arXiv subjects

A. M. Bobkov

Publications and source records attributed to A. M. Bobkov.

At least 37 records · Page 2Linked to original sources

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

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

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↗

arXiv2022

Magnetoelectric effects in Josephson junctions

The review is devoted to the fundamental aspects and characteristic features of the magnetoelectric effects, reported in the literature on Josephson junctions (JJs). The main focus of the review is on the manifestations of the direct and inverse magnetoelectric effects in various types of Josephson systems. They provide a coupling of the magnetization in superconductor/ferromagnet/superconductor JJs to the Josephson current. The direct magnetoelectric effect is a driving force of spin torques acting on the ferromagnet inside the JJ. Therefore it is of key importance for the electrical control of the magnetization. The inverse magnetoelectric effect accounts for the back action of the magnetization dynamics on the Josephson subsystem, in particular, making the JJ to be in the resistive state in the presence of the magnetization dynamics of any origin. The perspectives of the coupling of the magnetization in JJs with ferromagnetic interlayers to the Josephson current via the magnetoelectric effects are discussed.

cond-mat.supr-con↗

arXiv2022

Thermally induced spin-transfer torques in superconductor/ferromagnet bilayers

Thermally induced magnetization dynamics is currently a flourishing field of research due to its potential application in information technology. We study the paradigmatic system of a magnetic domain wall in a thermal gradient which is interacting with an adjacent superconductor. The spin-transfer torques arising in this system due to the combined action of the giant thermoelectric effect and the creation of equal-spin pairs in the superconductor are large enough to give rise to high domain wall velocities $10^3$ times larger than previously predicted.

cond-mat.supr-con↗

arXiv2022

Long-range interaction of magnetic moments in a coupled system of S/F/S Josephson junctions with anomalous ground-state phase shift

A mechanism of a superconductivity-mediated interaction of two magnets in a system of coupled superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions (JJs) with spin-orbit interaction is proposed. The predicted indirect magnetic interaction favors the antiparallel orientation of the magnets. Its spatial scale is not restricted by the proximity length scales of the superconductor. Our estimates suggest that the interaction strength is not reduced considerably even at the macroscopic scales of the order of millimeters. At larger distances $l$ between the magnets the coupling constant exhibits the long-range power law $1/l$ behavior. The mechanism of the interaction is based on two key ingredients: (i) the anomalous ground state phase shift in the S/F/S JJ provides a magnetoelectric coupling between the condensate phase and the magnetization and (ii) the interaction is mediated by the condensate phase of the superconducting region connecting both JJs. In addition we demonstrate high tunability of the total magnetic configuration of the system by the externally controlled superconducting phase between the leads.

cond-mat.supr-con↗

arXiv2021

Dynamic spin-triplet order induced by alternating electric fields in superconductor-ferromagnet-superconductor Josephson junctions

Dynamic states offer extended possibilities to control the properties of quantum matter. Recent efforts are focused on studying the ordered states which appear exclusively under the time-dependent drives. Here we demonstrate a class of systems which feature dynamic spin-triplet superconducting order stimulated by the alternating electric field. The effect is based on the interplay of ferromagnetism, interfacial spin-orbital coupling (SOC) and the condensate motion driven by the field, which converts hidden static p-wave order, produced by the joint action of the ferromagnetism and the SOC, into dynamical s-wave equal-spin triplet correlations. We demonstrate that the critical current of Josephson junctions hosting these states is proportional to the electromagnetic power, supplied either by the external irradiation or by the ac current source. Based on these unusual properties we propose the scheme of a Josephson transistor which can be switched by the ac voltage and demonstrates an even-numbered sequence of Shapiro steps. Combining the photo-active Josephson junctions with recently discovered Josephson phase batteries we find photo-magnetic SQUID devices which can generate spontaneous magnetic fields while being exposed to irradiation.

cond-mat.supr-con↗

arXiv2020

Thermally induced spin torque and domain wall motion in superconductor/antiferromagnetic insulator bilayers

