arXiv subjects

I. V. Bobkova

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

At least 37 records · Page 2Linked to original sources

arXiv2025

Gate-tunable nonlocal Josephson effect through magnetic van der Waals bilayers

It is well-known that the proximity effect at superconductor/ferromagnet (S/F) interfaces produces damped oscillatory behavior of the Cooper pair wave function within the ferromagnetic regions, which is analogous to the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity. It is often called the mesoscopic FFLO state and gives rise to $0-π$-transitions in S/F/S Josephson junctions. This paper offers an analysis of the proximity effect at interfaces between superconductors and magnetic van der Waals (vdW) bilayers. The specific feature of the proximity effect in the vdW bilayer systems is the presence of non-local Cooper pairs. We predict that the mesoscopic FFLO state formed by such pairs is sensitive to the difference between on-site energies of the monolayers composing the bilayer and, thus, can be controlled by applying a gating potential to one of the monolayers. This opens the possibility of implementing gate-controlled $0-π$ transitions in Josephson junctions through the magnetic vdW bilayer weak links.

cond-mat.supr-con↗

arXiv2024

Magnetic proximity effect in superconductor/ferromagnet van der Waals heterostructures: dependence on the number of superconducting monolayers

The magnetic proximity effect in superconductor/ferromagnet (S/F) heterostructures with a large number of atomic layers leads to a suppression of the superconducting order parameter and appearance of Zeeman-like spin splitting of the local density of states (LDOS). Here we study the magnetic proximity effects in van der Waals S/F heterostructures with a few atomic layers and demonstrate that the corresponding physics is very different from the classical results. We find that the dependence of the superconducting order parameter exhibits dips as a function of the ferromagnetic exchange field and gating. The number of dips is determined by the number of monolayers in the heterostructure and, in general, the superconductivity is not suppressed by large values of the exchange field. The spin splitting of the LDOS cannot be described by an effective Zeeman field and manifests a multiple peak structure, where each peak is connected to a unique spin splitting of one of the superconducting bands, which also can be tuned by gating.

cond-mat.supr-con↗

arXiv2024

Voltage-driven dynamics of $φ_0$-S/F/S Josephson junctions chains

Superconductor/ferromagnet/superconductor Josephson junctions with anomalous phase shift $φ_0$ ($φ_0$-S/F/S JJs) implement a coupling between the superconducting phase and the spin degrees of freedom. Here we investigate the dynamics of voltage-biased coupled chains of $φ_0$-S/F/S JJs and predict that the presence of $φ_0$ makes the conventional regime corresponding to the linear growth of the superconducting phase at each of the junctions and oscillating Josephson current unstable. New stable regimes are found and investigated. The changes of the dynamic behavior are clearly seen in the IV-characteristics of the system and can serve as a fingerprint of the presence of the magnetoelectric coupling $φ_0$. Moreover, the collective magnetic excitations of the chains of $φ_0$-S/F/S JJs, which were reported by G.A. Bobkov {\it et. al.} [JETP Lett. {\bf 119}, 251 (2024)] also manifest themselves in the IV-characteristics. This provides a method for their experimental detection and direct measurement of the constant quantifying the strength of the coupling between the superconducting phase and the spin degrees of freedom.

cond-mat.supr-con↗

arXiv2024

Gate-controlled proximity effect in superconductor/ferromagnet van der Waals heterostructures

The discovery of 2D materials opens up unprecedented opportunities to design new materials with specified properties. In many cases, the design guiding principle is based on one or another proximity effect, i.e. the nanoscale-penetration of electronic correlations from one material to another. In a few layer van der Waals (vdW) heterostructures the proximity regions occupy the entire system. Here we demonstrate that the physics of magnetic and superconducting proximity effects in 2D superconductor/ferromagnet vdW heterostructures is determined by the effects of interface hybridization of the electronic spectra of both materials. The degree of hybridization can be adjusted by gating, which makes it possible to achieve a high degree of controllability of the proximity effect. In particular, we show that this allows for electrical switching of superconductivity in such structures on and off, as well as for control of the amplitude and sign of the Zeeman splitting of superconducting spectra, opening interesting opportunities for spintronics and spin caloritronics.

