Cationic distribution in nanoparticles of ferrites of ZnxFe3-xO4 composition

Nanodispersed powders of zinc-substituted magnetite ZnxFe3-xO4 with the content of zinc ions x = 0.0 ÷ 0.5 were synthesized by the method of chemical condensation. X-ray spectra showed single-phase powders and their belonging to the cubic structure of spinel-type ferrite. According to the results of X-ray and electron microscopic studies, the particle sizes for all synthesized systems were determined. The average particle size of ferrites according to data obtained from X-ray spectra using the Selyakov-Shearer formula was ~ 7 nm, with a maximum particle size of about 10 nm. According to microscopy, the particle size range was 3 ÷ 13 nm with an average value of 6.5 nm. The study of cation distribution was carried out in two ways. The Poix method was chosen as the first method, which is based on the relationship between the lattice parameter a and the characteristic cation-anion distances. The second method was chosen to determine the cation distribution by measuring the magnetization. The formula for coupling the specific magnetization at 0 K with the number of Bohr magnetons per formula unit was used. An amendment was introduced into the formula due to the small particle size and, accordingly, the large share contribution of the near-surface region with a "canted" magnetic structure. The parameters of the crystal lattice and the magnetization were measured. The obtained data formed the basis of cation distribution calculations, according to which ferrites with a concentration of x ≤ 0.2 have an inverted spinel structure, ie zinc ions are localized only in octahedral positions, and at a concentration of 0.3 ≤ x ≤ 0.5 – a mixed spinel structure with a minimum degree of inversion of 80% at a concentration of zinc ions x = 0.5.

Pressure effect on the electrical resistance of Y0.77Pr0.23Ba2Cu3O7-δ single crystals

The effect of hydrostatic pressure up to 12 kbar on the electrical resistance in the basal ab-plane of optimally oxygen-doped (δ<0.1) single crystals Y1–xPrxBa2Cu3O7–δ moderately doped with praseodymium (x≈0.23) with a critical temperature Tc≈67 K. Compared to undoped single-crystal YBa2Cu3O7–δ, doping with praseodymium led to a decrease in the critical temperature by ≈24 K with a simultaneous increase in ρab (300 K) by ≈130 μΩcm. In the region of the transition to the superconducting state, several clearly pronounced peaks are observed on the dρ/dT – T curves, which indicates the presence of several phases with different critical temperatures in the sample. The application of high hydrostatic pressure leads to an increase in Tc by about 3 K. This increase slows down with increasing pressure, and the baric derivatives, dTc/dP, decrease from 0.44 K/kbar at atmospheric pressure to 0.14 K/kbar at 11 kbar. The comparatively weak change in the critical temperature under the action of hydrostatic pressure is due to the relatively small value of the orthorhombic distortion, (a–b)/a. The change in the baric derivative dTc/dP upon all-round compression of the sample is due to the fact that, along with an increase in the Debye temperature, the matrix element of the electron-phonon interaction also increases. Possible mechanisms of the effect of high pressure on Tc are discussed taking into account the presence of features in the electronic spectrum of carriers.

The classical boundary problem of the transition of a spherical type-I superconductor to the normal state with increasing of the applied uniform magnetic field

A complicated boundary value problem of the transition of a macroscopic massive spherical type-I superconductor to the intermediate and normal state with increasing of the applied uniform magnetic field has been solved. Taking into account a penetration effect and exact boundary conditions the boundary problem has been solved completely and rigorously within the framework of the classical (non-quantum) electrodynamics of continuous mediums and the modified (simplified) nonlocal Pippard electrodynamics of spatially homogenous type-I superconductors. The principal object of this work is a self-consistent and exact setting of the boundary value problem and also its mathematically rigorous solution taking into account surface effects and nonlocality of Pippard type-I superconductors. The solution novelty is a description of the surface effects within the framework the modified (simplified) nonlocal Pippard electrodynamics. It is shown that disregarding for the surface effects in a theory of low-temperature superconductors can lead not only to computational mistakes, but also to incorrect qualitative conclusions. The conclusions about nature of a macroscopic spherical type-I superconductor to the intermediate and normal state have been drawn on the ground of a rigorous solution of the boundary problem and determination of the total magnetic field distribution in the whole space (inside and outside the superconducting sphere). These conclusions are in agreement with those, which have been drawn earlier by other authors on the ground of different approximate models and methods. Since the scientific results have been obtained by the authors on the basis of rigorous and self-consistent solution of the exactly set boundary problem, the work is undoubtedly of theoretical and methodical interest.

