Osipyan Institute of Solid State Physics RAS

We report measurements of the temperature dependence of the critical current, I(c), in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature I(c) generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I(c) rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a pi phase state at low temperatures.

We describe variations in the resistance of $\mathrm{Co}/\mathrm{Cu}$ multilayers, induced by means of a high current density $\ensuremath{\approx}{10}^{8}\mathrm{A}/{\mathrm{cm}}^{2}$ injected into the multilayer through a point contact. We propose that the observed resistance changes are due to excitations of zero-wave-number spin waves in the magnetic layers. As predicted, such current-driven excitation of a magnetic multilayer occurs for only one direction of current flow and has a current threshold which increases linearly with the applied magnetic field.

A theory of electron counting statistics in quantum transport is presented. It involves an idealized scheme of current measurement using a spin 1/2 coupled to the current so that it precesses at the rate proportional to the current. Within such an approach, counting charge without breaking the circuit is possible. As an application, we derive the counting statistics in a single channel conductor at finite temperature and bias. For a perfectly transmitting channel the counting distribution is Gaussian, both for zero-point fluctuations and at finite temperature. At constant bias and low temperature the distribution is binomial, i.e., it arises from Bernoulli statistics. Another application considered is the noise due to short current pulses that involve few electrons. We find the time-dependence of the driving potential that produces coherent noise-minimizing current pulses, and display analogies of such current states with quantum-mechanical coherent states.

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained. © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.

We investigate analytically and numerically the melting of the vortex lattice moving in an inhomogeneous environment under the applied current $j$. We predict the existence of a dynamic phase transition at some characteristic current $j={j}_{t}$ (crystallization current) from the motion of the amorphous vortex configuration at $j<{j}_{t}$ to the motion of the vortex crystal at $j>{j}_{t}$. The crystallization current ${j}_{t}$ exceeds essentially the critical current ${j}_{c}$ for strongly disordered systems and diverges as temperature approaches the melting temperature ${T}_{m}$ of the undisturbed lattice.

Abstract The discovery of superconducting H 3 S with a critical temperature T c ∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted T c s among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH 3 , YH 4 , YH 6 and YH 9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH 6 and P6 3 /mmc YH 9 phases with maximal T c s of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH 10 with the highest predicted T c > 300 K was not observed in our experiments, and instead, YH 9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

In this work, we report on a single ZnO nanowire-based nanoscale sensor fabricated using focused ion beam (FIB/SEM) instrument. We studied the diameter dependence of the gas response and selectivity of ZnO nanowires (NWs) synthesized by chemical vapor phase growth method. The photoluminescence (PL) measurements were used to determine the deep levels related to defects which are presented in the ZnO nanomaterial as well as to evaluate the effect of thermal treatment in H2 atmosphere on the emission from ZnO nanowires. We show that sample annealed in hydrogen leads to passivation of recombination centers thus modifying the NWs properties. We studied the gas response and selectivity of these ZnO nanowires to H2, NH3, i-Butane, CH4 gases at room temperature. Our results indicated that zinc oxide NWs hold a high promise for nanoscale sensor applications due to its capability to operate at room-temperature and its ability to tune the gas response and selectivity by the defect concentration and the diameter of ZnO nanowire. A method is proposed to reduce the nanosensor's recovery time through the irradiation with an ultraviolet radiation pulse. The sensing mechanisms of ZnO nanowires will be discussed.

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

We report observation of an inverse energy cascade in second sound acoustic turbulence in He II. Its onset occurs above a critical driving energy and it is accompanied by giant waves that constitute an acoustic analogue of the rogue waves that occasionally appear on the surface of the ocean. The theory of the phenomenon is developed and shown to be in good agreement with the experiments.

The influence of quantum dot (QD) asymmetry on the emission of single three-dimensionally confined biexcitons in II-VI semiconductor nanostructures has been studied by magnetophotoluminescence spectroscopy. Investigating both the biexciton and the single-exciton transition in the same single QD, we obtain a unified picture of the impact of electron-hole exchange interaction on the fine structure and the polarization properties of optical transitions in QDs. The exchange splitting is demonstrated to have a strong influence on the derivation of the biexciton binding energy, which we determine to be about 17 meV, much less than the separation between exciton and biexciton lines ( $\ensuremath{\approx}24$ meV) in the spectra.

