We consider a problem of persistent magnetization precession in a single-domain ferromagnetic nanoparticle under the driving by the spin-transfer torque. We find that the adjustment of the electronic distribution function in the particle renders this state unstable. Instead, abrupt switching of the spin orientation is predicted upon increase of the spin-transfer torque current. On the technical level, we derive an effective action of the type of Ambegaokar-Eckern-Schön action for the coupled dynamics of magnetization [gauge group SU(2)] and voltage [gauge group U(1)].

We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths w ranging from 2 to 1000m and film thicknesses d4−6 and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves R(B,T) separate at a well-defined and field-dependent temperature T∗(B) with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width w. The associated excess conductivity in all the bridges scales as 1/w, which may suggest either the presence of a highly conducting region that is dominating the electric transport, or a change in the vortex dynamics in narrow enough bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.

We study the density of states (DOS) in diffusive superconductors with pointlike magnetic impurities of arbitrary strength described by the Poissonian statistics. The mean-field theory predicts a nontrivial structure of the DOS with the continuum of quasiparticle states and (possibly) the impurity band. In this approximation, all the spectral edges are hard, marking distinct boundaries between spectral regions of finite and zero DOS. Considering instantons in the replica sigma-model technique, we calculate the average DOS beyond the mean-field level and determine the smearing of the spectral edges due interplay of fluctuations of potential and nonpotential disorder. The latter, represented by inhomogeneity in the concentration of magnetic impurities, affects the subgap DOS in two ways: via fluctuations of the pair-breaking strength and via induced fluctuations of the order parameter. In limiting cases, we reproduce previously reported results for the subgap DOS in disordered superconductors with strong magnetic impurities.

We study the effect of superconducting fluctuations on the conductivity of metals at arbitrary temperatures T and impurity scattering rates τ−1. Using the standard diagrammatic technique but in the Keldysh representation, we derive the general expression for the fluctuation correction to the dc conductivity applicable for any space dimensionality and analyze it in the case of the film geometry. We observe that the usual classification in terms of the Aslamazov-Larkin, Maki-Thompson, and density-of-states diagrams is to some extent artificial since these contributions produce similar terms, which partially cancel each other. In the diffusive limit, our results fully coincide with recent calculations in the Keldysh technique. In the ballistic limit near the transition, we demonstrate the absence of a divergent term (Tτ)2 attributed previously to the density-of-states contribution. In the ballistic limit far above the transition, the temperature-dependent part of the conductivity correction is shown to grow as Tτ/ln(T/Tc), where Tc is the critical temperature.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of BaFe1.9Ni0.1As2. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps L ≈ 3.2−4.5 meV, S ≈ 1.2−1.6 meV (the ranges correspond to the minimum and maximum value of the coupling energy in the kxky plane). The 25%−30% anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps L ≈ 3.2 meV and S ≈ 1.6 meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe1−xSx (x = 0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature Tc, anisotropy, upper critical field Hc2, and critical current density Jc. The upper critical field Hc2(T ) and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth λab(T ) and specific heat, we show clear evidence for strong coupling two-gap s-wave superconductivity. The temperature dependence of λab(T ) calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with s-wave-like gaps. We find that a d wave and single-gap BCS theory under the weak-coupling approach cannot describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.

We report magnetic and superconducting properties of the modified spin valve system CoOx/Fe1/Cu/Fe2/Cu/Pb. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin valve effect and of the manifestation of the long-range triplet component of the superconducting condensate.

This article represents a mini review of our obtained results about of superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers. We have analysed and compared the superconducting properties of the spin-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In these experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $\Delta T_c$ assisted by the triplet superconducting spin-valve effect $\Delta T_c^{trip}$. In the second case, we observed a giant magnitude of $\Delta T_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented. We find a good agreement between our experimental data and theoretical results.

We demonstrate that in sufficiently long diffusive superconducting-normal-superconducting (SNS) junctions dc Josephson current is exponentially suppressed by electron-electron interactions down to zero temperature. This suppression is caused by the effect of Cooper pair dephasing which occurs in the normal metal and defines a new fundamental length scale Lφ in the problem. This length is fully consistent with that derived earlier from the analysis of dissipative electron transport across NS interfaces at subgap energies. Provided the temperature length exceeds Lφ this dephasing length can be conveniently extracted from equilibrium measurements of the Josephson current.

The spin-galvanic (inverse Edelstein) and inverse spin-Hall effects are calculated for a hybrid system that combines thin superconductor and Rashba-metal layers. These effects are produced by a nonequilibrium spin polarization which is injected into the normal metal layer. This polarization gives rise to an electric potential that relaxes within some characteristic length, which is determined by the Andreev reflection. Within this length the dissipative electric current of quasiparticles in the normal layer converts into the supercurrent. This process involves only subgap states and at the low temperature inelastic electron-phonon interactions are not important. It is discussed how such a hybrid system can be integrated into a SQID where it produces the effect similar to a magnetic flux.

A nonuniform in-plane Zeeman field can induce spontaneous supercurrents of spin-orbit-coupled electrons in superconducting two-dimensional systems and thin films. In this work it is shown that current vortices can be created at the ends of a long homogeneously magnetized strip of a ferromagnetic insulator, which is deposited on the surface of a three-dimensional topological insulator. The s-wave superconductivity on its surface is assumed to have an intrinsic origin, or to be induced by the proximity effect. It is shown that vortices with the odd vorticity can localize Majorana zero modes. The latter may also be induced by sufficiently narrow domain walls inside the strip, which opens a way for manipulating these modes by moving the walls. It is shown that the vorticity can be tuned by varying the magnetization and width of the strip. A stability of the strip magnetization with respect to the Berezinsky-Kosterlitz-Thouless transition has been analyzed.

