The vanadium-rich Heusler alloys V3X (X = Al, Si, Ge) have been synthesized and investigated. The crystal structure, optical and electronic structures were studied and compared. Using X-ray diffraction analysis, it was found that V3Al alloy has the D03 structure, and the V3Si and V3Ge compounds have the A15 one, also called the beta-tungsten structure. It was shown that depending on both the p-element and crystal structure alloys exhibit different properties. The V3Al alloy crystallized in the D03 structure is characterized by the electronic structure of a gapless semiconductor with vanadium magnetic moments equal to 1.36(-1.36) mu B completely compensated zero total magnetic moment, whereas, V3Si and V3Ge crystallized in the A15 structure exhibit metallic properties with zero magnetic moments. The optical characteristics of V3X (X = Al, Si, Ge) were studied for the first time using the ellipsometry method. The optical conductivity of the alloys was found to be mostly produced by transitions involving V 3d electrons, was found in good agreement with the theoretical results reproducing specific features of each alloy.

We employed X-ray diffraction analysis, electron transport measurements, and optical ellipsometry to investigate the topological insulator Bi2Se3 before and after argon ion irradiation with an energy of 15 keV and a fluence of 5.1015 cm-2. The electrical resistivity in the temperature range of 2-300 K and optical properties in the spectral range of 1250-40000 cm-1 at room temperature were measured. After irradiation, the rocking curve exhibited five local maxima, suggesting the crystal had fragmented into five mosaic blocks. The temperature-dependent electrical resistivity was modeled using a phenomenological approach accounting for electron-phonon and electron-electron scattering mechanisms. Irradiation led to enhancements in both scattering processes and an increase in electrical resistivity. Notable alterations were observed in the optical properties after irradiation: a more than twofold decrease in maximum optical conductivity, emergence of additional features in the infrared region of the imaginary part of dielectric permittivity, and reduction in reflectivity magnitude across the entire spectral range. We assume that such changes are associated with a change in the electronic structure of the irradiated layer.

The electrical conductivity sigma 0 (T) of single -crystal and polycrystalline samples of the intrinsic magnetic topological insulator MnBi 2 Te 4 was measured in the temperature range from 5 to 300 K. The optical characteristics sigma( omega), epsilon 1 ( omega) , epsilon 2 ( omega) and R( lambda) of MnBi 2 Te 4 were studied in the spectral range from 1250 to 36000 cm -1 at room temperature. The anisotropy of the electrical conductivity of MnBi 2 Te 4 was found, which arises due to the additional contribution from scattering on layer boundaries. The optical spectrum of MnBi 2 Te 4 is formed predominantly due to interband absorption of a light wave. Despite the qualitatively similar behavior of the optical characteristics, there is some difference between poly- and single crystals in infrared region.

The commercial Ti–6Al–4V alloy was obtained in an almost single-phase state, formed by finely dispersed globular α-grains with an average size of 12 μm, using thermomechanical processing, including hot rolling. The microtexture and structure of the alloy were studied using X-ray diffractometry and transmission and scanning electron microscopy, including orientation microscopy. It is found that for α-grains the Burgers orientation relationships are satisfied, and twin orientations are ensured in the rolling plane. A significant scattering of the crystallographic orientations of α-grains relative to each other (up to 10°–15°) is established for each group of close Burgers orientations as a result of plastic deformation by rolling at high temperatures. Clusters of microtexture regions in the layered microstructure of grains and the formation mechanisms and mutual crystallographic misorientations of microtexture regions and grains in the alloy have been identified.

The Hall effect in single crystals of topological semimetals WTe2 and MoTe2 is studied in the temperature range from 2 to 100 K and in magnetic fields up to 9 T. It is established that the Hall resistivity of WTe2 shows a nonlinear dependence on the magnetic field at temperatures below 100 K. At the same time, the Hall resistivity of MoTe2 depends linearly on the magnetic field in the temperature range from 2 to 25 K and a nonlinear contribution appears at 50 K. Along with the known mechanism of compensation/decompensation of electron and hole charge carriers, the nonlinear dependence of the Hall resistivity of WTe2 and MoTe2 single crystals on the magnetic field is associated with the scattering of charge carriers on the surface.

