The Berry phase, a fundamental geometric phase in quantum systems, has become a crucial tool for probing the topological properties of materials. Quantum oscillations, such as Shubnikov-de Haas (SdH) oscillations, are widely used to extract this phase, but its unambiguous determination remains challenging. This work highlights the inherent ambiguities in interpreting the oscillation phase solely from SdH data, primarily due to the influence of the spin factor RS, which depends on the Landé g-factor and effective mass. While the Lifshitz-Kosevich (LK) theory provides a framework for analyzing oscillations, the unknown g-factor introduces significant uncertainty. For instance, a zero oscillation phase could arise either from a nontrivial Berry phase or a negative RS. We demonstrate that neglecting RS in modern studies, especially for topological materials with strong spin-orbit coupling, can lead to doubtful conclusions. Through theoretical analysis and numerical examples, we show how the interplay between the Berry phase and Zeeman effect complicates phase determination. Additionally, we also discuss another underappreciated mechanism - the magnetic field dependence of the Fermi level. Our discussion underscores the need for complementary experimental techniques to resolve these ambiguities and calls for further research to refine the interpretation of quantum oscillations in topological systems.

Magnetic and electronic transport properties of Co2MnZ (Z = Al, Ga, Ge, Si, Sn) Heusler alloys were experimentally investigated. Electrical resistivity, in the temperature range from 4.2 to 300 K, as well as field dependences of the Hall effect and magnetization at T = 4.2 K in magnetic fields of up to 100 kOe and 70 kOe, respectively, were measured. Experimental data are in good agreement with the results of the theoretical DFT calculations of the electronic structure and magnetic moments. In the band structure of Co2MnSi, half-metallicity is formed with the full spin polarization and the half-metallic gap of about 0.6 eV. In Co2MnZ (Z = Al, Ge, Sn), it is shifted from the Fermi energy by the hole pockets at point & Gcy;, preventing thereby the formation of the halfmetallic state. In a peculiar case of Co2MnGa, the antisite defects are expected to determine structural and electronic properties. For the Co2MnAl and Co2MnGa topological semimetals, Weyl topological points are found at the Fermi energy; however, for Z = Si, Ge, Si, these features are located deeper within to the valence band. The results show that Co2MnGe and Co2MnSn are usual ferromagnets, Co2MnAl and Co2MnGa alloys are topological semimetals that can find application in microelectronics, while Co2MnSi is a half-metallic ferromagnet that is in high demand in spintronics.

The paper presents the findings of an experimental investigation into the optical properties of Mn3Z (Z = Al, Ga, Si, Sn) Heusler alloys. The behavior of the optical characteristics of the alloy in the IR region of the spectrum, anomalous for metallic systems, was revealed, specifically, the absence of contribution from intraband absorption and the presence of intense interband absorption. The structure of the optical conductivity spectra is explained qualitatively on the basis of the available first-principles calculations of the band structure.

The paper presents a brief overview of the works related to a new and promising direction: solid-state cooling technology based on caloric effects. A historical excursion into the problem of caloric cooling is considered. Problems and possible solutions are highlighted. The latest research results of Russian scientists working in this area are presented.

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.

The electrical resistivity, magnetization, and Hall effect in Co2FeZ (Z = Al, Si, Ga, Ge, Sn, Sb) ferromagnetic Heusler alloys have been investigated. It has been demonstrated that there are a number of correlations between the electronic and magnetic characteristics of the alloys under study, which are manifested by changes in the atomic number of the Z component. For Co2FeAl and Co2FeSi alloys, which are HMFs, the magnetization value agrees with the Slater–Pauling rule. The electrical resistivity of Co2FeAl, Co2FeSi, and Co2FeGe compounds exhibits quadratic temperature dependence at temperatures below 30 K and above 65 K. In the range of intermediate temperatures (40 to 65 K), a power-law dependence of Tb with an exponent of 3.5 ≤ b ≤ 4 has been revealed, which may be attributed to two-magnon scattering processes.

The structural, magneto-optical, and magnetotransport properties as well as the electronic band structure of the bulk crystalline topological insulator (TI) Sb2Te2Se, grown by the vertical Bridgman technique, were studied. The high structural quality of the grown crystals was established by x-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy revealed a single Dirac cone with the Dirac point, away from the valence as well as conduction bands, 0.22 eV above the Fermi level. The magnetotransport data exhibited a distinct single-frequency Shubnikov-de Haas oscillation in the magnetic fields above B = 10 T. The Lifshitz-Kosevich analysis of the data suggests that this oscillation originates from the Dirac-type states. A sharp fundamental absorption edge in the mid-infrared transmission spectra measured at 4.2 K demonstrated a direct band gap of 0.377 eV located at a momentum of 0.1 & Aring;-1 along the P- K directions of the Brillouin zone. A two-band model developed for massive Dirac electrons in the bulk of topological insulators with the direct band gap at a non-& Gcy; point suggests hyperbolic dispersion relations for the conduction and valence bands displaying the full electron-hole symmetry. Equal magnitude of the electron and hole effective masses me mh 0.21m0 and g factors ge gh 10 were estimated.