We theoretically investigate domain wall motion in an antiferromagnetic insulator layer caused by thermally generated spin currents in an adjacent spin-split superconductor layer. An uncompensated antiferromagnet interface enables the two crucial ingredients underlying the mechanism - spin splitting in the superconductor and absorption of spin currents by the antiferromagnet. Treating the superconductor using the quasiclassical theory and the antiferromagnet via Landau-Lifshitz-Gilbert description, we find domain wall propagation along the thermal gradient with relatively large velocities $\sim 100$ m/s. Our proposal exploits the giant thermal response of spin-split superconductors in achieving large spin torques towards driving domain wall and other spin textures in antiferromagnets.

cond-mat.supr-con↗

arXiv2020

Odd triplet superconductivity induced by the moving condensate

It has been commonly accepted that magnetic field suppresses superconductivity by inducing the ordered motion of Cooper pairs. We demonstrate that magnetic field can instead provide a generation of superconducting correlations by inducing the motion of superconducting condensate. This effect arises in superconductor/ferromagnet heterostructures in the presence of Rashba spin-orbital coupling. We predict the odd-frequency spin-triplet superconducting correlations called the Berezinskii order to be switched on at large distances from the superconductor/ferromagnet interface by the application of a magnetic field. This is shown to result in the unusual behaviour of Josephson effect and local density of states in superconductor/ferromagnet structures.

cond-mat.supr-con↗

arXiv2020

Magnetization reversal in S/F/S Josephson junctions on a 3D topological insulator

We study a magnetization reversal by an electric current pulse in a superconductor/insulating ferromagnet/superconductor Josephson junction placed on top of a 3D topological insulator. It is demonstrated that such a system is perspective for low-dissipative spintronics because of the strong spin-momentum locking in the TI surface states. This property provides an ideally strong coupling between the orbital and spin degrees of freedom thus giving a possibility of efficient reversal of the magnetic moment by current pulse with amplitude lower than the critical current, that results in strongly reduced energy dissipation. The underlying physical mechanism of the reversal is discussed. The influence of the magnetic anisotropy on the controllability of the reversal by the pulse duration is investigated. In addition, a way of a simultaneous electrical detection of the reversal is proposed.

cond-mat.supr-con↗

arXiv2020

Electrical response of S-F-TI-S junctions on magnetic texture dynamics

We consider a hybrid structure consisting of superconducting or normal leads with a combined ferromagnet-3D topological insulator interlayer. We compare responses of a Josephson junction and a normal junction to magnetic texture dynamics. In both cases the electromotive force resulting from the magnetization dynamics generates a voltage between the junction leads. For an open circuit this voltage is the same for normal and superconducting leads and allows for electrical detection of magnetization dynamics and a structure of a given magnetic texture. However, under the applied current the electrical response of the Josephson junction is essentially different due to the strong dependence of the critical Josephson current on the magnetization direction and can be used for experimental probing of this dependence. We propose a setup, which is able to detect a defect motion and to provide detailed information about the structure of magnetic inhomogeneity. The discussed effect could be of interest for spintronics applications.

cond-mat.mes-hall↗

arXiv2019

Resistive state of SFS Josephson junctions in the presence of moving domain walls

We describe resistive states of the system combining two types of orderings - superconducting and ferromagnetic one. It is shown that in the presence of magnetization dynamics such systems become inherently dissipative and in principle cannot sustain any amount of the superconducting current because of the voltage generated by the magnetization dynamics. We calculate generic current-voltage characteristics of a superconductor/ferromagnet/superconductor Josephson junction with an unpinned domain wall and find the low-current resistance associated with the domain wall motion. We suggest the finite slope of Shapiro steps as the characteristic feature of the regime with domain wall oscillations driven by the ac external current flowing through the junction.

cond-mat.supr-con↗

arXiv2019

Anomalous phase shift in a Josephson junction via an antiferromagnetic interlayer

The anomalous ground state phase shift in S/AF/S Josephson junctions in the presence of the Rashba SO-coupling is predicted and numerically investigated. It is found to be a consequence of the uncompensated magnetic moment at the S/AF interfaces. The anomalous phase shift exhibits a strong dependence on the value of the SO-coupling and the sublattice magnetization with the simultaneous existence of stable and metastable branches. It depends strongly on the orientation of the Neel vector with respect to the S/AF interfaces via the dependence on the orientation of the interface uncompensated magnetic moment, what opens a way to control the Neel vector by supercurrent in Josephson systems.

cond-mat.supr-con↗