We studied electron spin resonance in a quantum magnet NiCl2−4SC(NH2)2, demonstrating a field-induced quantum phase transition from a quantum-disordered phase to an antiferromagnet. We observe two branches of the antiferromagnetic resonance of the ordered phase, one of them has a gap, and the other is a Goldstone mode with zero frequency at a magnetic field along the fourfold axis. This zero-frequency mode acquires a gap at a small tilting of the magnetic field with respect to this direction. The upper gap was found to be reduced in the Br-substituted compound Ni(Cl1−xBrx)2−4SC(NH2)2 with x=0.21. This reduction is unexpected because of the previously reported rise in the main exchange constant in a substituted compound. Furthermore, a nonresonant diamagnetic susceptibility χ′ was found for the ordered phase in a wide frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as large as the dynamic susceptibility of the paramagnetic resonance. We speculate that it originates from a two-magnon absorption band of the low-frequency dispersive magnon branch.

Multiple condensates in a superconducting material can interfere constructively or destructively and this leads to unconventional effects not inherent in single-band superconductors. Such effects can be pronounced when the spatial scales (healing lengths) of different band condensates deviate from each other. Here we show that, contrary to usual expectations, this deviation can be considerable even far beyond the regime of nearly decoupled bands, being affected by difference between the band Fermi velocities. Our study is performed within the extended Ginzburg-Landau (GL) formalism that goes to one order beyond the GL theory in the perturbation expansion of the microscopic equations over the proximity to Tc. The formalism makes it possible to obtain closed analytical results for the profiles of the band condensates and for their healing lengths and, at the same time, captures the difference between the healing lengths which does not appear in the standard GL domain.

We have carried out the electron spin resonance (ESR) study of the multiferroic triangular antiferromagnet CuCrO2 in the presence of an electric field. The shift of ESR spectra by the electric field was observed; the value of the shift exceeds that in materials with linear magnetoelectric coupling. It was shown that the low-frequency dynamics of magnetically ordered CuCrO2 is defined by joint oscillations of the spin plane and electric polarization. The results demonstrate an agreement with theoretical expectations of a phenomenological model [V. I. Marchenko, J. Exp. Theor. Phys. 119, 1084 (2014)].

We explore the evolution of wave-function statistics on a finite Bethe lattice (Cayley tree) from the central site (“root”) to the boundary (“leaves”). We show that the eigenfunction moments Pq=N⟨|ψ|2q(i)⟩ exhibit a multifractal scaling Pq∝N−τq with the volume (number of sites) N at N→∞. The multifractality spectrum τq depends on the strength of disorder and on the parameter s characterizing the position of the observation point i on the lattice. Specifically, s=r/R, where r is the distance from the observation point to the root, and R is the “radius” of the lattice. We demonstrate that the exponents τq depend linearly on s and determine the evolution of the spectrum with increasing disorder, from delocalized to the localized phase. Analytical results are obtained for the n-orbital model with n≫1 that can be mapped onto a supersymmetric σ model. These results are supported by numerical simulations (exact diagonalization) of the conventional (n=1) Anderson tight-binding model.

We report the results of the numerical study of the non-dissipative quantum Josephson junction chain with the focus on the statistics of many-body wave functions and local energy spectra. The disorder in this chain is due to the random offset charges. This chain is one of the simplest physical systems to study many-body localization. We show that the system may exhibit three distinct regimes: insulating, characterized by the full localization of many-body wavefunctions, fully delocalized (metallic) one characterized by the wavefunctions that take all the available phase volume and the intermediate regime in which the volume taken by the wavefunction scales as a non-trivial power of the full Hilbert space volume. In the intermediate, non-ergodic regime the Thouless conductance (generalized to many-body problem) does not change as a function of the chain length indicating a failure of the conventional single-parameter scaling theory of localization transition. The local spectra in this regime display the fractal structure in the energy space which is related with the fractal structure of wave functions in the Hilbert space. A simple theory of fractality of local spectra is proposed and a new scaling relationship between fractal dimensions in the Hilbert and energy space is suggested and numerically tested.

