arXiv subjects

T. Karabassov

Publications and source records attributed to T. Karabassov.

11 recordsLinked to original sources

arXiv2026

Proximity-induced orbital antiferromagnetism in Ising superconductors

We predict a fundamentally new superconducting state in superconductor/antiferromagnet heterostructures with Ising spin--orbit coupling: proximity-induced orbital antiferromagnetism. In this state, the order parameter acquires a periodic phase modulation locked to the magnetic lattice, generating atomic-scale loop currents with opposite orbital moments on neighboring unit cells. Its emergence requires at least three nonequivalent magnetic sublattices per unit cell and finite spin--orbit coupling. Using NbSe$_2$/MnPS$_3$ as a concrete example, we combine first-principles and Bogoliubov--de Gennes calculations to demonstrate that the proximity-induced exchange field leads to robust phase modulation. Unlike FFLO and helical states, the phase gradient is atomic-scale, the state is current-carrying, and it remains uniquely stable over the full parameter range. The state manifests as characteristic finite-energy dips in the local density of states, accessible by STM.

cond-mat.supr-con↗

arXiv2026

Hybrid magnon -- Nambu-Goldstone excitations in topological superconductor/ferromagnetic insulator thin-film heterostructures

We address a previously unexplored type of dynamical proximity effect that occurs in s-wave topological superconductor/ferromagnetic insulator (TS/FI) heterostructures. It is predicted that magnons in the FI and the Nambu-Goldstone (NG) collective superconducting phase mode in the TS are coupled, forming composite magnon-NG excitations. The mechanism of this coupling is associated with the complete spin-momentum locking of electrons in the helical surface state of the TS. The strength of the magnon-NG coupling is strongly anisotropic with respect to the mutual orientation of the magnon wave vector and the equilibrium magnetization of the FI. This effect provides a mechanism for the interconversion of spin signals and the spinless signals carried by collective superconducting excitations, thereby giving new impetus to the development of superconducting spintronics.

cond-mat.supr-con↗

arXiv2026

Hybrid collective excitations in topological superconductor/ferromagnetic insulator heterostructures

We develop a linear response theory for the dynamical proximity effect in topological superconductor/ferromagnetic insulator (TS/FI) hybrids. Our approach integrates the nonequilibrium quasiclassical Keldysh-Usadel formalism for the TS with the Landau-Lifshitz-Gilbert equation for the FI's magnetization dynamics. This framework reveals a proximity-induced coupling between magnons and superconducting collective modes. Crucially, we find that spin-momentum locking in the TS surface state drives a hybridization between magnons and the superconducting Nambu-Goldstone (phase) mode, giving rise to composite magnon-Nambu-Goldstone excitations. We analyze the coupling strength's dependence on key parameters both analytically and numerically. In contrast, we demonstrate that the Higgs (amplitude) mode does not couple to magnons at linear order and is thus excluded from the hybrid excitation spectrum. The hybridization between magnons and the superconducting phase mode provides a mechanism for the interconversion of spin signals and the spinless signals carried by collective superconducting excitations, thereby giving new impetus to the development of superconducting spintronics.

cond-mat.supr-con↗

arXiv2026

The physics of superconductor-ferromagnet hybrid structures

In this review, we summarize the foundations underlying a variety of phenomena in superconductor-ferromagnet hybrid structures, with a focus on recent advances in several key areas. These include: (i) the fundamental understanding of proximity effects in superconductor-ferromagnet based systems; (ii) spin-valve effects in superconductor-ferromagnet and superconductor-ferromagnet-superconductor Josephson junctions; and (iii) the design and realization of superconducting memory elements, particularly in hybrid Josephson junctions. We also discuss the experimental progress in fabricating and characterizing spin-valve structures.

cond-mat.supr-con↗

arXiv2025

Competitive helical bands and highly efficient diode effect in F/S/TI/S/F hybrid structures

The diode effect in superconducting materials has been actively investigated in recent years. Plenty of different devices have been proposed as a platform to observe the superconducting diode effect. In this work we discuss the possibility of a highly efficient superconducting diode design with controllable polarity. We propose the mesoscopic device that consists of two separated superconducting islands with proximity induced ferromagnetism deposited on top of the three-dimensional topological insulator. Using the quasiclassical formalism of the Usadel equations we demonstrate that the sign of the diode efficiency can be controlled by magnetization tuning of a single superconducting island. Moreover, we show that the diode efficiency can be substantially increased in such device. We argue that the dramatic increase of the diode efficiency is due to competing contribution of the two superconducting islands to the supercurrent with single helical bands linked through the topological insulator surface.

cond-mat.supr-con↗

arXiv2023

Phase diagrams of the superconducting diode effect in topological hybrid structures

Recently the superconducting diode effect (SDE) has attracted a lot of attention due to new possibilities in the field of superconducting electronics. One of the possible realizations of the SDE is the implementation in superconducting hybrid structures. In this case the SDE is achieved by means of the proximity effect. However, the optimal conditions for the SDE quality factor in hybrid devices remain unclear. In this study we consider the Superconductor/Ferromagnet/Topological insulator (S/F/TI) hybrid device and investigate the diode quality factor at different parameters of the hybrid structure. Consequently, we reveal important parameters that have crucial impact on the magnitude of the SDE quality factor.

