The discovery of extreme magnetoresistance (MR) in nonmagnetic materials attracted attention to WTe2 semimetal. We studied MR in a single crystal of tungsten ditelluride in the range of magnetic fields up to 14 T. MR increased with increasing field following a close-to-quadratic law without saturation. The Shubnikov–de Haas oscillations were observed. Four fundamental frequencies were found in the oscillation spectrum, which correspond to two electron and two hole pockets, caused by strong spin–orbit coupling.

The tungsten ditelluride WTe2 was suggested to belong to the Weyl semimetal family. We studied 125Te spin-lattice relaxation and NMR spectra in a WTe2 single crystal within a large range from 28 K up to room temperature. Measurements were carried out on a Bruker Avance 500 NMR pulse spectrometer for two orientations of the crystalline c axis, parallel and perpendicular to magnetic field. Relaxation proved to be single-exponential. The relaxation time varied depending on the sample position in magnetic field and frequency offset. The relaxation rate increased about linearly with temperature below 70 K however the dependence became nearly quadratic at higher temperatures. The relaxation rate within the total temperature range was fitted using a theoretical model developed in [41] for Weyl semimetals and assuming the decrease of the chemical potential with increasing temperature. The results obtained for 125Te spin-lattice relaxation evidence in favor of the topological nontriviality of the WTe2 semimetal. The 125Te NMR spectra agreed with the occurrence of nonequivalent tellurium sites and varied insignificantly with temperature.

The discovery of extreme magnetoresistance (XMR) in non-magnetic materials attracted attention to the WTe2 semimetal. We have carried out studies of magnetoresistance in a tungsten ditelluride single crystal in the magnetic field range up to 14 T. Magnetoresistance increased with increasing field following a near quadratic law without saturation. The Shubnikov-de Haas oscillations were observed. Four fundamental frequencies were found in the oscillations spectrum, which correspond to two electron and two hole pockets caused by strong spin-orbit coupling.

The phenomenological Landau theory was applied for treating the phase transitions in a thin multiferroic film. The multiferroic phase in the relevant bulk was assumed to emerge following two successive phase transitions of the second order. The coupling between the order parameters was implied to be biquadratic as for a multiferroic-magnetoelectric. The variation of the free energy density functional yielded a system of two nonlinear differential Euler-Lagrange equations with four boundary conditions. The boundary value problem was analyzed numerically. It was shown that the phase diagram in the film can differ radically from that in bulk for particular sets of the phenomenological parameters.

The paper presents the results of studying the crystallization and melting processes of Ga–In eutectic alloys, which are embedded in opal matrices, using acoustic and NMR methods. The indium concentrations in the alloys were 4, 6, 9, and 15 at %. Measurements were performed upon cooling from room temperature to complete crystallization of the alloys and subsequent heating. It is revealed how the size effects and alloy composition influence the formation of phases with α- and β-Ga structures and on changes in the melting-temperature ranges. A difference was observed between the results obtained using acoustic and NMR methods, which was attributed to different temperature measurement conditions.

The results of acoustic investigations of the melting and crystallization of indium-gallium alloys of different compositions embedded in porous glass matrices with a pore size of 18 nm are presented. It was shown that the formation of the α and β crystalline modifications of gallium is possible depending on the alloy composition in nanopores. The fraction of β-Ga increased as the indium concentration in the alloy increased. Stabilization of β-Ga in conditions of confined geometry was revealed. Broadening of the melting region of the α phase of gallium and narrowing of the melting region of β-Ga, as compared with the corresponding bulk alloys, were observed.

The transverse Ising model was developed for an array of coupled ferroelectric small particles. Two contributions to exchange integral were included: dipole-dipole interaction and short-range coupling. General analytical equations were found for the ferroelectric phase transition temperature and order parameter. The equations obtained were analyzed numerically for cubic and tetragonal lattices of particles and cells. The shift of the ferroelectric phase transition temperature was considered as a function of the interparticle distance, tunneling constant, and geometry of particle arrangement.

Second order ferroelectric phase transition for a thin film with arbitrary boundary conditions is considered within the framework of the Landau theory. General phase-plane portraits for the relevant Euler-Lagrange equation are constructed and different types of trajectories are analyzed. Full set of order parameter profiles is found. The method of reconstructing the admissible order parameter profiles for particular film thickness and extrapolation lengths is discussed.