Multiple Mn2+ spin-flip Raman scattering (SFRS) in Voigt geometry was observed in self-organized disk-shaped quantum dots (QDs) of CdSe/Zn0.99Mn0.01Se, where magnetic ions and QD carriers are spatially separated and therefore the exchange interaction between them is expected to be weak. Many lines (about ten) were observed in SFRS spectra, yet the overlapping of the hole wave function with Mn2+ ions is very small, in agreement with both the absence of observable Zeeman splitting of the photoluminescence line and the calculation. Interesting polarization properties of SFRS spectra were observed which could be affected by tilting the sample out of normal alignment and changing the temperature. These polarization properties were attributed to the selection rules in SFRS in Voigt geometry. It has been found that the theoretical model suggested by Stühler *et al.* [J. Cryst. Growth **159**, 1001 (1996)] does not describe the SFRS spectra in systems with weak exchange interaction between charge carriers and magnetic ions. A qualitative model is suggested here for description of SFRS in such systems.

We study the thermodynamics of the three-dimensional Hubbard model at half filling on approach to the Néel transition by means of large-scale unbiased diagrammatic determinant Monte Carlo simulations. We obtain the transition temperature in the strongly correlated regime, as well as the temperature dependence of the energy, entropy, double occupancy, and nearest-neighbor spin correlation function. Our results improve the accuracy of previous unbiased studies and present accurate benchmarks in the ongoing effort to realize the antiferromagnetic state of matter with ultracold atoms in optical lattices.

We study the superconducting properties of the bulk states of a doped topological insulator. We obtain that hexagonal warping stabilizes the nematic spin-triplet superconducting phase with Eu pairing and the direction of the nematic order parameter which opens the full gap is the ground state. This order parameter exhibits non-BCS behavior. The ratio of the order parameter to the critical temperature of Δ(0)/Tc differs from the BCS ratio. It depends on the chemical potential and the value of the hexagonal warping. We discuss the relevance of the obtained results for the explanation of the experimental observations.

The transition of molecular hydrogen to atomic ionized state with the increase of temperature and pressure poses still unresolved problems for experimental methods and theory. Here we analyze the dynamics of this transition and show its nonequilibrium nonadiabatic character overlooked in both interpreting experimental data and in theoretical models. The nonadiabatic mechanism explains the strong isotopic effect [M. Zaghoo, R. J. Husband, and I. F. Silvera, Phys. Rev. B 98, 104102 (2018).] and the large latent heat [M. Houtput, J. Tempere, and I. F. Silvera, Phys. Rev. B 100, 134106 (2019).] reported recently. We demonstrate the possibility of the formation of intermediate excitonlike molecular states at heating of molecular hydrogen that can explain the puzzling experimental data on reflectivity and conductivity during the insulator-to-metal transition.

Anomalous diffusion in some bcc metals is the long-standing topic in material science. In this work, I obtain the temperature dependence of the self-diffusion coefficient in bcc titanium directly from molecular dynamics (MD) calculation. MD simulations indicate that both vacancies and self-interstitials contribute to diffusivity in bcc Ti. The resultant self-diffusion coefficient is non-Arrhenius, but shows less curvature than observed in most experiments.

We developed the model of the internal phonon bottleneck to describe the energy exchange between the acoustically soft ultrathin metal film and acoustically rigid substrate. Discriminating phonons in the film into two groups, escaping and nonescaping, we show that electrons and nonescaping phonons may form a unified subsystem, which is cooled down only due to interactions with escaping phonons, either due to direct phonon conversion or indirect sequential interaction with an electronic system. Using an amplitude-modulated absorption of the sub-THz radiation technique,we studied electron-phonon relaxation in ultrathin disordered films of tungsten silicide.We found an experimental proof of the internal phonon bottleneck. The experiment and simulation based on the proposed model agree well, resulting in τe−ph ∼ 140–190 ps at TC = 3.4K, supporting the results of earlier measurements by independent techniques.

