inside both the $s_{\pm}$ and $s_{++}$ domains with unusual properties due to softening of normal modes.

cond-mat.supr-con↗

arXiv2016

Microscopically derived multi-component Ginzburg-Landau theories for $s+is$ superconducting state

Starting with the generic Ginzburg-Landau expansion from a microscopic $N$-band model, we focus on the case of a 3-band model which was suggested to be relevant to describe some iron-based superconductors. This can lead to the so-called $s+is$ superconducting state that breaks time-reversal symmetry due to the competition between different pairing channels. Of particular interest in that context, is the case of an interband dominated pairing with repulsion between different bands. For that case we consider in detail the relevant reduced two-component Ginzburg-Landau theory. We provide detailed analysis of the ground state, length scales and topological properties of that model.

cond-mat.supr-con↗

arXiv2016

Vortex motion and flux-flow resistivity in dirty multiband superconductors

The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory. Both the limits of high and low fields are considered, when the magnetic induction is close to or much smaller than the critical field strength $H_{c2}$, respectively. It is shown that in contrast to single-band superconductors the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. The low-field magneto-resistance can strongly exceed Bardeen-Stephen estimation in a quantitative agreement with experimental data for two-band superconductor MgB$_2$.

cond-mat.supr-con↗

arXiv2016

Excess magneto-resistance in multiband superconductors due to the viscous flow of composite vortices

By using the time-dependent Ginzburg-Landau theory, we show that extremely diverse experimental data on flux-flow resistivity in multiband superconductors can be qualitatively explained by a composite nature of Abrikosov vortices consisting of elementary fractional vortices in different bands. In composite vortices, the ratio of a core size to electric field relaxation length is found to vary in wide limits depending on system parameters. As a result, the flux-flow magneto-resistance can strongly exceed the single-component Bardeen-Stephen estimation provided that moving vortices generate electric field stretching strongly outside the vortex cores.

cond-mat.supr-con↗

arXiv2016

Thermoelectric Signatures of Time-Reversal Symmetry Breaking States in Multiband Superconductors

We show that superconductors with broken time-reversal symmetry have very specific magnetic and electric responses to inhomogeneous heating. A local heating of such superconductors induces a magnetic field with a profile that is sensitive to the presence of domain walls and crystalline anisotropy of superconducting states. A nonstationary heating process produces an electric field and a charge imbalance in different bands. These effects can be measured and used to distinguish $s+is$ and $s+id$ superconducting states in the candidate materials such as Ba$_{1-x}$K$_x$Fe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2015

Unconventional thermoelectric effect in superconductors that break time-reversal symmetry

We demonstrate that superconductors which break time-reversal symmetry can exhibit thermoelectric properties, which are entirely different from the Ginzburg mechanism. As an example, we show that in the $s+is$ superconducting state there is a reversible contribution to thermally induced supercurrent, whose direction is not invariant under time-reversal operation. Moreover in contrast to Ginzburg's mechanism it has a singular behavior near the time-reversal symmetry breaking phase transition. The effect can be used to confirm or rule out the $s+is$ state, which is widely expected to be realized in pnictide compounds Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and stoichiometric LiFeAs.

cond-mat.supr-con↗

arXiv2014

Lindblad equation approach for the full counting statistics of work and heat in driven quantum systems

We formulate the general approach based on the Lindblad equation to calculate the full counting statistics of work and heat produced by driven quantum systems weakly coupled with a Markovian thermal bath. The approach can be applied to a wide class of dissipative quantum systems driven by an arbitrary force protocol. We show the validity of general fluctuation relations and consider several generic examples. The possibilities of using calorimetric measurements to test the presence of coherence and entanglement in the open quantum systems are discussed.

quant-ph↗

arXiv2013

Unusual mechanism of vortex viscosity generated by mixed normal modes in superconductors with broken time reversal symmetry

We show that under certain conditions multiband superconductors with broken time-reversal symmetry have a new vortex viscosity-generating mechanism which is different from that in conventional superconductors. It appears due to the existence of mixed superfluid phase-density mode inside vortex core. This new contribution is dominant near the time reversal symmetry breaking phase transition. The results could be relevant for three band superconductor $Ba_{1-x}K_{x}Fe_2As_2$.

cond-mat.supr-con↗

arXiv2013

Nucleation of domain walls by $Z_2$ symmetry breaking transition in $p_x+ip_y$ superconductors

We show that time reversal symmetry breaking $p_x+ip_y$ wave superconductors undergo several phase transitions subjected to external magnetic field or supercurrent. In such system the discrete $Z_2$ symmetry can recover before the complete destruction of the order parameter. The topological defects associated with $Z_2$ symmetry - domain walls can be created in a controllable way by magnetic field or current sweep according to the Kibble-Zurek scenario. Such domain wall generation can take place in exotic superconductors like $Sr_2RuO_4$ and some heavy fermion compounds.

cond-mat.supr-con↗

arXiv2013

Type-1.5 superconductivity in muliband and other multicomponent systems

Usual superconductors are classified into two categories as follows: type-1 when the ratio of the magnetic field penetration length (λ) to coherence length (ξ) with Ginzburg-Landau parameter κ=λ/ξ<1/\sqrt{2} and type-2 when κ>1/\sqrt{2}. The boundary case κ=1/\sqrt{2} is also considered to be a special situation, frequently termed as "Bogomolnyi limit". Here we discuss multicomponent systems which can possess three or more fundamental length scales and allow a separate superconducting state, which was recently termed "type-1.5". In that state a system has the following hierarchy of coherence and penetration lengths ξ_1<\sqrt{2}λ<ξ_2. We also briefly overview the works on single-component regime $κ\approx 1/\sqrt{2}$ and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.

cond-mat.supr-con↗