cond-mat.supr-con↗

arXiv2022

Hybrid helical state and superconducting diode effect in S/F/TI heterostructures

It is well-known that the ground state of homogeneous superconducting systems with spin-orbit coupling (SOC) in the presence of the Zeeman field is the so-called helical state, which is characterized by the phase modulation of the order parameter, but zero supercurrent density. In this work we investigate the realization of the helical state in a hybrid system with spatially separated superconductivity and exchange field by considering S/F bilayer on top of a 3D topological insulator. This system is characterized by strong spin-momentum locking and, consequently, provides the most favorable conditions for the helical state generation. The analysis is based on the microscopic theory in terms of the quasiclassical Green's functions. We demonstrate that in the bilayer the helical state survives if the exchange field has non-zero component perpendicular to the S/F interface even in spite of the fact that the superconducting order parameter and the exchange field are spatially separated. At the same time, in this spatially inhomogeneous situation the helical state is accompanied by the spontaneous currents distributed over the bilayer in such a way as to have zero average. Further, we show that this hybrid helical state gives rise to nonreciprocity in the system. We demonstrate the realization of the superconducting diode effect and show that the degree of nonreciprocity is very large in the considered system due to the strongest possible value of the spin-momentum locking.

cond-mat.supr-con↗

arXiv2020

Reentrant superconductivity in proximity to a topological insulator

In the following paper we investigate the critical temperature $T_c$ behavior in the two-dimensional S/TI (S denotes superconductor and TI - topological insulator) junction with a proximity induced in-plane helical magnetization in the TI surface. The calculations of $T_c$ are performed using the general self-consistent approach based on the Usadel equations in Matsubara Green's functions technique. We show that the presence of the helical magnetization leads to the nonmonotonic behavior of the critical temperature as a function of the topological insulator layer thickness.

cond-mat.supr-con↗

arXiv2020

Anomalous current-voltage characteristics of SFIFS Josephson junctions with weak ferromagnetic interlayers

We present a quantitative study of the current-voltage characteristics (CVC) of SFIFS Josephson junctions (S denotes bulk superconductor, F - metallic ferromagnet, I - insulating barrier) with weak ferromagnetic interlayers in the diffusive limit. The problem is solved in the framework of the nonlinear Usadel equations. We consider the case of a strong tunnel barrier such that the left SF and the right FS bilayers are decoupled. We calculate the density of states (DOS) in SF bilayers using a self-consistent numerical method. Then we obtain the CVC of corresponding SFIFS junctions, and discuss their properties for different set of parameters including the thicknesses of ferromagnetic layers, the exchange field, and the magnetic scattering time. We observe the anomalous nonmonotonic CVC behavior in case of weak ferromagnetic interlayers, which we ascribe by DOS energy dependencies in case of small exchange fields in F layers.

cond-mat.supr-con↗

arXiv2019

Competitive 0 and π states in S/F multilayers: multimode approach

We have investigated the critical temperature behavior in periodic superconductor/ ferromagnet (S/F) multilayers as a function of the ferromagnetic layer thickness $d_f$ and the interface transparency. The critical temperature $T_c(d_f)$ exhibits a damped oscillatory behavior in these systems due to an exchange field in the ferromagnetic material. In this work we have performed $T_c$ calculations using the self-consistent multimode approach, which is considered to be exact solving method. Using this approach we have derived the conditions of 0 or $π$ state realization in periodic S/F multilayers. Moreover, we have presented the comparison between the single-mode and multimode approaches and established the limits of applicability of the single-mode approximation, frequently used by experimentalists.

cond-mat.supr-con↗

arXiv2018

Relaxation of Nonequilibrium Quasiparticles in Mesoscopic Size Superconductors

Rapid development of micro- and nanofabrication methods have provoked interest and enabled experimental studies of electronic properties of a vast class of (sub)micrometer-size solid state systems. Mesoscopic-size hybrid structures, containing superconducting elements, have become interesting objects for basic research studies and various applications, ranging from medical and astrophysical sensors to quantum computing. One of the most important aspects of physics, governing the behavior of such systems, is the finite concentration of nonequilibrium quasiparticles, present in a superconductor even well below the temperature of superconducting transition. Those nonequilibrium excitations might limit the performance of a variety of superconducting devices, like superconducting qubits, single-electron turnstiles and microrefrigerators. On the contrary, in some applications, like detectors of electromagnetic radiation, the nonequilibrium state is essential for their operation. It is therefore of vital importance to study the mechanisms of nonequilibrium quasiparticle relaxation in superconductors of mesoscopic dimensions, where the whole structure can be considered as an `interface'. At early stages of research the problem was mostly studied in relatively massive systems and at high temperatures close to the critical temperature of a superconductor. We review the recent progress in studies of nonequilibrium quasiparticle relaxation in superconductors including the low temperature limit. We also discuss the open physical questions and perspectives of development in the field.

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