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Project titlePhase transitions in Cd3As2 Dirac semimetals. Superconductivity and other modifications of the electron spectrum.

KeywordsDirac semimetals, Cd3As2, Phase transitions, Superconductivity, Weyl semimetals, Magnetotransport, The structure of superconducting order parameters, Size effects, Anisotropy.

Annotation
About a century ago, the equations describing nontrivial properties of massless fermions were introduced in the field of particle physics. The analogous fermions were first observed in the solid matter only twenty years ago. After that, the number of similar research works began to increase dramatically. As a result, in the field of condensed matter physics a new research area was established – topological materials. The interest to this area stems from the combination of unique fundamental properties of such materials and enormous prospects of their use for applications. During last 10-15 years, these systems were considered as exceptionally exotic ones, but the theoretical study reported in 2019 suggested that about a quarter of all known stable inorganic compounds potentially belong to one or another subclass of topological materials. And now the results obtained in this research field are being generalized over a huge number of existing materials. The distinctive feature of topological materials is the presence of nontrivial electronic states with chiral gapless quasilinear spectrum. Topological materials include systems with different dimensionality of the spectrum of nontrivial states. The systems hosting 3D nontrivial states form subclasses of the Dirac (DSM) and Weyl (WSM) semimetals. The active study of such systems began about five years ago, and during that time the basic properties were adequately characterized only for a handful of materials. Among them, we have Cd3As2, which has a relatively simple band structure, is highly stable and has outstanding transport characteristics. Using Cd3As2 as a basic system, we plan to investigate topological transitions induced by different structural modifications (including material composition variations). This project is focused on the detailed investigation of magnetotransport properties (including studies of quantum effects and of magnetoresistance anisotropy) of the samples with different composition and effective dimensionality (bulk crystals and thin monocrystalline films). We plan to pay special attention to the investigation of the superconductivity in these materials. In our preliminary studies we made first experimental observation of the superconductivity in Cd3As2 films obtained by magnetron sputtering without the application of external stimuli. Later we observed superconductivity in Cd3As2 films obtained by another method - vacuum thermal deposition. Both series of films showed similar features of the superconducting state, which confirmed the reproducibility of this phenomenon. An analysis of the obtained results indicates the topological nature of the observed superconductivity. It is worth mentioning that the realization of topological superconductivity is one of the most topical problems in this field of research. However, to unambiguously confirm the topological nature of the observed superconductivity, one needs to study the effects of numerous factors, which form a considerable part of this project. Suggested project includes a full research cycle - working out the growth regimes and synthesis of bulk Cd3As2 crystals and of Cd3As2 thin films with different types of impurities, structural studies and investigations of magnetotransport properties. The low-level doping by isovalent and magnetic impurities should turn system into a trivial phase, with possible formation of some intermediate phase. In our preliminary studies we investigated Cd3As2 films with a manganese impurity, however, the obtained results were not completely unambiguous. In this project, we plan to consider other magnetic impurities, which, together with further improvement of the quality of the synthesized films, should make it possible to obtain reliable phase diagrams of the systems under study. In addition, we plan to choose an efficient impurity, which can be used to decrease typically high electron densities in Cd3As2 that suppresses the relative amplitudes of the effects related to the DSM phase. We also plan not only to study the effects of synthesis parameters (including the effects driven by the paraelectric substrate) on the native superconductivity, observed earlier, but also to consider alternative methods to obtain superconducting Cd3As2 - based systems by doping with corresponding impurities. Thus, basing on the analysis of the quantum effects in magnetotransport, we intend to construct a detailed picture of topological transitions (including the arising superconductivity) in Cd3As2-based systems with various dopants. We also plan to reveal basic factors that determine values of superconducting parameters, and to determine the pairing type in the Cd3As2- based systems. It should be noted that superconductivity in Dirac semimetals was discovered quite recently and has hardly been studied. And in this sense, the proposed studies are unique, since we plan to study equilibrium systems (without the application of external pressure), which implies the potential possibility of using a wider range of techniques, as well as the absence of the need to take into account the effects of deformation associated with external stimuli. In addition, topological superconductors are of great interest from the point of view of applied science (in particular, for the creation of quantum computers), which also guarantees the relevance of these studies.