The non-trivial topology of electronic bands in Weyl semimetals originates from band inversion due to strong spin–orbit coupling. The Weyl semimetals have pairs of Weyl gap-less nodes in the bulk Brillouin zone. The tungsten ditelluride WTe2 likely belongs to type II Weyl semimetals. Doping WTe2 with magnetic ions could induce magnetic ordering in this crystal, which provides prospects for practical applications. We studied the magnetic properties of the iron-doped single crystals Fe0.03W0.97Te2, annealed and unannealed, in comparison with the undoped WTe2. Measurements of the dc magnetization were carried out from 1.8 to 400 K. We revealed pronounced ferromagnetic ordering that was affected by annealing. Anomalies associated with antiferromagnetism and paramagnetism were also found. The magnetic order was suppressed by a field of 60 kOe. The rise in susceptibility with increasing temperature was observed at high temperatures in all samples and was treated using a model developed for Weyl semimetals. The Curie–Weiss law fit at 60 kOe showed that the effective magnetic moment was close to that of Fe2+. Metamagnetism was demonstrated for the unannealed doped WTe2 crystal. The data for the heat capacity of the iron-doped sample agreed with results for the undoped WTe2.

Electrical resistivity, magnetoresistivity, and the Hall effect have been studied in a topological semimetal WTe2 single crystal in the temperature range from 12 to 200 K under magnetic fields up to 9 T. It has been found that quadratic temperature dependences of the electrical resistivity in the absence of a magnetic field and the conductivity in a magnetic field are observed at low temperatures, which is apparently associated with contributions from various scattering mechanisms. Single-band and two-band models were used to analyze data on the Hall effect and magnetoresistivity. These results indicate electron-hole compensation with a slight predominance of electron charge carriers.

Electronic transport and magnetic properties of bulk and rapid melt quenched samples of the Mn$_3$Al Heusler alloy were studied. A correlation between the magnetic and structural states was established. For a cast sample, there is no ferromagnetic moment, and the behavior of the magnetic susceptibility (break at low temperatures and the Curie-Weiss law with high values of the paramagnetic Curie temperature) indicates a frustrated antiferromagnetic state. At the same time, for a rapid melt quenched sample, a ferrimagnetic state is observed with a moment close to compensation. The results of measurements of the electrical resistivity and the Hall effect evidence as well in favor of the implementation of these magnetic states.

The electrical resistivity, magnetic, and galvanomagnetic properties of the cast and rapid melt quenched Mn 3 Al Heusler alloy have been studied. Rapid melt quenching was found to result in changing the microstructure of the Mn 3 Al alloy, which leads to substantial changes in its electronic transport and magnetic properties. It was suggested that for the cast and rapid melt quenched Mn 3 Al alloy frustrated antiferromagnetic and almost compensated ferrimagnetic state could appear, respectively. It is shown that the preparation and treatment of the Mn 3 Al compound plays a substantial role in the formation of its electronic and magnetic characteristics.

The electrical resistivity and the Hall effect of topological insulator Bi2Te3 and Bi2Se3 single crystals were studied in the temperature range from 4.2 to 300 K and in magnetic fields up to 10 T. Theoretical calculations of the electronic structure of these compounds were carried out in density functional approach, taking into account spin–orbit coupling and crystal structure data for temperatures of 5, 50 and 300 K. A clear correlation was found between the density of electronic states at the Fermi level and the current carrier concentration. In the case of Bi2Te3, the density of states at the Fermi level and the current carrier concentration increase with increasing temperature, from 0.296 states eV−1 cell−1 (5 K) to 0.307 states eV−1 cell−1 (300 K) and from 0.9 × 1019 cm−3 (5 K) to 2.6 × 1019 cm−3 (300 K), respectively. On the contrary, in the case of Bi2Se3, the density of states decreases with increasing temperature, from 0.201 states eV−1 cell−1 (5 K) to 0.198 states eV−1 cell−1 (300 K), and, as a consequence, the charge carrier concentration also decreases from 2.94 × 1019 cm−3 (5 K) to 2.81 × 1019 cm−3 (300 K).

