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.

We present the thermodynamic properties and the magnetic phase diagram of Gd2BaNiO5 investigated by means of specific heat and magnetization measurements performed along the main crystallographic axes of the single crystal. Our data imply antiferromagnetic ordering at TN = 55 K and a spontaneous spin-reorientation transition at TSR = 24 K. Upon application of external magnetic fields, a spin-flop transition is observed at T < TSR in magnetic fields B || b-axis and at TSR < T < TN in magnetic fields B || a-axis. The magnetic phase diagram in Gd2BaNiO5 is established from these data. Considering Gd-Gd and Gd-Ni interactions allows estimating the magnetic contribution of the gadolinium subsystem to the magnetization and the specific heat. The analysis of the experimental data shows that the behavior of the nickel subsystem differs from that of a classical antiferromagnet.

We investigate the thermoelectric current and heat conductance in a chiral Josephson contact on a surface of a three-dimensional topological insulator, covered with superconducting and magnetic insulator films. The contact consists of two junctions of Majorana and Dirac channels next to two superconductors. Geometric asymmetry results in a supercurrent without a phase bias. The interference of Dirac fermions causes oscillations of the electric and heat currents with an unconventional period 2Φ0=h/e as functions of the Aharonov-Bohm flux. Due to the gapless character of Majorana modes, there is no threshold for the thermoelectric effect, and the current-flux relationship is nonsinusoidal. Depending on the magnetic flux, the direction of the electric current can be both from the hot to the cold lead and vice versa.

Three-particle complexes consisting of two holes in the completely filled zero-electron Landau level and an excited electron in the unoccupied first Landau level are investigated in a quantum Hall insulator. The distinctive features of these three-particle complexes are an electron-hole mass symmetry and the small energy gap of the quantum Hall insulator itself. Theoretical calculations of the trion energy spectrum in a quantizing magnetic field predict that, besides the ground state, trions feature a hierarchy of excited bound states. In agreement with the theoretical simulations, we observe new photoluminescence lines related to the excited trion states. A relatively small energy gap allows the binding of three-particle complexes with magnetoplasma oscillations and the formation of plasmarons. The plasmaron properties are investigated experimentally.

We extend the basic theory of Andreev reflection (AR) in a normal metal/superconductor junction to the situation with an arbitrary time-dependent bias voltage V(t) across the junction. The central element of the theory is the fact that the Fourier transform of the AR amplitude has a casual structure. As an example, the theory is used to describe the current response to short pulses of the bias voltage, which create coherent superposition of quasiparticle states with different energies. The current oscillates in time, with the gap frequency Δ/ℏ, and also as a function of the pulse area ∫V(t)dt, with the period of the single-electron flux quantum e/h.

We provide topological classification of possible phases with the symmetry of the planar phase of superfluid He3. Compared to the B phase [class DIII in classification of A. Altland and M. R. Zirnbauer, Phys. Rev. B 55, 1142 (1997)], it has an additional symmetry, which modifies the topology. We analyze the topology in terms of explicit mappings from the momentum space and also discuss explicitly topological invariants for the B phase. We further show how the bulk-boundary correspondence for the three-dimensional (3D) B phase can be inferred from that for the 2D planar phase. A general condition is derived for the existence of topologically stable zero modes at the surfaces of 3D superconductors with class-DIII symmetries.

Recent advances in nanophotonics have brought about coherent light sources with chaotic circular polarization; a low-dimensional chaotic evolution of optical spin was evidenced in laser diodes. Here we propose a mechanism that gives rise to light with a spatiotemporal spin chaos resembling turbulent states in hydrodynamics. The spin-chaotic radiation is emitted by exciton polaritons under resonant optical pumping in arbitrarily sized planar microcavities, including, as a limiting case, pointlike systems with only three degrees of freedom. The underlying mechanism originates in the interplay between spin symmetry breakdown and scattering into pairs of Bogolyubov excitations. As a practical matter, it opens up the way for spin modulation of light on the scale of picoseconds and micrometers.

We report numerical and analytical studies of the donor-acceptor recombination in compensated semiconductors. Our calculations take into account random electric fields of charged impurities that are important in the nonzero compensation case. We show that the donor-acceptor optical spectrum can be described as a sum of two components: monomolecular and bimolecular. In the low compensation limit, we develop two analytical models for both types of recombination. Also our numerical simulation predicts that these two components of the photoluminescence spectra can be resolved under certain experimental conditions.

We report results of systematic measurements of charge transport properties of the 20.5-nm-wide HgTe-based quantum well in perpendicular magnetic field, performed under hydrostatic pressures up to 15.1 kbar. At ambient pressure, transport is well described by the two-band semiclassical model. In contrast, at elevated pressure, we observed nonmonotonic pressure dependence of resistivity at the “charge neutrality point.” For pressures lower than ≈9kbar, resistivity grows with pressure, in accord with expectations from the band structure calculations and the model incorporating effects of disorder on transport in two-dimensional (2D) semimetals with indirect band overlap. For higher pressures, the resistivity saturates and starts decreasing upon further increase of pressure. Above ≈14kbar the resistance value and the magnetoresistance character sharply change, which may indicate significant change of electronic structure due to new electronic phase formation or some structural transitions. The data also reveal strong influence of disorder on transport in 2D electron-hole system with a small band overlap.

In the reported experiment, a picosecond strain pulse induces a sharp transition between the steady states in a bistable cavity-polariton system. The strain pulse of 10-ps duration, generated in the GaAs substrate and injected into a high-Q GaAs/AlAs microcavity, modulates the exciton resonance energies of the embedded quantum wells and correspondingly of the polariton resonances. When the microcavity is pumped by a laser with the photon energy slightly above the lower-polariton resonance, the strain-induced energy shift triggers the irreversible switching of the bistable polariton system from the lower to the upper state. This transition is accompanied by an instant increase of the optical emission from the microcavity by more than an order of magnitude.