cond-mat.supr-con↗

arXiv2024

Spin supercurrent in superconductor/ferromagnet van-der-Waals heterostructures

We study dissipationless spin transport induced by a charge supercurrent in a monolayer van der Waals superconductor under the applied magnetic field and in a bilayer superconductor/ferromagnet (S/F) heterostructure with no external field. It is shown that in both cases a combined action of the Ising-type spin-orbit coupling and the Zeeman field results in appearance of a nonunitary superconducting triplet correlations with nonzero average Cooper pair spin, which carry spin current in the presence of a condensate motion. Properties of this dissipationless spin current are investigated. In particular, it is shown that it manifests a rectification effect. In addition, in S/F heterostructures the amplitude and the sign of the spin current are controlled by gating.

cond-mat.supr-con↗

arXiv2024

Neel proximity effect in superconductor/antiferromagnet heterostructures

It is well-known that the cornerstone of the proximity effect in superconductor/ferromagnet heterostructures is a generation of triplet Cooper pairs from singlet Cooper pairs inherent in a conventional superconductor. This proximity effect brought a lot of new exciting physics and gave a powerful impulse to development of superconducting spintronics. Nowadays a new direction of spintronics is actively developing, which is based on antiferromagnets and their heterostructures. It is called antiferromagnetic spintronics. By analogy with an important role played by triplet Cooper pairs in conventional superconducting spintronics based on ferromagnets the question arises: does the triplet proximity effect exist in superconductor/antiferromagnet heterostructures and, if so, what are the properties of the induced triplet correlations and the prospects for use in superconducting spintronics? Recent theoretical findings predict that despite the absence of a net magnetization, the Neel magnetic order of the antiferromagnet does give rise to specific spin-triplet correlations at superconductor/antiferromagnet interfaces. They were called Neel triplet correlations. The goal of this review is to discuss the current understanding of the fundamental physics of these Neel triplet correlations and their physical manifestations.

cond-mat.supr-con↗

arXiv2024

Superconducting spin valves based on antiferromagnet/superconductor/antiferromagnet heterostructures

Proximity effect at superconductor/antiferromagnet (S/AF) interfaces, which manifests itself as generation of Neel-type triplet correlations, leads to sensitivity of the superconducting critical temperature to the mutual orientation of the AF Neel vectors in AF/S/AF trilayers, which is called the spin-valve effect. Here we predict that the spin-valve effect in AF/S/AF heterostructures crucially depends on the value of the chemical potential of the superconducting interlayer due to the occurrence of the finite-momentum Neel triplet correlations. In addition we investigate equal-spin triplet correlations, which appear in AF/S/AF structures for non-aligned Neel vectors of the AFs, and their role in the nonmonotonic dependence of the superconducting critical temperature of the AF/S/AF structure on the mutual orientation of the AF Neel vectors. The influence of impurities on the spin-valve effect is also investigated.

cond-mat.supr-con↗

arXiv2024

Controllable magnetic states in chains of coupled phi-0 Josephson junctions with ferromagnetic weak links

A superconductor/ferromagnet/superconductor Josephson junction with anomalous phase shift (phi_0-S/F/S JJ) is a system, where the anomalous ground state shift phi_0 provides a direct magnetoelectric coupling between a magnetic moment and a phase of the superconducting condensate. If a chain of such phi_0-S/F/S JJs are coupled via superconducting leads, the condensate phase, being a macroscopic quantity, mediates a long-range interaction between the magnetic moments of the weak links. We study static and dynamic magnetic properties of such a system. It is shown that it manifests properties of n-level system, where the energies of the levels are only determined by projections of the total magnetic moment onto the easy magnetic axis. It is similar to a magnetic atom in a Zeeman field, but the role of the field is played by the magnetoelectric coupling. However, unlike an atom in a magnetic field, the relative order of energies of different states is controlled by electrical means. It is also demonstrated that the total magnetic moment can be fully controlled by a supercurrent and the response of the magnetic system to local external perturbations is highly nonlocal.