Quasi-one-dimensional systems demonstrate Van Hove singularities in the density of states and the resistivity ρ, occurring when the Fermi level E crosses a bottom E_N of some subband of transverse quantization. We demonstrate that the character of smearing of the singularities crucially depends on the concentration of impurities. There is a crossover concentration n_c ∝ |λ|,λ ≪ 1 being the dimensionless amplitude of scattering. For n ≫ n_c the singularities are simply rounded at ε ≡ E − E_N ∼ τ ^{−1} – the Born scattering rate. For n ≪ nc the single-impurity non-Born effects in scattering become essential despite λ ≪ 1. The peak of the resistivity is asymmetrically split in a Fano-resonance manner (however with a more complex structure). Namely, for ε > 0 there is a broad maximum at ε∝λ^2 while for ε<0 there is a deep minimum at |ε|∝n^2 ≪λ^2. The behaviour of ρ below the minimum depends on the sign of λ. In case of repulsion ρ monotonically grows with |ε| and saturates for |ε| ≫ λ^2. In case of attraction ρ has sharp maximum at |ε| ∝ λ^2. The latter feature is due to resonant scattering at quasistationary bound states that inevitably arise just below the bottom of each subband for any attracting impurity.

Clean quasi-one-dimensional systems demonstrate Van Hove singularities in the density of states ν and resistivity ρ, occurring when the Fermi level crosses the bottom of some transversal quantization subband. However, taking the scattering on impurities into the account should smear the singularities. As we have shown in our previous work [Phys. Rev. B **99**, 035414 (2019)], for the case of clean conducting tubes, the character of smearing strongly depends on the impurity concentration n. For n≫nc, the singularities are simply rounded, while for n≪nc, the initial peak is asymmetrically split into two for the case of attracting impurities, nc being a crossover concentration. In this work, we extend our consideration to “strips”—quasi-one-dimensional structures in 2D conductors. Here also for n≪nc, an original Van Hove singularity is asymmetrically split into two peaks. However, in contrast to the tube case, the amplitudes of scattering at impurities depend on their positions and these peaks are inhomogeneously broadened. The strongest broadening occurs in the left peak, arising, for attracting impurities, due to the scattering at the quasistationary levels that form a relatively broad impurity band with a weak quasi-Van Hove feature on its lower edge. Different parts of ρ(ɛ) are dominated by different groups of impurities: close to the minimum the most effective scatterers, paradoxically, are the “weakest” impurities located close to nodes of the electronic wave function. The quasi-Van Hove feature at the left maximum is dominated by the strongest impurities located close to antinodes.

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction of magnetic impurities in a superconductor exponentially decreases when the distance between them is larger than the superconductor's coherence length, because this interaction is mediated by quasiparticles, which have a gap in their energy spectrum. At the same time, the spin-singlet superconducting condensate was always assumed to stay neutral to magnetic impurities. Due to a spin-orbit coupling (SOC), however, Cooper pairs gain an admixture of spin-triplet correlated states, which provide for a link between impurity spins and an s-wave condensate. It is shown that perturbations of its phase mediate the 1/r^2 interaction of these spins in two-dimensional (2D) systems. This effect is considered within two models: of a clean 2D s-wave superconductor with the strong Rashba SOC and of a bilayer system which combines a 2D Rashba coupled electron gas and an s-wave superconducting film. The predicted long-range interaction can have a strong effect on spin orders in superconductor-magnetic impurity systems that are expected to host Majorana fermions.

We use the *N*-terminal scheme for studying the edge-state transport in two-dimensional topological insulators. We find the universal nonlocal response in the ballistic transport approach. This macroscopic exhibition of the topological order offers different areas for applications.

