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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 WTe2. 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 WTe2. 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 cm2/ $\text{V}\cdot \text{s}$ 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 WTe2. 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. [ABSTRACT FROM AUTHOR]

The dichalcogenide Bi2Te3 and Bi2Se3 crystals belong to the 3D topological insulator family. Doping the crystals with magnetic ions can break the time-reversal symmetry and open an energy gap at surface Dirac points. This provides the opportunity of manipulating the surface transport and observing the anomalous quantum Hall effect. We studied magnetic properties of three single-crystalline Bi CrxSe3 samples with , 0.03, and 0.06 within a temperature range from 2 to 300 K. The dc magnetization revealed the coexistence of antiferromagnetic and ferromagnetic ordering and paramagnetism. Their significance depends on the chromium content. The ferromagnetic phase transition was suppressed by high enough magnetic field. The antiferromagnetic transition near 80 K did not shift visibly up to 50 kOe. The Curie-Weiss law approximation gave the effective magnetic moment close to , which corresponds to divalent chromium ions. Metamagnetic phenomena were found for crystals with and 0.06. [ABSTRACT FROM AUTHOR]

A topological semimetal PtSn4 single crystal is grown and the following properties are studied: its electrical resistivity in the temperature range from 4.2 to 300 K, galvanomagnetic properties at temperatures from 4.2 to 80 K and in magnetic fields of up to 100 kOe, and optical properties at room temperature. Theoretical calculations of the electronic structure are performed. It is shown that the residual resistivity is rather small: ρ0 = 0.47 μΩ cm, which is characteristic of a "good" metal. The ρ(T) dependence is of the metallic type, increasing monotonically with temperature. Analysis of the temperature dependences of the magnetoresistivity suggests that the Fermi surface of the PtSn4 compound may contain closed sheets. Studies of the Hall effect and the estimates made in the single-band model allowed us to conclude that the predominant type of current carriers are holes with concentration n = 6.8 × 1021 cm–3 and mobility μ ≈ 1950 cm2/(V s) at T = 4.2 K. It is shown that the optical properties of PtSn4 have features characteristic of "bad" metals. The calculation of the electronic structure of PtSn4 showed that, in general, this compound has a structure characteristic of metallic systems with a sufficiently large number of electronic states at the Fermi level, which is consistent with the experimental results on the electronic transport and optical properties of the PtSn4 single crystal. [ABSTRACT FROM AUTHOR]

Low-dimension ferromagnetic maghemite γ-Fe2O3 was synthesized through a precursor route, using basic iron formate Fe(OH)(HCOO)2 as a precursor. Conditions of formation of γ-Fe2O3 and the temperature range of its existence on heating in air were determined. The saturation magnetization of γ-Fe2O3 produced through heating the precursor at 350°C 57.5 (T = 4.2 K) and 43.8 emu/g (T = 300 K). [ABSTRACT FROM AUTHOR]

The electrical resistivity of thin films of topological insulator (TI) Bi2Se3 10 to 75 nm thick is measured in the temperature range of 4.2 to 300 K. A size effect is observed in the electrical conductivity of the Bi2Se3 films; i.e., there is a linear dependence of a film's conductivity on its inverse thickness. It is assumed that a similar effect can be observed in other TIs and in systems with nonuniform distributions of current over the cross section of a sample. [ABSTRACT FROM AUTHOR]