arXiv subjects

I. V. Bobkova

Publications and source records attributed to I. V. Bobkova.

At least 73 records · Page 4Linked to original sources

arXiv2017

Quasiclassical theory of magnetoelectric effects in superconducting heterostructures in the presence of the spin-orbit coupling

The quasiclassical theory in terms of equations for the Green's functions (Eilenberger equations) is generalized in order to allow for quantitative description of the magneto-electric effects and proximity-induced triplet correlations in the presence of spin-orbit coupling in hybrid superconducting systems. The formalism is valid under the condition that the spin-orbit coupling is weak with respect to the Fermi energy, but exceeds the superconducting energy scale considerably. On the basis of the derived formalism it is shown that the triplet correlations in the spin-orbit coupled normal metal can be induced by proximity to a singlet superconductor without any exchange or external magnetic field. They contain an odd-frequency even-momentum component, which is stable against disorder. The value of the proximity-induced triplet correlations is of the order of $Δ_{so}/\varepsilon_F$, that is absent in the framework of the standard quasiclassical approximation, but can be described by our theory. The spin polarization, induced by the Josephson current flowing through the superconductor/Rashba metal/superconductor junction, is also calculated.

cond-mat.supr-con↗

arXiv2017

Thermospin effects in superconducting heterostructures

Recently thermally created pure spin currents were predicted for Zeeman-split superconductor/normal metal heterostructures. Here it is shown that this "thermospin" current can lead to an accumulation of a pure spin imbalance in a system. The thermally induced spin imbalance can reach the value of Zeeman splitting of the superconducting DOS and strongly influences superconductivity in the heterostructure. Depending on the temperature difference between the superconductor and the normal reservoir it can enhance the critical temperature of the superconductor or additionally suppress the zero-temperature superconducting state. The last possibility gives rise to an unusual superconducting state, which starts to develop at finite temperature.

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arXiv2017

Electrically controllable spin filtering based on superconducting helical states

The magnetoelectric effects in the surface states of the 3D TI are extremely strong due to the full spin-momentum locking. Here the microscopic theory of S/3D TI bilayer structures in terms of quasiclassical Green's functions is developed. On the basis of the developed formalism it is shown that the DOS in the S/TI bilayer manifests giant magnetoelectric behavior and, as a result, S/3D TI heterostructures can work as non-magnetic fully electrically controllable spin filters. It is shown that due to the full spin-momentum locking the amplitudes of the odd-frequency singlet and triplet components of the consensate wave function are equal. The same is valid for the even frequency singlet and triplet components. We unveil the connection between the odd-frequency pairing in S/3D TI heterostructures and magnetoelectric effects in the DOS.

cond-mat.supr-con↗

arXiv2017

Generalized quasiclassical theory of the long-range proximity effect and spontaneous currents in superconducting heterostructures with strong ferromagnets

We present the generalized quasiclassical theory of the long-range superconducting proximity effect in heterostructures with strong ferromagnets, where the exchange splitting is of the order of Fermi energy. In the ferromagnet the propagation of spin-triplet Cooper pairs residing on the spin-split Fermi surfaces is shown to be governed by the spin-dependent Abelian gauge field which results either from the spin-orbital coupling or from the magnetic texture. The additional gauge field enters into the quasiclassical equations in superposition with the usual electromagnetic vector potential and results in the generation of spontaneous superconducting currents and phase shifts in various geometries which provide the sources of long-range spin-triplet correlations. We derive the Usadel equations and boundary conditions for the strong ferromagnet and consider several generic examples of the Josephson systems supporting spontaneous currents.