Computer simulations of hydrostatic pressure influence on screw dislocation slip in Mg

Atomistic modeling of hydrostatic pressure influence on critical resolved shear stress was performed for glide of screw <a> dislocation in magnesium. It was found that application of pressure can change the resolved critical stress for basal and prismatic slip. The effect is dependent on dislocation core structure. It can be connected to the pressure dependence transient dilatation of the dislocation core.

Structural and electroresistive properties of layered compounds based on the 1-2-3 HTSC system and transition metal dichalcogenides under extreme external influences (review)

The problem of the influence of extreme external influences (high pressure, sharp temperature drops, structural relaxation, and strong magnetic fields) on various mechanisms of electric transport of HTSC compounds Re1Ba2Cu3O7-δ (Re = Y or another rare-earth ion) and dichalcogenides of transition metals are considered. The features of the crystal structure and the effect of structural defects of various morphologies on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states are discussed. A review of the experimental data obtained in the study of the effect of high hydrostatic pressure and other extreme effects on various mechanisms of electric transport of Re1Ba2Cu3O7-δ compounds of various compositions and transition metal dichalcogenides of various technological backgrounds is carried out. Various theoretical models devoted to the effect of high pressure on the electrical conductivity of HTSC compounds of the 1-2-3 system and transition metal dichalcogenides are discussed, and a comprehensive comparative analysis of their magnetoresistive characteristics under extreme external influences is performed. In particular, it was shown, that the relatively weak effect of pressure on the Tc value of optimally doped samples can be explained within the framework of a model assuming the presence of a Van Hove singularity in the spectrum of charge carriers which is characteristic of strongly coupled lattices. This is confirmed by the observation similar features of the behavior of the baric derivatives dTc/dP depending on the change composition in NbSe2 single crystals, which also belong to systems of two-dimensional lattices and have a similar anisotropy parameter. Nevertheless, it is still possible to formulate a number of questions that have not yet found a final experimental and theoretical solution. Namely, what is the role of the crystal lattice and structural defects and, in particular, twinning planes? What is the reason for the broadening of the resistive transitions of HTSC compounds into the superconducting state under pressure, and what is the relationship between this broadening and charge transfer and the nature of the redistribution of the vacancy subsystem? What is the role of phase separation in the implementation of different modes of longitudinal and transverse transport? Obviously, more research, both experimental and theoretical, is needed to answer these questions.

Search for new superconducting compounds with a critical transition temperature Tc close to room temperature under pressure

A new chemical composition of superconducting compounds formed on the basis of elements of the fifth group (semimetals) is proposed within the framework of the quantum Bardin-Cooper-Shriffer quantum theory of superconductivity (BCS-theory) using physical chemistry methods for analyzing equilibrium crystal structures. These compounds satisfy all the conditions for transition to the superconducting state at temperatures close to room temperature and a pressure of ≈107 Pa. As initial chemical elements from which superconducting compounds can be synthesized under pressure, in addition to hydrides, substances that allow the "collectivization" of electrons can be used. The most suitable substances in this sense are the elements of the fifth group of the periodic system or the so-called semimetals, which include Bi, Sb, As, graphite, etc. These elements, by their electrical properties, occupy an intermediate position between metals and semiconductors. They are characterized by a slight overlap of the valence and conduction zones, which leads, on one hand, to the fact that they remain good conductors of electricity up to absolute zero temperature, and on the other hand, they have a significantly lower carrier density compared to metals charge. Moreover, in these substances in a wide temperature range at atmospheric pressure, the stability of the solid phase is maintained and, very importantly, a partial “collectivization” of valence electrons inherent in metals is already realized in the initial state. It is shown that, under the action of pressure p``≈107Pa, semimetals can turn into metals characterized by a specific energy spectrum of electrons. A change in the semimetals structure and in parameters of the electronic subsystem energy spectrum is accompanied by an increase in the electron pairing constant and by the density of electronic states at the Fermi level. In turn, an increase in these parameters makes it possible to transfer semimetals to the superconducting state at temperature ≈300К.

