The recent experimental studies of extremely long-lived macroscopic ensembles of spin-cyclotron excitons (magnetoexcitons) which have to obey the Bose-Einstein statistics signal the emergence of an excitonic coherent phase. In the present paper the theory of a weakly interacting Bose gas of spin-cyclotron excitations is developed in terms of a virial correction to the single-magnetoexciton energy. The condition for coherent-incoherent phase transition is discussed. It is expected to be strongly related to the studied long-distance interexcitonic correlations. The results obtained theoretically are discussed in terms of their agreement with specific experimental data.

We demonstrate coherent dynamics of quantized magnetic fluxes in a superconducting loop with a weak link, a nanobridge patterned from the same thin NbN film as the loop. The bridge is a short, rounded shape constriction, close to 10 nm long and 20–30 nm wide, having minimal width at its center. It superposes neighboring fluxoid states of the loop. Quantum state control and coherent oscillations in the driven time evolution of the tunnel-junctionless system are achieved. Decoherence and energy relaxation in the system are studied using a combination of microwave spectroscopy and direct time-domain techniques. The effective flux noise behavior suggests inductance fluctuations as a possible cause of the decoherence.

We demonstrate evidence of coherent magnetic flux tunneling through superconducting nanowires patterned in a thin highly disordered NbN film. The phenomenon is revealed as a superposition of flux states in a fully metallic superconducting loop with the nanowire acting as an effective tunnel barrier for the magnetic flux, and reproducibly observed in different wires. The flux superposition achieved in the fully metallic NbN rings proves the universality of the phenomenon previously reported for InOx .We perform microwave spectroscopy and study the tunneling amplitude as a function of the wire width, compare the experimental results with theories, and estimate the parameters for existing theoretical models.

We report an observation of coherent phonons of E-g(1), E-u(1), A(1g)(1), and E-g(2) symmetry generated in a single-crystal film of Bi2Se3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the E-g(2) mode. The generation of E-g(2) phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear E-u(1) oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of E-g(2). The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of E-g(1) (1.1 THz) and A(1g)(1) (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi2Se3.

We report an observation of coherent phonons of E-g(1), E-u(1), A(1g)(1), and E-g(2) symmetry generated in a single-crystal film of Bi2Se3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the E-g(2) mode. The generation of E-g(2) phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear E-u(1) oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of E-g(2). The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of E-g(1) (1.1 THz) and A(1g)(1) (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi2Se3.

We develop a theory of collective modes in a model of strongly disordered s-wave superconductor with a localization-induced pseudogap ΔP, that is much larger than superconducting gap Δ. Then we applied the obtained results to the calculation of the ultrasound decay rate α(ω) at low-frequencies ω≪kBT/ℏ. We show that at low temperatures T≪Tc the magnitude of the decay rate α(ω) is controlled by the ratio of T/Δ, while single-particle gap ΔP does enter the result for α(ω). Thus, we propose a new method to measure the collective gap Δ in a situation when strong pseudogap is present.

We investigate the behavior of the critical temperature T_c in superconductor/ferromagnet/superconductor (S/F/S) trilayers in the dirty limit as a function of the ferromagnetic layer thickness d_f and the S/F interface transparency. We perform T_c calculations using the general self-consistent multimode approach based on the Usadel equations in Matsubara Green’s functions technique, and compare the results with the singlemode approximation, widely used in literature. Both methods produce similar results for sufficiently low interface transparency. For transparent interfaces, we obtain a qualitatively different T_c(d_f) behavior. Using the multimode approach, we observe multiple 0-π transitions in critical temperature, which cannot be resolved by the single-mode approximation. We also calculate the critical S layer thickness at given d_f when an S/F/S trilayer still has a nonzero critical temperature. Finally, we establish the limits of applicability of the single-mode approximation.