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arXiv2016

Magnetoelectrics in Disordered Topological Insulator Josephson Junctions

We theoretically study the coupling of electric charge and spin polarization in an equilibrium and nonequilibrium electric transport across a two dimensional Josephson configuration comprised of disordered surface channels of a three dimensional topological insulator. In the equilibriun state of the system we predict the Edelstein effect, which is much more pronounced than its counterpart in conventional spin orbit coupled materials. Employing a quasiclassical Keldysh technique, we demonstrate that the ground state of system can be experimentally shifted into arbitrary macroscopic superconducting phase differences other than the standard `$0$' or `$π$', constituting a $ϕ_0$-junction, solely by modulating a quasiparticle flow injection into the junction. We propose a feasible experiment where the quasiparticles are injected into the topological insulator surface by means of a normal electrode and voltage gradient so that oppositely oriented stationary spin densities can be developed along the interfaces and allow for directly making use of the spin-momentum locking nature of Dirac fermions in the surface channels. The $ϕ_0$-state is proportional to the voltage difference applied between the injector electrode and superconducting terminals that calibrates the injection rate of particles and, therefore, the $ϕ_0$ shift.

cond-mat.str-el↗

arXiv2015

Injection of nonequilibrium quasiparticles into Zeeman-split superconductors: a way to create long-range spin imbalance

A theory of spin transport and spin detection in Zeeman-split superconducting films at low temperatures is developed. It is shown that an injection of spin-unpolarized quasiparticles into a Zeeman-split superconductor gives rise to a spin imbalance. The relaxation length of such a spin signal is determined by the energy relaxation length and can be extremely large as compared to the relaxation length of spin-polarized quasiparticles. There can exist two types of signals: due to nonthermalized quasiparticle distribution and due to thermalized overheated electron distribution. They have different decay lengths and can be distinguished by their different dependencies on the applied voltage. The decay length of the nonthermalized signal is determined by the electron-electron scattering rate, renormalized due to superconductivity. The decay length of the thermalized signal is determined by the length on which energy leaves the electronic subsystem and can be very large under special conditions. Applications of the theory to recent experimental data on spin relaxation in Zeeman-split and exchange-split superconductors are discussed. In particular, it can explain the extremely high spin relaxation lengths, experimentally observed in Zeeman-split superconductors, and their growth with the magnetic field and with the applied voltage.

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arXiv2015

Long-range spin imbalance in mesoscopic superconductors under a Zeeman splitting

We develop a theory of spin relaxation in Zeeman-splitted superconducting films at low temperatures. A new mechanism of spin relaxation, specific only for Zeeman-splitted superconductors is proposed. It can explain the extremely high spin relaxation lengths, experimentally observed in Zeeman-splitted superconductors, and their strong growth with the magnetic field. In the framework of this mechanism the observed spin signal is formed by the spin-independent nonequilibrium quasiparticle distribution weighted by the spin-split DOS. We demonstrate that the relaxation length of such a spin signal is determined by the energy relaxation length at energies of the order of the superconducting gap.

cond-mat.supr-con↗

arXiv2014

Recovering of superconductivity in S/F bilayers under spin-dependent nonequilibrium quasiparticle distribution

We study theoretically the influence of spin accumulation on superconductivity in a superconductor/ferromagnet bilayer. It is well-known that the superconductivity in S/F bilayers is suppressed by the proximity to a ferromagnet. The spin accumulation by itself is also a depairing factor. But here we show that creation of the spin accumulation on top of effective exchange depairing, caused by the proximity to a ferromagnet, can lead to an opposite result. The superconductivity can be partially recovered by spin-dependent quasiparticle distribution. The systems with realistic parameters are considered and the possible experimental setup is proposed.

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arXiv2014

Nonlocal supercurrent in mesoscopic multiterminal SNS Josephson junction in the low-temperature limit

Nonlocal supercurrent was observed in mesoscopic planar SNS Josephson junction with additional normal metal electrodes when nonequilibrium quasiparticles were injected from the normal metal electrode in one of the superconducting banks of the Josephson junction in the absence of net transport current through the junction. We claim that the effect observed is due to a supercurrent counterflow appearing to compensate the quasiparticle flow in the SNS weak link. We have measured SNS junction response for different distances from the quasiparticle injector to the SNS junction at temperatures far below the superconducting transition temperature. The charge-imbalance relaxation length was estimated by using modified Kadin, Smith and Skocpol scheme in the case of planar geometry. The model developed allows to describe the interplay of the charge imbalance and Josephson effects in the nanoscale proximity system in detail.

cond-mat.supr-con↗

arXiv2014

Enhancing of the in-plane FFLO-state critical temperature in heterostructures by the orbital effect of the magnetic field