At present, the question of the relationship between the characteristic martensitic transformation temperatures (MTT) and the electronic parameters of a system has not been fully studied. In the present work, an attempt to establish a similar relationship using the example of the concentration of charge carriers, n, was made. The field dependences of Hall resistivity ρH and magnetization M of the magnetocaloric Ni47−xMn41+xIn12 (x = 0, 1, 2) alloys were measured at T = 4.2 K and in magnetic fields of up to 80 kOe. The MTT were obtained from the temperature dependences of electrical resistivity and magnetization. It was observed that the MTT correlate strongly with both the valence electron concentration e/a and the electronic transport characteristics, which are the coefficient of the normal (NHE) R0 and anomalous (AHE) RS Hall effect and the concentration of charge carriers n.

The anisotropy of the electrical resistivity and optical properties of the antiferromagnetic topological insulator MnBi2Te4 single crystal has been studied. It is shown that the electrical resistivity of MnBi2Te4 measured perpendicular to the (00l) plane is an order of magnitude higher than the resistivity measured in this plane. The behavior of optical conductivity is qualitatively similar for the cases when light is directed at the surface (00l) and at a surface perpendicular to the plane (00l). It has been shown that the optical spectrum of MnBi2Te4 is formed predominantly by interband transitions of charge carriers.

The peculiarities of electronic transport of the WTe2 single crystal, in particular, the minimum on the temperature dependence of the resistivity in a magnetic field and quadratic field dependence of the Hall resistivity, were studied. The values of the mean free path were estimated. It is suggested that the observed quadratic field dependence of the Hall resistivity may be associated with the strong electron-surface scattering mechanism, which was previously observed in compensated metals with a closed Fermi surface, under conditions of inhomogeneous distribution of electric current over the conductor cross section.

The results of the optical properties study and the electronic structure calculation of ${\text{Co}}_{2}Y\text {Si}$ ( $Y = \text {Ti}$ , V, Cr, Mn, Fe) Heusler alloys are presented. Temperature dependences of the electrical resistivity were studied in the temperature range from 4.2 to 300 K. Field dependences of magnetization were measured in the magnetic fields of up to 5 T. The main attention is paid to the spectral dependence of the real and imaginary parts of the dielectric constant in the wavelength range from 0.3 to $8~\mu \text{m}$ by the ellipsometric Beattie method at room temperature. Anomalous behavior of optical conductivity at infrared (IR) frequencies in Co 2 TiSi and Co 2 VSi alloys was found, which significantly differs from that in Co 2 MnSi, Co 2 FeSi, and Co 2 CrSi. The results obtained are discussed based on calculations of the electronic structure. Simultaneously, the correlations between the changes in these characteristics can be an indicator of the states of a half-metallic ferromagnet (HMF) or a spin gapless semiconductor (SGS). A high degree of charge carriers’ spin polarization is possible in such materials, and therefore, they are promising materials for spintronics.

Temperature dependences of electrical resistivity of Co2MnZ (Z = Al, Si, Ga, Ge, Sn) Heusler alloys were measured in a temperature range of 78–300 K; their magnetization was measured at 5 and 300 K in fields up to 30 kOe. It was found that, for the Co2MnAl and Co2MnGa alloys, the Mooij rule [Mooij J.H. Phys. Stat. Sol. (a). 1973. V. 17. P. 521] does not hold. A correlation between electronic and magnetic characteristics of the Co2MnZ alloys and atomic number of element Z is found.

PtSn4 and WTe2 single crystals were grown, and the resistivity was studied in the temperature range from 4.2 to 80 K in various magnetic fields up to 10 T in detail. It is suggested that the observed quadratic temperature dependence of the electrical resistivity at low temperatures in zero field can be due to, in addition to electron-electron scattering, the “electron-phonon-surface” interference scattering mechanism. The transition from high effective magnetic fields to weak ones, which is observed in compensated conductors with a closed Fermi surface, was proposed as a possible explanation for the minimum on the temperature dependence of the resistivity of PtSn4 and WTe2 in a magnetic field. The values of the mean free path of current carriers were estimated in these materials.

Electrical resistivity, magnetoresistivity, and the Hall effect have been studied in a single crystal of topological semimetal WTe 2 in the temperature range from 12 to 200 K under magnetic fields up to 9 T. A quadratic temperature dependence of the electrical resistivity in the absence of field and conductivity in a magnetic field is found at low temperatures, which appears to be associated with contributions from various scattering mechanisms. Single-band and two-band models were used to analyze data on the Hall effect and magnetoresistivity. These results indicate electron–hole compensation with a slight predominance of electron charge carriers.