cond-mat.supr-con↗

arXiv2024

Magnetic eigenmodes in chains of coupled phi0-Josephson junctions with ferromagnetic weak links

A coupled chain of superconductor/ferromagnet/superconductor (S/F/S) with anomalous ground phase shift phi_0 represents a system realizing long-range interaction between magnetic moments of the weak links. The interaction is of magnetoelectric origin and is mediated by the condensate phase of superconductors. The system is a paradigmic platform for investigation of collective magnetic states governed by the superconducting phase. Here we study the magnetic eigenmodes of such a system and demonstrate that the eigenfrequencies are determined by the magnetic configuration of the whole system and are controlled by the superconducting phase. Depending on the orientation of the magnetic easy axis the eigenmodes can be very different ranging from individual oscillations of different magnets to highly-cooperative behavior.

cond-mat.supr-con↗

arXiv2023

Magnon influence on the superconducting density of states in superconductor$-$ferromagnetic-insulator bilayers

Superconductor$-$ferromagnetic-insulator heterostructures are paradigmatic systems for studying the mutual influence of superconductivity and magnetism via proximity effects. In particular, spin-split superconductivity is realized in such structures. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such heterostructures that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection, and sensing. In this work we investigate the influence of the electron-magnon interaction at the superconductor$-$ferromagnetic-insulator interface on the spin-split superconductivity. It is predicted that due to the magnon-mediated electron spin-flip processes the spin-split quasiparticle branches are partially mixed and reconstructed, and the BCS-like spin-split shape of the superconducting density of states, which is typical for superconductors in the effective exchange field, is strongly modified. An odd-frequency superconducting order parameter admixture to the leading singlet order parameter is also found. These findings expand the physical picture of spin-split superconductivity beyond the mean-field description of the ferromagnet exchange field.

cond-mat.supr-con↗

arXiv2023

Oscillatory superconducting transition temperature in superconductor/antiferromagnet heterostructures

One of the most famous proximity effects at ferromagnet/superconductor (F/S) interfaces is partial conversion of singlet superconductivity to triplet pairing correlations. Due to the presence of macroscopic exchange field in the ferromagnet the Cooper pairs penetrating into the ferromagnet from the superconductor acquire a finite momentum there. The finite-momentum pairing manifests itself, in particular, as a nonmonotonic dependence of the critical temperature of the bilayer on the thickness of the F layer. Here we predict that despite the absence of the macroscopic exchange field the critical temperature of the antiferromagnet/superconductor (AF/S) bilayers also exhibit nonmonotonic (oscillating) dependence on the AF layer thickness. It is a manifestation of the proximity-induced Neel-type triplet correlations, which acquire finite total pair momentum and oscillate in the AF layer due to the Umklapp electron scattering processes at the AF/S interface. Our prediction can provide a possible explanation for a number of recently published experimental observations of the critical temperature of AF/S bilayers.

cond-mat.supr-con↗

arXiv2023

Complete $T_c$ suppression and Néel triplets-mediated exchange in antiferromagnet-superconductor-antiferromagnet trilayers

An antiferromagnetic insulator (AFI) bearing a compensated interface to an adjacent conventional superconductor (S) has recently been predicted to generate Néel triplet Cooper pairs, whose amplitude alternates sign in space. Here, we theoretically demonstrate that such Néel triplets enable control of the superconducting critical temperature in an S layer via the angle between the Néel vectors of two enclosing AFI layers. This angle dependence changes sign with the number of S monolayers providing a distinct signature of the Néel triplets. Furthermore, we show that the latter mediate a similarly distinct exchange interaction between the two AFIs' Néel vectors.