We predict an existence of a dissipation channel leading to attenuation of ultrasound in disordered conductors and superconductors with perfect electroneutrality. It is due to slow diffusion of thermal energy. We show that in doped silicon ultrasound attenuation may be enhanced by a factor about 100. A similar effect is also studied for s-wave and d-wave superconductors. The latter case is applied to BSCCO family where a strong enhancement of the ultrasound attenuation is predicted. For usual s-wave superconductors, this dissipation channel might be important for very low-electron-density materials near the BCS-BEC crossover.

The magnetic phase diagram of a spin-12 chain antiferromagnet Sr2CuO3 is studied by an ultrasound phase-sensitive detection technique. The system is in the extreme proximity of the Luttinger-liquid quantum-critical point and we observe an unusually strong effect of magnetic field, which is very weak compared to the in-chain interaction, on the Néel ordering temperature. Inside the ordered phase, we detect an unexpected, field-induced continuous phase transition. The transition is accompanied by softening of magnetic excitation observed by electron-spin resonance, which in previous work [E. G. Sergeicheva *et al.*, Phys. Rev. B **95**, 020411(R) (2017)] was associated with a longitudinal (amplitude) mode of the order parameter. These results suggest a transition from a transverse collinear antiferromagnet to an amplitude-modulated spin-density-wave phase in a very weak magnetic field, which is unexpected for a system of weakly coupled Heisenberg spin-12 chains.

The magnetic phase diagram of a spin-12 chain antiferromagnet Sr2CuO3 is studied by an ultrasound phase-sensitive detection technique. The system is in the extreme proximity of the Luttinger-liquid quantum-critical point and we observe an unusually strong effect of magnetic field, which is very weak compared to the in-chain interaction, on the Néel ordering temperature. Inside the ordered phase, we detect an unexpected, field-induced continuous phase transition. The transition is accompanied by softening of magnetic excitation observed by electron-spin resonance, which in previous work [E. G. Sergeicheva *et al.*, Phys. Rev. B **95**, 020411(R) (2017)] was associated with a longitudinal (amplitude) mode of the order parameter. These results suggest a transition from a transverse collinear antiferromagnet to an amplitude-modulated spin-density-wave phase in a very weak magnetic field, which is unexpected for a system of weakly coupled Heisenberg spin-12 chains.

We report on an electron spin resonance (ESR) study of a nearly one-dimensional (1D) spin-12 chain antiferromagnet, Sr2CuO3, with extremely weak magnetic ordering. The ESR spectra at T>TN, in the disordered Luttinger-spin-liquid phase, reveal nearly ideal Heisenberg-chain behavior with only a very small, field-independent linewidth, ∼1/T. In the ordered state, below TN, we identify field-dependent antiferromagnetic resonance modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet. Additionally, we observe a major resonant mode with unusual and strongly anisotropic properties, which is not anticipated by the conventional theory of Goldstone spin waves. We propose that this unexpected magnetic excitation can be attributed to a field-independent magnon mode renormalized due to its interaction with the high-energy amplitude (Higgs) mode in the regime of weak spontaneous symmetry breaking.

The Majorana representation for spin operators enables efficient application of field-theoretical methods for the analysis of spin dynamics. Moreover, a wide class of spin correlation functions can be reduced to Majorana correlations of the same order, simplifying their calculation. For the spin-boson model, direct application of this method in the lowest order allows for a straightforward computation of the transverse-spin correlations, however, for the longitudinal-spin correlations it apparently fails in the long-time limit. Here we indicate the reason and discuss, how this method can be used as a convenient and accurate tool for generic spin correlations. Specifically, we demonstrate that accurate results are obtained by avoiding the use of the longitudinal Majorana fermion, and that correlations of the remaining transverse Majorana fermions can be easily evaluated using an effective Gaussian action.

We use an external magnetic field to probe the detection mechanism of a superconducting nanowire single-photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter Delta across the whole width of the superconducting nanowire after absorption of the photon) does not explain observed weak-field dependence of the photon count rate (PCR) for photons with lambda = 450 nm and noticeable decrease of PCR (with increasing the magnetic field) in a range of the currents for photons with wavelengths lambda = 450-1200 nm. Found experimental results for all studied wavelengths can be explained by the vortex hot spot model (which assumes partial suppression of Delta in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed Delta.

We consider ballistic SQUIDs with spin filtering inside half-metallic ferromagnetic arms. A singlet Cooper pair cannot pass through an arm in this case, so the Josephson current is entirely due to the Cooper pair splitting, with two electrons going to different interferometer arms. In order to elucidate the mechanisms of Josephson transport due to split Cooper pairs, we assume the arms to be single-channel wires in the short-junction limit. Different geometries of the system (determined by the length of the arms and the phases acquired by quasiparticles during splitting between the arms) lead to qualitatively different behavior of the SQUID characteristics (the Andreev levels, the current-phase relation, and the critical Josephson current) as a function of two control parameters, the external magnetic flux and misorientation of the two spin filters. The current-phase relation can change its amplitudeandshape,inparticular,turningtoaπ junctionformoracquiringadditionalzerocrossings.Thecritical current can become a nonmonotonic function of the misorientation of the spin filters and the magnetic flux (on half of period). Periodicity with respect to the magnetic flux is doubled, in comparison to conventional SQUIDs.