The galvanomagnetic properties [magnetoresistivity (MR) and Hall Effect] and resistivity at temperatures from 2 to 300 K in magnetic fields of up to 9 T as well as optical characteristics (real and imaginary parts of the complex permittivity, optical conductivity, and reflectivity) at room temperature were studied. It was shown that the temperature dependence of the electrical resistivity has a “metallic” type and is quadratic at low temperatures (< 60 K) that can be caused by the “electron–phonon–surface” interference scattering mechanism in WTe 2 . Whereas an applied magnetic field induces a minimum in the temperature dependence of the resistivity at low temperatures that shifts to a higher temperature with increasing field, one of the possible explanation of which is the transition from high to weak effective magnetic fields. The MR increases with the magnetic field according to the quadratic law, reaching 1750% at 2 K, which is due to the compensation of charge carriers in WTe 2 . The Hall effect studies showed that the majority charge carriers are electrons with the concentration of $\sim 10^{19}$ cm −3 and mobility of 7500 cm 2 / $\text{V}\cdot \text{s}$ at 2 K. At the same time, optical investigations did not reveal the feature characteristics of metals. The optical conductivity spectrum is a broadband centered at 3.4 eV and formed by interband transitions. The presence of peaks in the infrared region indicates the formation of low-energy gaps in the band spectrum of WTe 2 . The obtained data on the real and imaginary parts of the complex permittivity also evidence the absence of a contribution from free carriers up to 0.2 eV. The optical characteristics are shown to be in good agreement with the data on electronic transport properties. The revealed features are a manifestation of the topological nature of the material.

Structural and phase transformations in the microstructure and new metastable baro- and deformation-induced phases of the Ni50Mn28.5Ga21.5 alloy, typical of the unique class of ferromagnetic shape memory Heusler alloys, have been systematically studied for the first time. Phase X-ray diffraction analysis, transmission and scanning electron microscopy, and temperature measurements of electrical resistivity and magnetic characteristics in strong magnetic fields were used. It was found that in the course of increasing the pressure from 3 to 12 GPa, the metastable long-period structure of martensite modulated according to the 10M-type experienced transformation into a final non-modulated 2M structure. It is proved that severe shear deformation by high pressure torsion (HPT) entails grainsize refinement to a nanocrystalline and partially amorphized state in the polycrystalline structure of the martensitic alloy. In this case, an HPT shear of five revolutions under pressure of 3 GPa provided total atomic disordering and a stepwise structural-phase transformation (SPT) according to the scheme 10M → 2M → B2 + A2, whereas under pressure of 5 GPa the SPT took place according to the scheme 10M → 2M → B2 → A1. It is shown that low-temperature annealing at a temperature of 573 K caused the amorphous phase to undergo devitrification, and annealing at 623–773 K entailed recrystallization with the restoration of the L21 superstructure in the final ultrafine-grained state. The size effect of suppression of the martensitic transformation in an austenitic alloy with a critical grain size of less than 100 nm at cooling to 120 K was determined. It was established that after annealing at 773 K, a narrow-hysteresis thermoelastic martensitic transformation was restored in a plastic ultrafine-grained alloy with the formation of 10M and 14M martensite at temperatures close to those characteristic of the cast prototype of the alloy.

The electro- and magneto-transport as well as magnetic properties of Co2MeSi (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys were studied. The electroresistivity was measured from 4.2 to 300 K, the galvanomagnetic properties (magnetoresistivity and Hall effect) were measured at T = 4.2 K in magnetic fields of up to 100 kOe, and the magnetization at T = 4.2 and 300 K in fields of up to 70 kOe. The normal and anomalous Hall coefficients, saturation magnetization, residual resistivity, current carrier concentration, coefficients at linear contributions into the electroresistivity and magnetoresistivity were obtained. It was shown that on the one hand, there is quite clear correlation between the electronic and magnetic characteristics of Heusler alloys studied, and the spin polarization coefficients of current carriers, taken from well know literature data, on the other hand. The obtained results can be used for creation of new materials for spintronics.