arXiv subjects

G. A. Bobkov

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

30 records · Page 2Linked to original sources

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

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

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↗

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↗

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↗