cond-mat.mes-hall↗

arXiv2023

Proximity effect in superconductor/antiferromagnet hybrids: Neel triplets and impurity suppression of superconductivity

Two possible physical mechanisms of superconductivity suppression at superconductor/antiferromagnet (S/AF) interfaces, which work even for interfaces with compensated antiferromagnets, were reported. One of them suggests that the Neel order of the AF induces rapidly oscillating spin-triplet correlations in the S layer. They are called Neel triplets, and they suppress singlet superconductivity. Nonmagnetic disorder destroys this type of triplet correlations. As a result, the critical temperature of the S/AF bilayer grows with impurity strength. The second mechanism, on the contrary, suggests that nonmagnetic impurity scattering suppresses superconductivity in S/AF hybrids. The predictions were made in the framework of two different quasiclassical approaches [G. A. Bobkov et al. Phys. Rev. B 106, 144512 (2022) and E. H. Fyhn et al. arXiv:2210.09325]. Here we suggest the unified theory of the proximity effect in thin-film S/AF structures, which incorporates both pictures as limiting cases, and we study the proximity effect at S/AF interfaces for arbitrary impurity strength, chemical potential, and the value of the Neel exchange field.

cond-mat.supr-con↗

arXiv2023

Magnon-Cooparons in magnet-superconductor hybrids

Generation and detection of spinful Cooper pairs in conventional superconductors has been intensely pursued by designing increasingly complex magnet-superconductor hybrids. Here, we demonstrate theoretically that magnons with nonzero wavenumbers universally induce a cloud of spinful triplet Cooper pairs around them in an adjacent conventional superconductor. The resulting composite quasiparticle, termed magnon-cooparon, consists of a spin flip in the magnet screened by a cloud of the spinful superfluid condensate. Thus, it inherits a large effective mass, which can be measured experimentally. Furthermore, we demonstrate that two magnetic wires deposited on a superconductor serve as a controllable magnonic directional coupler mediated by the nonlocal and composite nature of magnon-cooparons. Our analysis predicts a quasiparticle that enables generation, control, and use of spinful triplet Cooper pairs in the simplest magnet-superconductor heterostructures.

cond-mat.supr-con↗

arXiv2023

Magnetic anisotropy of superconducting transition in S/AF heterostructures with spin-orbit coupling

The influence of Rashba spin-orbit coupling (SOC) on superconducting correlations in thin-film superconductor/antiferromagnet (S/AF) structures with compensated interfaces is studied. A unique effect of anisotropic enhancement of proximity-induced triplet correlations by the SOC is predicted. It manifests itself in the anisotropy of the superconducting critical temperature Tc with respect to orientation of the Neel vector relative to the S/AF interface, which is opposite to the behaviour of Tc in superconductor/ferromagnet structures. We show that the anisotropy is controlled by the chemical potential of the superconductor and, therefore, can be adjusted in (quasi)2D structures.

cond-mat.supr-con↗

arXiv2023

Renormalization of antiferromagnetic magnons by superconducting condensate and quasiparticles

The ability to modify and tune the spin-wave dispersion is one of the most important requirements for engineering of magnonic networks. In this study we demonstrate the promise of synthetic thin-film hybrids composed of an antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal for tuning and modifying the spin-wave dispersion in antiferromagnetic insulators. The key ingredient is the uncompensated magnetic moment at the AF/S(N) interface, which induces an effective exchange field in the adjacent metal via the interface exchange interaction. The exchange field spin polarizes quasiparticles in the metal and induces spinful triplet Cooper pairs screening the magnon. The quasiparticle and Cooper pair polarization renormalizes the magnon dispersion. The renormalization results in the splitting of the otherwise degenerate AF magnon modes with no need to apply a magnetic field. It is also proposed that measurements of the renormalized dispersion relations can provide the amplitude of the effective exchange field induced by the AF in the adjacent metal.

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↗