Condensation of pairs formed by spatially separated electrons and holes in a system of two isolated graphene layers is studied beyond the mean-field approximation. Suppression of the screening of the pairing interaction at large distances, caused by the appearance of the gap, is considered self-consistently. A mutual positive feedback between the appearance of the gap and the enlargement of the interaction leads to a sharp transition to a correlated state with a greatly increased gap above some critical value of the coupling strength. At a coupling strength below the critical value, this correlation effect increases the gap approximately by a factor of 2. The maximal coupling strength achievable in experiments is close to the critical value. This indicates the importance of correlation effects in closely spaced graphene bilayers at weak substrate dielectric screening. Another effect beyond the mean-field approximation considered is the influence of vertex corrections on the pairing, which is shown to be very weak.

We analyze symmetries and magnetic properties of Cooper pairs appearing as subdominant pairing correlations in inhomogeneous superconductors, on the basis of the quasiclassical Green-function theory. The frequency symmetry, parity, and the type of magnetic response of such subdominant correlations are opposite to those of the dominant pairing correlations in the bulk state. Our conclusion is valid even when we generalize the theory of superconductivity to recently proposed diamagnetic odd-frequency superconductors. As a consequence, Cooper pairs are classified into eight classes in terms of their symmetries and magnetic properties. Anomalous magnetic properties of subdominant components can be probed by studying the Meissner effect.

The high-frequency conductivity of the two dimensional electron system formed at a ZnO/MgZnO heterojunction was investigated in the 4-18 GHz frequency range at high Landau level fillings and in the regime of both integer quantum Hall effect. The conductivity was probed by measuring the transmission of the broadband coplanar waveguide deposited on the sample surface. At low magnetic fields in case the sample was additionally subjected to the exciting radiation of the 60-140 GHz frequencies, the high-frequency conductivity exhibited well developed microwave-induced oscillations. Remarkably, these oscillations were detected in the whole range of probe frequencies studied - up to 18 GHz - in contrast to GaAs/AlGaAs heterostructures, where microwave-induced oscillations were reported to smear out at probe frequencies around 1 GHz. Furthermore, for each exciting frequency the phase and the period of the microwave-induced oscillations revealed no dependence on the probe frequency and coincided with the period and phase of the microwave-induced resistance oscillations studied independently by dc transport measurements in the same sample. The characteristic probe frequency at which the oscillations start to degrade corresponds to the characteristic scattering time in the region between quantum scattering rates deduced from the cyclotron resonance linewidth and that extracted from the magnetic field dependence of the microwave-induced resistance oscillations.

Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb.We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.

We have studied the amplified emission properties of nanoislands with CdSe quantum dots in ZnSe/CdSe/ZnSe heterostructures surrounded by metallic antennas. It has been found that variations of the optical antenna length give rise to periodic amplification of the integral emission intensity. The period of the discovered oscillations corresponds to the wavelength of the surface plasmon-polariton mode propagating in the metallic antenna. The nature of observed periodicity was confirmed by results of numerical simulations for linear antennas. It has been established that the velocity of surface polaritons depends not only on the parameters of the dielectric constants of the metal and of the semiconductor substrate but also on the width of the metallic antenna. The influence of antenna antisymmetry (its helicity) on selective amplification of the degree of circular polarization of photoexcitation has been investigated. We found that plasmon-polariton standing waves induced in S-type (curved) antennas by circularly polarized light, which was used for quantum dot photoexcitation, result in enhanced polarization selectivity of the quantum dot emission. The selectivity of the polarization of photoexcitation is a periodic function of the helical antenna length.

The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenide monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton. It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane. Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in hexagonal boron nitride modifies the exciton longitudinal-transverse splitting. We calculate the exciton spin-valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to fivefold, enhancement of the exciton valley polarization lifetime.

We study Josephson junctions with weak links consisting of two parallel disordered arms with magnetic properties: ferromagnetic, half-metallic, or normal with magnetic impurities. In the case of long links, the Josephson effect is dominated by mesoscopic fluctuations. In this regime, the system realizes a phi_0 junction with sample-specific phi_0 and critical current. Cooper pair splitting between the two arms plays a major role and leads to 2*Phi_0 periodicity of the current as a function of flux between the arms. We calculate the current and its flux and polarization dependence for the three types of magnetic links.