It is well-known that the orbital effect of the magnetic field suppresses superconducting $T_c$. We show that for a system, which is in the Larkin-Ovchinnikov-Fulde-Ferrell (FFLO) state at zero external magnetic field, the orbital effect of an applied magnetic field can lead to the enhancement of the critical temperature higher than $T_c$ at zero field. We concentrate on two systems, where the in-plane FFLO-state was predicted recently. These are equilibrium S/F bilayers and S/N bilayers under nonequilibrium quasiparticle distribution. However, it is suggested that such an effect can take place for any plane superconducting heterostructure, which is in the in-plane FFLO-state (or is close enough to it) at zero applied field.

cond-mat.supr-con↗

arXiv2014

Bistable state in superconductor/ferromagnet heterostructures

A novel state is predicted for a S/F heterostructure. It is shown that the equilibrium F/S bilayer can be in a bistable state: one state is ground and the other is metastable. One of these states is always normal and the other is superconducting. One can switch between these states by an external control parameter, for example, by an applied magnetic field. This phenomenon can manifest itself through the hysteresis behavior upon varying the applied magnetic field or, even, temperature. It can also lead to transition of the heterostructure into the superconducting state upon increase of the temperature.

cond-mat.supr-con↗

arXiv2013

In-plane FFLO instability in superconductor-normal metal bilayer system under non-equilibrium quasiparticle distribution

It is predicted that a new class of systems - superconductor/normal metal (S/N) heterostructures can reveal the in-plane Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) instability under nonequilibrium conditions at temperatures close to the critical temperature. It does not require any Zeeman interaction in the system. For S/N heterostructures under non-equilibrium distribution there is a natural easily adjustable parameter - voltage, which can control the FFLO-state. This FFLO-state can be of different types: plane wave, stationary wave and, even, 2D-structures are possible. Some types of the FFLO-state are accompanied by the magnetic flux, which can be observed experimentally. All the types of the FFLO-state can reveal through the temperature dependence of the linear response of the system on the applied magnetic field near $T_c$, which strongly differs from that one for the homogeneous state.

cond-mat.supr-con↗

arXiv2012

Recovering of superconductivity in exchange fields exceeding Pauli limiting field under spin-dependent quasiparticle distribution

We study theoretically the simultaneous influence of spin accumulation potential $eV_\uparrow-eV_\downarrow$ and the Zeeman exchange field on singlet superconductivity. It is shown that the pair-breaking effect of the Zeeman field can be fully compensated by creation of the appropriate spin accumulation potential in the superconductor. Moreover, superconductivity can be recovered for exchange fields well exceeding the Pauli limiting field. It is proposed that the effect can be experimentally realized on the basis of voltage biased junction consisting of a thin superconducting film sandwiched between two half metals.

cond-mat.supr-con↗

arXiv2012

Long-range proximity effect for opposite-spin pairs in S/F heterostructures under non-equilibrium quasiparticle distribution

By now it is known that in a singlet superconductor/ferromagnet (S/F) structure the superconducting correlations carried by opposite-spin pairs penetrate into the ferromagnet over a short distance of the order of magnetic coherence length. The long-range proximity effect (LRPE), taking place on the length scale of the normal metal coherence length, can only be maintained by equal-spin pairs, which can be generated by magnetic inhomogeneities in the system. In this work we have predicted a new type of LRPE, which can take place in S/F heterostructures under non-equilibrium conditions. The superconducting correlations in the F region are generated by opposite-spin Cooper pairs and equal-spin pairs are not involved. The possibility for an opposite-spin pair to penetrate into the ferromagnet over a large distance is provided by creation of the proper non-equilibrium quasiparticle distribution there. This leads to a sharp increase (up to a few orders of magnitude) of the critical Josephson current through a S/F/S junction at some values of the voltage controlling the nonequilibrium distribution in the F interlayer.

cond-mat.supr-con↗

arXiv2011

Time-reversal symmetry breaking state near the surface of $s_{\pm}$-superconductor