To investigate the complex magnetic structure and interatomic interactions of low-temperature martensite, nonstoichiometric Ni43Mn46Sn11 Heusler alloy was prepared and characterized by DC and AC magnetization under intensive variables including temperature, magnetic field, time and frequency. A cluster-type reentrant spin-glass (RSG) transition is detected at T = 123.7 K, just below the ferromagnetic transition in martensite phase. Unexpectedly, the hump in the thermomagnetic curves suggests another intriguing magnetic transition at T = 25.2 K, which results in a second RSG transition at a slightly lower temperature. The frequency dispersion of the two RSG transitions was examined using the dynamical scaling law. The obvious exchange bias at low temperatures is a combined action of the separated spin-glass and ferromagnetic martensitic phases. This work claims an unusual freezing state of double spin glass transitions, which may be useful for a further understanding of the physical nature of spin glass state.

The paper presents a concise overview of a novel and promising area, namely, magnetic cooling technology that utilizes the magnetocaloric effect (MCE). The nature of the effect and the main relevant publications in this field are succinctly examined. The materials in which the MCE is observed are reported, and corresponding charts reflecting the number of main works of the last five years are presented. Additionally, recent research findings of Russian scientists in this field are considered.

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 temperature dependences of the electrical resistivity of topological insulator Bi2Se3 thin films with thicknesses of 20 and 40 nm were measured in the temperature range from 4.2 to 80 K. Their resistivity was shown to depend on thickness. A method was proposed for "separation" of the bulk and surface resistivity of films, with the help of which corresponding estimates were made. It was demonstrated that the surface resistivity is more than two orders of magnitude less than the bulk resistivity at T = 4.2 K.

This article presents a brief review of works devoted to the latest research by a number of Russian groups involved in the development, creation, and study of the physical properties of new magnetic materials and the phenomena observed in them. These studies are of both great fundamental and practical interest for various fields of science, engineering, and new technologies.

A series of cadmium oxide samples (CdOs) were synthesized by thermal treatment method with various annealing temperatures (TA = 500 degrees C - 900 degrees C). All samples crystallize in a pure cubic structure with Fm3m symmetry. The crystallite size, lattice parameter and volume change slightly with TA, and show an unexpected inflection point for the sample with TA = 800 degrees C (CdO-800), while the average particle size increases monotonically from -0.4 mu m to -3.0 mu m. Distinct ferromagnetic features accompanied by diamagnetic and/or paramagnetic phases were detected. The XPS data indicate the valence of cadmium and oxygen vacancy concentration varies with TA, which are responsible for the ferromagnetic origin in CdOs. CdO-800 shows the maximum magnetization, significant coercive force and residual magnetization. Oxygen vacancy plays a crucial role in the ferromagnetic performance, which can be effectively controlled by changing the sintering temperature. This work claims new characteristics for CdO besides optical and semi-conducting, which may be important in the preparation and possible applications for CdO dilute magnetic semiconductors.

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 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.

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.

Abstract— A brief review of works related to a new and promising direction – magnetic cooling technology based on the magnetocaloric effect (MCE) is presented. The essence of the effect and the main publications related to this area are briefly considered. The materials in which the MCE is observed are reported and the corresponding diagrams of the main publications on the MCE for the last 5 years are presented. The latest results of research by Russian scientists working in this field are presented.

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.

The structure and electrical properties of the Cu-5.9 at.% Pd alloy were studied after its annealing for two months at 250 degrees C. As a result of such a long-term thermal treatment, in the XRD patterns taken from the alloy one can observe a weak superstructural (100) reflection, which seems abnormal, since the alloy can only be in the state of a disordered single-phase solid solution (as it follows from the generally accepted Cu-Pd phase diagram). Quantitative analysis of the X-ray diffraction patterns reveals the presence of two new phases with different contents of Pd. An assumption is made that L12 superstructure can be formed in the Pd-enriched phase. The specific features observed in the temperature dependence of electrical resistivity, as well as the TEM results, confirm this set-forth hypothesis of ours. Using the resistometric method, the temperature of the order-disorder phase transition in the alloy was estimated as Tc approximate to 340 degrees C. It is concluded that the position of the A1-(A1+ L12) phase boundary in the Cu-Pd phase diagram requires more precise definition in the region of compositions on the side of the low Pd content.