A weak parallel Zeeman field combined with the spin-orbit coupling can induce the supercurrent in an s-wave two-dimensional superconductor. At the same time, the thermodynamically equilibrium state of such a system is characterized by the helix phase where the order parameter varies in space as exp(iQr). In this state the electric current that is induced by the Zeeman interaction is exactly counterbalanced by the current produced by the gradient of the order-parameter. We studied the interplay of the helix state and magnetoelectric current in the case of a varying in space Zeeman field, as it might be realized in hybrid heterostructures with magnetic and superconducting layers. The theoretical analysis was based on Usadel equations for Green functions in a dirty superconductor. It is shown that even a weak inhomogeneity produces a strong long-range effect on the magnetoelectric current and the order-parameter phase. Consequently, depending on the macroscopic shape of such an inhomogeneity, either the helix state with the zero supercurrent, or a locally uniform state with the finite supercurrent are realized. A mixture of these two extreme situations is also possible. It is also shown that the current can be induced at a large distance from a ferromagnetic island embedded into a superconductor. Quantum effects associated with the magnetoelectric effect are briefly discussed for multiply connected systems. The theory proposes a new point of view on interplay of the magnetoelectric effect and helix phase in spin-orbit coupled superconductors. It also suggests an interesting method allowing to couple superconducting and magnetic circuits.

A nonlocal supercurrent was observed in mesoscopic planar SNS Josephson junctions with additional normal-metal electrodes, where nonequilibrium quasiparticles were injected from a normal-metal electrode into one of the superconducting banks of the Josephson junction in the absence of a net transport current through the junction. We claim that the observed effect is due to a supercurrent counterflow, appearing to compensate for the quasiparticle flow in the SNS weak link. We have measured the responses of SNS junctions for different distances between the quasiparticle injector and the SNS junction at temperatures far below the superconducting transition temperature. The charge-imbalance relaxation length was estimated by using a modified Kadin, Smith, and Skocpol scheme in the case of a planar geometry. The model developed allows us to describe the interplay of charge imbalance and Josephson effects in the nanoscale proximity system in detail.

We study how the non-Fermi-liquid two-phase state reveals itself in transport properties of high-mobility Si-MOSFETs. We have found features in zero-field transport, magnetotransport, and thermodynamic spin magnetization in a 2D correlated electron system that may be directly related with the two-phase state. The features manifest above a density-dependent temperature T* that represents a high-energy scale, apart from the Fermi energy. More specifically, inmagnetoconductivity, we found a sharp onset of the regime delta sigma (B, T)alpha(B/T)(2) above a density-dependent temperature T-kink(n), a high-energy behavior that "mimics" the low-temperature diffusive interaction regime. The zero-field resistivity temperature dependence exhibits an inflection point T-infl(n). In thermodynamic magnetization, the weak-field spin susceptibility per electron partial derivative chi/partial derivative n changes sign at T-dM/dn (n). All three notable temperatures, T-kink, T-infl,T- and T-dM/dn behave critically alpha(n - n(c)), are close to each other, and are intrinsic to high-mobility samples solely; we therefore associate them with an energy scale T* caused by interactions in the 2DE system.

Microwave induced resistance and photovoltage oscillations were investigated in MgxZn1−xO/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35–120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m ∗ = (0.35 ± 0.01)m0, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps.

The high-frequency transport of a two-dimensional (2D) electron system was investigated by measuring the rf power transmitted through a pair of emitter/detector T-shaped antennas capacitively coupled to the 2D channel. The frequency range covered amounted to 10-100 MHz. The distinctive feature of such a setup is that neither were Ohmic contacts formed to the electron system, nor were metallic pads deposited on the sample surface. We demonstrate that in such an arrangement the microwave-induced resistance oscillations could be observed in case the sample was additionally excited by a microwave radiation of 60-100 GHz frequencies. The amplitude of the first oscillation is clearly comparable to the amplitude of the Shubnikov-de Haas oscillations resolved at relatively high magnetic fields. Furthermore, introducing Ohmic contacts to the 2D channel or pads deposited directly on the sample surface did not alter significantly the amplitude of the detected microwave-induced resistance oscillations. © 2020 American Physical Society.