The structure of superconducting order parameter near the surface of a two-band superconductor with $s_{\pm}$ order parameter in the bulk is theoretically investigated. The main parameter of the surface, which determines the appropriate physics is the coefficient of the interband scattering $R_{12}$. For small $R_{12}$ the superconducting order parameter is only suppressed to some extent near the surface for the both bands. For intermediate and strong interband scattering there are two possible non-trivial surface states of the order parameter: (i) purely real solution, where the symmetry of the superconducting state near the surface is changed from $s_{\pm}$ to conventional $s_{++}$ and (ii) time-reversal symmetry breaking (TRB) state. In this state the order parameters in the two bands acquire phases $ϕ_{1,2}(x) \neq (0,π)$ upon approaching the surface. We argue that at low temperatures the TRB surface state can be more energetically favorable than the $s_{\pm} \to s_{++}$ time reversal symmetry conserving state (TR). For higher temperatures up to $T_c$ only the TR state can exist. The transition between the two temperature regions is rather sharp. Signatures of the transition between the TRB and the TR surface states can be detected by the measurements of the local density of states and the angle-resolved density of states.

cond-mat.supr-con↗

arXiv2011

Influence of the spin-dependent quasiparticle distribution on the Josephson current through a ferromagnetic weak link

The Josephson current flowing through weak links containing ferromagnetic elements is studied theoretically under the condition that the quasiparticle distribution over energy states in the interlayer is spin-dependent. It is shown that the interplay between the spin-dependent quasiparticle distribution and the triplet superconducting correlations induced by the proximity effect between the superconducting leads and ferromagnetic elements of the interlayer, leads to the appearence of an additional contribution to the Josephson current. This additional contribution $j_t$ can be extracted from the full Josephson current in experiment. The features of the additional supercurrent $j_t$, which are of main physical interest are the following: (i) We propose the experimental setup, where the contributions given by the short-range (SRTC) and long-range (LRTC) components of triplet superconducting correlations in the interlayer can be measured separately. It can be realized on the basis of S/N/F/N/S junction, where the interlayer is composed of two normal metal regions with a spiral ferromagnet layer sandwiched between them. For the case of tunnel junctions the measurement of $j_t$ in such a system can provide direct information about the energy-resolved anomalous Green's function components describing SRTC and LRTC. (ii) In some cases the exchange field-suppressed supercurrent can be not only recovered but also enhanced with respect to its value for non-magnetic junction with the same interface resistances by the presence of spin-dependent quasiparticle distribution. This effect is demonstrated for S/N/S junction with magnetic S/N interfaces. In addition, it is also found that under the considered conditions the dependence of the Josephson current on temperature can be nontrivial: at first the current rises upon temperature increasing and only after that starts to decline.

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arXiv2010

Triplet contribution to the Josephson current in the nonequilibrium SFS junction

The Josephson current through a long s-wave superconductor/weak ferromagnet/s-wave superconductor weak link is studied theoretically in the regime of nonequilibrium spin-dependent occupation of electron states in the ferromagnetic intelayer. While under the considered nonequilibrium conditions the standard supercurrent, carried by the singlet part of current-carrying density of states, practically is not modified, the additional supercurrent flowing via the triplet part of the current-carrying density of states appears. Depending on voltage, controlling the particular form of spin-dependent nonequilibrium in the interlayer, this additional current can enhance or reduce the usual current of the singlet component and also switch the junction between $0$- and $π$-states.

cond-mat.supr-con↗

arXiv2010

Order parameter sign-reversal near $s_\pm$-superconductor surface

The superconducting order parameter and LDOS spectra near an impenetrable surface are studied on the basis of selfconsistent calculations for a two band superconductor with nodeless extended s-+wave order parameter symmetry, as possibly realized in Fe-based high-temperature superconductors. It is found that for a wide range of parameters the spatial behavior of the order parameter at a surface is not reduced to a trivial suppression. If the interband scattering at a surface is of the order of the intraband one or dominates it, it can be energetically favorable to change the symmetry of the superconducting state near the surface from $s_{\pm}$ to conventional s-wave. The range of existing this surface conventional superconductivity is very sensitive to the relative values of interband and intraband pairing potentials. It is shown that the LDOS spectra near the surface can qualitatively differ upon calculating with and without taking into account the selfconsistency of the order parameter.

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