Анализируются температурные зависимости модуля упругости в мультиферроиках-магнитоэлектриках, в которых магнитное и сегнетоэлектрическое упорядочения возникают в результате двух последовательных фазовых переходов. Получены аналитические выражения для модуля упругости в области фазовых переходов в упорядоченные состояния при учете либо стрикционных, либо биквадратичных вкладов в магнитоупругое и электроупругое взаимодействия. Найдена явная зависимость модуля упругости в мультиферроидной фазе от константы магнитоэлектрической связи. Показано, что характерные аномалии упругих свойств в мультиферроиках допускают интерпретацию в рамках теории Ландау без учета флуктуаций. При рассмотрении учитывались изменения фазовых диаграмм за счет магнито- и электроупругих взаимодействий.

The Landau theory was applied to treat the phase diagrams for a multiferroic with two second order phase transitions taking into account the coupling of the primary order parameters with strain. Two order parameters are coupled biquadratically which corresponds to the magnetoelectric materials. The coupling with strain is assumed to be linear in strain and quadratic in order parameters. Three ordered phases are discussed. Analytic relationships were obtained for the phase transition temperatures and for elastic modulus changes through the phase transitions. Strong influence of the coupling with strain on the phase diagrams was shown.

The temperature dependences of the elastic modulus in multiferroics-magnetoelectrics are analyzed, in which magnetic and ferroelectric orderings appear as the result of two successive phase transitions. The analytical relationships for the elastic modulus near the phase transitions to ordered states are obtained for the cases of either linear-quadratic or biquadratic contributions to magneto- and electroelastic coupling. The explicit dependence of the elastic modulus in the multiferroic phase on the magnetoelectric coupling constant was found. It is shown that the characteristic elastic properties in multiferroics can be treated using the Landau theory without taking into account fluctuations. The analysis includes changes in the phase diagrams due to the magneto- and electroelastic coupling.

In this paper we present the results of studies on melting and crystallization of decane embedded in the pores of opal matrices filled with chemically pure hexadecane. In this studies measured the ultrasonic velocity by the pulse-interference method at frequencies of about 7 MHz in the temperature range 170–290 K.

The analytical solution is considered for the phenomenological theory of the second order ferroelectric or ferroelastic phase transition in a thin film with arbitrary boundary conditions. The general phase-plane portrait for the relevant Euler-Lagrange equation was analyzed. The order parameter distribution in the film was found for some particular sets of extrapolation lengths. The case of extrapolation lengths with opposite signs was also considered numerically. It was shown that the size effect on the order parameter and transition temperature is remarkably weakened when the extrapolation lengths have similar absolute values and opposite signs.

Studies of ac and dc magnetization and heat capacity in a superconducting lead-porous glass nanocomposite were carried out. Double anomalies were found on their temperature dependences at different magnetic field. The positions of anomalies of heat capacity and ac and dc magnetization correlated with each other. The additional, low-temperature anomalies shifted remarkably with increasing magnetic field. The FC and FCW curves observed upon cooling and warming, respectively, showed thermal hysteresis at the second step. The peak effect on magnetization loops was seen above 6 K. The low-temperature anomalies in the ac and dc magnetization were treated as a manifestation of transformation in the vortex system which is triggered by superconductivity in confined lead islands.

The phenomenological theory was developed for the ferroic phase transition in a thin film with asymmetric boundary conditions which simulate the real situation of a film on the substrate or a layer between two different materials. The solutions of the boundary value problem for the order parameter were obtained numerically for some particular sets of phenomenological parameters in the Landau expansion and extrapolation lengths. The main attention was focused on the case of opposite signs of the extrapolation lengths at film surfaces. It was shown that the mean order parameter is close to that in bulk and phase transition temperature remains weakly dependent on film thickness when the extrapolation lengths have similar absolute values and opposite signs.

The magnetization of the porous glass nanocomposite with CuO nanoparticles embedded in the pores has been studied in the temperature range from 1.8 to 350 K for different pore fillings. It has been shown that the magnetic properties of these nanocomposites depend significantly on pore filling. It has been found that, in the low-temperature range for the nanocomposite with a pore filling of 55% and for pressed CuO, the ZFC and FC susceptibilities diverge, a feature which has been almost absent in the nanocomposite with 21% filling. It has been demonstrated that the kink in the temperature dependence of magnetization, which corresponds to the paramagnetic-multiferroic phase transition, does not shift in the sample with a larger pore filling as compared to that observed in the bulk sample.

Dc magnetization and ac electric permittivity were measured for the CuO-porous glass nanocomposite made and for pressed powder CuO. Magnetization curves showed a bend between two linear segments for both the nanocomposite and bulk cupric oxide at 230K evidencing that the temperature of the transition from the paramagnetic into multiferroic phase did not change noticeably under nanoconfinement. Results suggested also a reduction of the temperature of the second transition into the collinear antiferromagnetic phase. ZFC and FC magnetizations were found to bifurcate for the nanocomposite and bulk CuO. The bifurcation was accompanied with peaks on ZFC magnetization.