We theoretically study the conductivity in arrays of metallic grains due to the variable-range multiple cotunneling of electrons with short-range (screened) Coulomb interaction. The system is supposed to be coupled to random stray charges in the dielectric matrix that are only loosely bounded to their spatial positions by elastic forces. The flexibility of the stray charges gives rise to a polaronic effect, which leads to the onset of Arrhenius-like conductivity behaviour at low temperatures, replacing conventional Mott variable-range hopping. The effective activation energy logarithmically depends on temperature due to fluctuations of the polaron barrier heights. We present the unified theory that covers both weak and strong polaron effect regimes of hopping in granular metals and describes the crossover from elastic to inelastic cotunneling.

We consider Coulomb blockade effects for tunneling through a piece of wire with large resistance R≫1. This system can not be treated as a zero-dimensional one, as the dynamics of internal inhomogeneous degrees of freedom is crucial. At moderately high temperatures the linear conductance G of the system is suppressed due to the one-dimensional Coulomb zero bias anomaly effect. At low T, besides the standard activational factor, there is an additional T-independent (though also exponentially strong) suppression of G. It arises due to the tunneling evolution of the charge in the wire to the equivipotential distribution. In the intermediate range of T the G(T) dependence is a power law, as in the phenomenological environmental theory. The effective “environmental resistance” entering the power exponent is found explicitly. It depends on the length of the wire and on the positions of the contacts.

We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use a two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of transformation from the exciton-photon basis to the lower-upper polariton basis, and obtain the exciton and photon occupation numbers of the lower and upper polariton branches for nonzero temperatures. For an inhomogeneous system, the set of coupled equations of the Bogoliubov–de Gennes type is derived. The equations govern the spectra and spatial distributions of noncondensate photons and excitons.

We study numerically the critical behavior at the localization transition in the Anderson model on infinite Bethe lattice and on random regular graphs. The focus is on the case of coordination number m+1=3, with a box distribution of disorder and in the middle of the band (energy E=0), which is the model most frequently considered in the literature. As a first step, we carry out an accurate determination of the critical disorder, with the result Wc=18.17±0.01. After this, we determine the dependence of the correlation volume Nξ=mξ (where ξ is the associated correlation length) on disorder W on the delocalized side of the transition W<Wc, by means of population dynamics. The asymptotic critical behavior is found to be ξ∝(Wc−W)−1/2, in agreement with analytical prediction. We find very pronounced corrections to scaling, in similarity with models in high spatial dimensionality and with many-body localization transitions.

We analyze the paradigmatic competition between intraband and crossband Cooper-pair formation in twoband superconductors, neglected in most works to date.We derive the phase-sensitive gap equations and describe the crossover between the intraband-dominated and the crossband-dominated regimes, delimited by a “gapless” state. Experimental signatures of crosspairing comprise notable gap splitting in the excitation spectrum, non-BCS behavior of gaps versus temperature, as well as changes in the pairing symmetry as a function of temperature. The consequences of these findings are illustrated on the examples of MgB2 and Ba0.6K0.4Fe2As2.

We theoretically address the problem of cubic B20 helimagnets with a small concentration *c* <<1 of defect
bonds in external magnetic field **H**, which is relevant to mixed B20 compounds at small dopant concentrations.
We assume that the Dzyaloshinskii-Moriya interaction and the exchange coupling constant are changed on
imperfect bonds which leads to distortion of the conical spiral ordering. In a one-impurity problem we find
that the distortion of the spiral pitch is long-ranged and it is governed by the Poisson equation for an electric
dipole. The variation of the cone angle is described by the screened Poisson equation for two electric charges
with the screening length being of the order of the spiral period. We calculate corrections to the spiral vector and
to the cone angle at finite *c*. The correction to the spiral vector is shown to be independent of *H*. We demonstrate
that diffuse neutron scattering caused by disorder appears in the elastic cross section as power-law decaying tails
centered at magnetic Bragg peaks.