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 magnetotransport properties of Mo0.7W0.3Te2 and WTe2 single crystals were studied at temperatures from 4.2 to 80 K and in magnetic fields up to 10 T. The concentrations and mobilities of electron and hole current carriers were estimated in the studied samples at a temperature of 4.2 K. It was found that the carrier mobility in the WTe2 single crystal is an order of magnitude higher than the values obtained for Mo0.7W0.3Te2, which is associated with its higher “electrical” purity. A minimum of the temperature dependence of the resistivity of WTe2 was found in a magnetic field of 10 T at a temperature of 60 K, which can be explained by the transition from effectively high magnetic fields to weak ones. The absence of such a minimum for the Mo0.7W0.3Te2 single crystal is due to the fact that the region of effectively high magnetic fields is not reached for it. The Hall resistivity of WTe2 was shown to depend quadratically on the magnetic field at a temperature of 4.2 K, which is associated with the decompensation of electrons and holes, as well as with the scattering of charge carriers on the surface of the sample. Whereas for Mo0.7W0.3Te2, along with the quadratic contribution, a linear contribution to the Hall resistivity was observed, the cause of which may be the presence of a large number of defects and impurities in the crystal, which leads to a decrease in the mean free path of carriers and, consequently, to a decrease in the contribution of electron-surface scattering.

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 Hall resistance. xy of thin films of the Bi2Se3 topological insulator with a thickness from 10 nm to 75 nm at a temperature of 4.2 K and in magnetic fields up to 10 T has been measured. The size effect was found, i.e. dependence of the Hall resistance and the Hall coefficient on the thickness of the studied films. Using a single-band model, the values of the current carrier concentration and their mobility are calculated, which also change with a change in the thickness of the samples.

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.

A WTe2 single crystal was grown by the chemical vapor transport method, and its electrical resistivity and galvanomagnetic properties were investigated. Single-band and two-band models were used to estimate the concentration and mobility of charge carriers in WTe2 at temperatures from 4.2 to 150 K.

fominykh@imp.uran.ru Abstract. A WTe2 single crystal was grown by the chemical vapor transport method, and its electrical resistivity and galvanomagnetic properties were investigated. Single-band and two -band models were used to estimate the concentration and mobility of charge carriers in WTe2 at temperatures from 4.2 to 150 K.

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 electrical resistance and structure of Ni42+xMn47-xSn11 alloys have been studied; where x=0, 1, 2, 3, 4, At room temperature, Ni42+xMn47-xSn11 alloys are ordered in the cubic L21 Ni2MnSn structure characteristic of Heusler alloys. The phase transition between the cubic austenite phase and martensite with lower symmetry, characteristic of ferromagnetic non-stoichiometric Heusler alloys, is observed at temperatures below room temperature. It was found that the partial replacement of Mn atoms by Ni atoms leads to an increase in the phase transition temperatures and to their shift to room temperature. Keywords: Heusler alloys, electrical resistance, phase transition.

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.

Исследованы электросопротивление и структура сплавов Ni42+xMn47-xSn11, где x=0, 1, 2, 3, 4. При комнатной температуре сплавы Ni42+xMn47-xSn11 упорядочены в кубическую структуру L21 Ni2MnSn, характерную для сплавов Гейслера. Характерный для ферромагнитных нестехиометрических сплавов Гейслера фазовый переход между кубической аустенитной фазой и мартенситом с более низкой симметрией наблюдается при температурах ниже комнатной. Обнаружено, что частичная замена атомов Mn атомами Ni приводит к повышению температур фазового перехода и к смещению их в область комнатных температур. Ключевые слова: сплавы Гейслера, электросопротивление, фазовый переход.