Hydrodynamics and proper vibrations of quantum liquids with conformational degrees of freedom

The order parameters are constructed for a Fermi liquid with conformational degrees of freedom. Based on them, additional thermodynamic parameters were introduced: the spin unit vector dα (determining the anisotropy in the spin subspace), the unit spatial vectors mi and ni (determining the anisotropy in space), and also three scalar parameters determining the shape of the Cooper pair u, v, q ( first two items are half-axes of ellipsoid of Cooper pair and last item is mutual orientation in space of these half axes). The symmetry properties of the order parameter operator are considered. The equations of ideal hydrodynamics of a Fermi liquid are derived taking into account the influence of conformational degrees of freedom. By conformational degrees of freedom should be understood the parameters associated with the shape and size of the Cooper pair. Expressions are obtained for the flows of thermodynamic quantities of such a Fermi liquid in terms of the density of the energy functional. The energy functional depends both on the additive integrals of motion (classical fluid parameters) and on conformational parameters. The dispersion equation of such a liquid is obtained for a model representation of the energy functional (the work was performed as part of the Fermi-liquid approach). The dispersion equation includes spin modes, first, second, and third sounds. The dispersion equation for the spatial subsystem, including the first, second, and third sounds characteristic of superfluid systems, is analyzed. Particular solutions of the dispersion equation are simulated using the Maple software package (several 3D figures are given for the angular dependence of the speeds of 1 and 2 sounds in a spherical coordinate system). All of the above allows us to conclude that such a Fermi liquid can be considered as a superfluid liquid crystal of a nematic type. The presence of conformational parameters distinguishes the considered phase from the F phase of a superfluid Fermi liquid.

Mechanisms of micro-voids formation caused by optical breakdown in KCl single crystals

The phenomenon of optical breakdown has been studied experimentally for KCl single crystals exposed to laser emission focused on the neodymium glass with modulated quality-factor, pulse duration 5·10-8 s, wavelength λ= 1054 nm, and pulse energy of the order 1 J in the regime of local intrinsic absorption of the laser emission by the single crystal. Evaluations of local heat flash energetic constituents and characteristic durations for both local area heating and relaxation processes and following comparison with experimental results have shown that the relaxation process takes place in two stages: the first is fast phasefollowed by crowdion mass transfer with shock wave participation, and the second is slow phase with participation also dislocation mass transfer. The energy losses for heat radiation and thermal conductivity are found to be by orders of value less than the absorption energy flux Iabs that provides fast local heating and plasma formation. From the viewpoint of the mechanics of continua the process under study where the pressure achieves value exceeding the theoretical strength limit for the time less 10-6 s, should be considered as explosion-like or shock process. The general scheme of plastic deformation arising from abovementioned estimations and observations is seemed as follows. In the beginning, under action of the shock wave the crowdions are generated which carry the substance from the high pressure area and move along close-packed atomic rows (<110> type directions in KCl crystals); the void is formed almost completely during the shock wave passing the relaxation zone crosssection. This time is of the order of τrel, i. e. 10-9…10-8 s. After falling temperature and pressure and vapor condensation into liquid, at the end of relaxation process, the void boundaries expand already under liquid melt pressure, and the mass transfer dislocation mechanism comes into action providing additionally some enlarging the void volume. This process continues also after stopping the laser emission, during the crystal cooling down to the melt crystallization in the void and formation of a pore with size observed.