We report results of experiments with liquid 3He confined in a high-porosity anisotropic nanostructure which we call planar aerogel. This aerogel consists of nanofibers (with diameters ∼10 nm) which are randomly oriented in the plane normal to the specific axis. We used two samples of planar aerogel prepared using different techniques. We have found that on cooling from the normal phase of 3He the superfluid transition in both samples occurs into an equal spin pairing superfluid phase. NMR properties of this phase qualitatively agree with the properties of the superfluid A phase in the anisotropic Larkin-Imry-Ma state. We have observed differences between results obtained in the presence and absence of solid paramagnetic 3He on the aerogel strands. We propose that these differences may be due, at least in part, to a magnetic scattering channel, which appears in the presence of solid paramagnetic 3He.

We analyze the critical behavior of the superfluid density ρs in strongly disordered superconductors near the superconductor-insulator transition and compare it with the behavior of the spectral gap Δ for collective excitations. We show that, in contrast to conventional superconductors, the superconductors with preformed pairs display an unusual scaling relation ρs∝Δ2 close to the superconductor-insulator transition. This relation has been reported in very recent experiments.

Precise angle-resolved magnetoresistance (ARM) measurements are applied to reveal the origin of symmetry
lowering in electron transport and the emergence of a huge number of magnetic phases in the ground state of the
antiferromagnetic metal HoB12 with fcc crystal structure. By analyzing the polar *H-θ-ϕ* magnetic phase diagrams
of this compound reconstructed from the experimental ARM data, we argue that nonequilibrium electron density
oscillations (dynamic charge stripes) are responsible for the suppression of the indirect Ruderman-Kittel-Kasuya-
Yosida exchange along the <110> directions between the nearest neighboring magnetic moments of Ho3+ ions in
this strongly correlated electron system.

We consider a superconductor with surface suppression of the BCS pairing constant $\lambda(x)$. We analytically find the gap in the surface density of states (DOS), behavior of the DOS $\nu(E)$ above the gap, a "vertical" peculiarity of the DOS around energy equal to the bulk order parameter $\Delta_0$, and perturbative correction to the DOS at higher energies. The surface gap in the DOS is parametrically different from the surface value of the order parameter due to difference between the spatial scale $r_c$, at which $\lambda(x)$ is suppressed, and coherence length. The vertical peculiarity implies infinite-derivative inflection point of the DOS curve at $E=\Delta_0$ with square-root behavior as $E$ deviates from $\Delta_0$. The coefficients of this dependence are different at $E<\Delta_0$ and $E>\Delta_0$, so the peculiarity is asymmetric.

We experimentally investigate charge transport through a single planar junction between a Cd3As2 Dirac semimetal and a normal Au lead. For nonsuperconducting bulk Cd3As2 samples we observe non-Ohmic dV/dI(V ) curves, which strongly resemble standard Andreev reflection with a well-defined superconducting gap. Andreev-like behavior is demonstrated for Cd3As2 samples with different surface and contact preparation techniques. We connect this behavior with surface superconductivity due to the flat-band formation in Cd3As2, which has been predicted theoretically. The conclusion on superconductivity is also supported by the gap suppression by magnetic fields or temperature.

By measuring magnetoresistance and the Hall effect in a classically moderate perpendicular magnetic field in a Si-MOSFET-type macroscopic antidot array, we found a nonlinear with field, temperature- and density-dependent Hall resistivity. We argue that this nonlinearity originates from low mobility shells of the antidots with a strong temperature dependence of the resistivity and suggest a qualitative explanation of the phenomenon.

We study a one-dimensional anisotropic XXZ Heisenberg spin-12 chain with weak random fields hizSiz by means of Jordan-Wigner transformation to spinless Luttinger liquid with disorder and bosonization technique. First, we reinvestigate the phase diagram of the system in terms of dimensionless disorder γ=h2/J2≪1 and anisotropy parameter Δ=Jz/Jxy, we find the range of these parameters where disorder is irrelevant in the infrared limit and spin-spin correlations are described by power laws, and compare it with previously obtained numerical and analytical results. Then we use the diagram technique in terms of plasmon excitations to study the low-temperature (T≪J) behavior of heat conductivity κ and spin conductivity σ in this power-law phase. The obtained Lorentz number L≡κ/σT differs from the value derived earlier by means of the memory function method. We argue also that in the studied region inelastic scattering is strong enough to suppress quantum interference in the low-temperature limit.

Motivated by recent experimental results for GdRu2Si2 [Khanh et al., Nat. Nanotechnol. 15, 444 (2020)], in which a nanometric square skyrmion lattice was observed, we propose a simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. Dipolar forces provide momentum-dependent biaxial anisotropy in reciprocal space. It is shown that in a tetragonal lattice, in the large part of the Brillouin zone, for mutually perpendicular modulation vectors in the *ab* plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, which leads to double-*Q* modulated spin structure stabilization. In the large part of its stability region, the latter turns out to be a square skyrmion lattice with a topological charge of ±1 per magnetic unit cell, which is determined by the frustrated exchange coupling and thus nanometer sized. Easy and middle axes can be swapped in the presence of additional single-ion easy-axis anisotropy. This results in the different phase diagram. It is argued that the latter case is relevant to GdRu2Si2.

We show that the magnetic susceptibility of a dilute ensemble of magnetic impurities in a conductor with a relativistic electronic spectrum is nonanalytic in the inverse temperature at

T−1→0

. We derive a general theory of this effect and construct the high-temperature expansion for the disorder averaged susceptibility to any order, convergent at all temperatures down to a possible ordering transition. When applied to Ising impurities on a surface of a topological insulator, the proposed general theory agrees with Monte Carlo simulations, and it allows us to find the critical temperature of the ferromagnetic phase transition.