In order to elucidate room-temperature (RT) ferromagnetism (FM) in ZnO, we have analyzed a multitude of experimental publications with respect to the ratio of grain-boundary (GB) area to grain volume. FM only appears if this ratio exceeds a certain threshold value ${s}_{\text{th}}$. Based on these important results nanograined pure and Mn-doped ZnO films have been prepared, which reveal reproducible RT FM and magnetization proportional to the film thickness, even for pure ZnO films. Our findings strongly suggest that grain boundaries and related vacancies are the intrinsic origin for RT ferromagnetism.

Thermal fluctuations of vortex lines are shown to be capable of strongly reducing the value of the critical current in the mixed state of high-${\mathit{T}}_{\mathit{c}}$ superconductors. The theory of pinning in the presence of thermal fluctuations is developed. The current relaxation law in the regime of single-vortex pinning is obtained.

Abstract We present an analytical treatment of the problem of pattern selection in a fully non-local symmetrical model of dendritic crystal growth. Simplifications of mathematical equations are based on the assumption that anisotropies of surface energy and kinetic effects are small. Selection rules for growth velocity and instability increments are derived at arbitrary Peclet numbers. For a dendrite growing in a channel, a double-valued velocity versus undercooling dependence is obtained. The upper branch of the solution is stable and changes into a free dendrite with increased channel width. Interplay between surface energy and kinetic effects results in morphological transition from surface energy dendrite to dense branching morphology and then to kinetic dendrite. In the framework of the boundary-layer model it is shown that at deep undercooling parabolic dendrite turns into angular dendrite and then into planar front.

The Kramers theory for the escape rate of a Brownian particle from a potential well is extended to the full damping range. It is shown that the most adequate description of the underdamped Brownian motion in a deep potential well is provided by a Green function of the Fokker–Planck equation in the energy-position variables. The problem of lifetime of a particle in a single potential well is reduced to an integral equation in energy variable, with the Green function being the kernel of this equation. The straightforward solution by the Wiener–Hopf method yields an explicit expression for the lifetime, which describes the crossover from the extremely underdamped regime to that of a moderate damping. With the use of Kramer’s result for moderate-to-large damping an expression for the lifetime is presented, which holds at arbitrary damping. The problem of the rate of transitions between the two minima of a double-well potential is reduced to a system of two integral equations, which is also solved by the Wiener–Hopf method. An explicit expression for the relaxation time of nonequilibrium populations of the two minima is given.

We present intrinsic tunneling spectroscopy measurements on small Bi2Sr2CaCu2O8+x mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the c-axis pseudogap. The superconducting gap vanishes at Tc, while the pseudogap exists both above and below Tc. Our observation implies that the superconducting and pseudogaps represent different coexisting phenomena.

We have investigated current-current correlations in a cross-shaped conductor made of graphene. The mean free path of charge carriers is on the order of the ribbon width which leads to a hybrid conductor where there is diffusive transport in the device arms while the central connection region displays near ballistic transport. Our data on auto and cross correlations deviate from the predictions of Landauer-Büttiker theory, and agreement can be obtained only by taking into account contributions from non-thermal electron distributions at the inlets to the semiballistic center, in which the partition noise becomes strongly modified. The experimental results display distinct Hanbury - Brown and Twiss (HBT) exchange correlations, the strength of which is boosted by the non-equilibrium occupation-number fluctuations internal to this hybrid conductor. Our work demonstrates that variation in electron coherence along atomically-thin, two-dimensional conductors has significant implications on their noise and cross correlation properties.

The current density and local magnetic field are calculated analytically for a strip of a type-II superconductor in perpendicular magnetic field Ha for constant critical current density. The penetrating flux front has vertical slope and the initial penetration depth, magnetization change and hysteretic losses are ∼ Ha2, ∼ Ha3 and ∼ Ha4, respectively. The analytical results differ from the widely used Bean model and explain numerous experiments in a natural way without the assumption of a surface barrier.

We report the first experimental observation of the two-node thickness dependence of the critical current in Josephson junctions with a ferromagnetic interlayer. Nodes of the critical current correspond to the transitions into the pi state and back into the conventional 0 state. From the experimental data the superconducting order parameter oscillation period and the pair decay length in the ferromagnet are extracted reliably. We develop a theoretical approach based on the Usadel equations taking into account the spin-flip scattering. Results of numerical calculations are in good agreement with experiments.

The question of how the lack of spatial reflection symmetry can affect properties of a superconductor is investigated. A novel magnetoelectric effect is predicted: The supercurrent in a metal of polar symmetry must be accompanied by the spin polarization of the carriers. The relevance to some known pyro- and antipyroelectric superconductors including a high-temperature system as well as the possibility of an experimental verification are briefly discussed.