Expected results
The expected results should correspond to the tasks and aims of the project, namely, clarifying the nature of superconductivity in Cd3As2 and the study of the composition effect on the systems properties in the superconducting and normal state, in particular, on its topological phase. In the framework of this project, the following results are expected. 1) We will develop the synthesis regimes for bulk crystals and thin films of pure Cd3As2 and solid solutions based on it, using various synthesis methods (crystals synthesis using Bridgman and vapor-phase methods, films synthesis using vacuum thermal and magnetron sputtering). Synthesized crystals and films will be used as a platform for studying the effect of introducing an isovalent and magnetic component on the electronic spectrum of the corresponding solid solutions. We will also consider the effect of charged impurity doping in order to reduce the concentration of free charge carriers, as well as of an impurity that potentially induces superconductivity in the system. 2) We will perform structural characterization of the obtained samples - the phase and elemental compositions will be determined, as well as the type of crystallinity, the defect structure will be characterized, we will also study the surface morphology and spectral features of inelastic light scattering on the sample (Raman spectroscopy), and the single-crystal samples will be oriented. On the basis of this data, growth regimes will be optimized, and the influence of structural factors on the results of magnetotransport studies will be assessed. 3) Studies of magnetotransport and its anisotropy for the obtained crystals and films will be carried out in fields up to 16 T and at temperatures from room temperature to 10 mK. Based on the analysis of quantum effects (antilocalization and Shubnikov-de Haas oscillations), as well as of other magnetotransport phenomena (linear and / or asymmetric magnetoresistance, anomalous Hall effect), the parameters of the conducting subsystem (concentration, mobility, and effective masses of charge carriers) will be determined and the topological phase of the systems under study will be characterized (DSM, WSM, trivial). 4) Based on the obtained structural and magnetotransport data, compositional phase diagrams will be elaborated for the systems under study, which will signify the transitions between various topological phases. The obtained results will be compared with the results of calculations within the framework of the density functional theory. We will perform the theoretical study of the effect of various impurities on the magnetoresistance and the Hall effect in Dirac semimetals and other systems with Dirac charge carriers. The obtained theoretical results will be compared with experiment, which will make it possible to specify the properties of doped Dirac semimetals. 5) The nature of superconductivity (and the symmetry of the order parameter) in Cd3As2 and solid solutions based on it will be studied. We will obtain the dependence of the parameters of the superconducting state in films on their structure, composition, thickness and type of substrate. We will compare the behavior of the intrinsic and impurity-induced superconducting states (for crystals and films). The relationship between the features of the observed superconducting state and the topological phase of the system will be analyzed. If possible, the properties of Josephson junctions based on the materials under study will be investigated, and we will carry out the "soft" point contact spectroscopy experiments. Based on the results of these experiments, the picture of the properties of superconducting Dirac semimetals will be refined and additional information will be obtained on the nature of this superconductivity.

Annotation of the results obtained in 2023
We study the effects a strong Coulomb disorder on the transverse magnetoresistance in Weyl semimetals at low temperatures. Using the diagrammatic technique and the Keldysh model to sum up the leading terms in the diagrammatic expansion, we find that the linear magnetoresistance exhibits a strong renormalization due to the long-range nature of the Coulomb interaction ~HlnH As disorder becomes even stronger (but still in the parametric range, where the Coulomb interaction can be treated as a long-range one), we find that the magnetoresistivity becomes quadratic in the magnetic field ~H^2. We study the effects of anisotropy on the magnetoresistance of Weyl semimetals in the ultraquantum regime. We find that anisotropy manifests itself in the strong dependence of the magnetoresistance on the polar and azimuthal angles determining the orientation of the anisotropy axis with respect to the applied magnetic field and electric current. We also predict that the ratio of magnetoresistances in the geometries, where the magnetic field and anisotropy axes are aligned and where they are orthogonal, scales as (v⊥/v ) where v⊥ and v are the corresponding Fermi velocities. A composite Cd0.95Ni0.05Sb crystal consisting of CdSb host material and NiSb inclusions was grown by the modified Bridgman method. Using the scanning electron microscopy, X-ray phase analysis, energy-dispersive X-ray spectroscopy, and the studies of Raman scattering spectra, we demonstrated that the CdSb host material was a single crystal (having the orthorhombic Pbca structure) containing microcrystalline needle-like inclusions of the second (NiSb) phase (having the hexagonal structure of the NiAs P63/mmc type). It has been found that the anisotropy of the single-crystalline NiSb needles in the Cd0.95Ni0.05Sb composite samples (with the needles directed along and across the applied electric current) produces a pronounced effect on the electrical resistance and magnetoresistance of the crystal. The interface between a needle-like inclusion and the host material exhibits a transition to the superconducting state, which affects the electrical conductivity of the composite as a whole. The contributions to the effective magnetic anisotropy of the constants of the first - K1 and second - K2 orders are distinguished, which are confirmed by the data of electron microscopic, energy dispersive elemental, X-ray diffraction analyzes and vibrational microspectrometry arising from the structuring of crystallized InSb-MnSb eutectic compositions in the form of acicular single-crystal MnSb inclusions in an InSb matrix in fields up to 50 kOe with a temperature change from 5 to 350 K. The dependences K1(T) and K2(T) are obtained, and the signs of K1 < 0, K2 > 0 are determined from the calculated temperature dependences of the saturation magnetization in the studied range of magnetic fields. It is shown that the rivalry between the contributions of K1 and K2 to the effective magnetic anisotropy constant determines the magnetic ordering of the “easy plane” type; extrapolation of K1(T) to the region of high temperatures at 530 K predicts the appearance of a spin-reorientation transition to the magnetic anisotropy of the “easy axis” type. With significant doping with impurities, the Shubnikov–de Haas steps in the magnetoresistance also acquire impurity concentrations, when the average distance between impurities is less than the Debye screening length, but greater than the magnetic length, the linear magnetoresistance acquires an additional (logarithmic in the magnetic field) prefactor. In the highly logarithmic renormalization mode. The results obtained are presented in the form of a phase diagram in the coordinates the average distance between impurities - magnetic field. We performed initial calculations of the (Cd0.96Cr0.04)3As2 alloy in non-magnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) spin states. The FM state was found to have the lowest energy, which is consistent with the prediction of the Zener double exchange mechanism. Calculations have demonstrated that the spin state of the (Cd1 − xCrx)3As2 alloy strongly affects the Dirac cone (DC).

Publications

1. A. I. Dmitriev, A. V. Kochura, A. P. Kuzmenko, Zaw Htet Aung, V. V. Rodionоv, S. F. Marenkin, B. A. Aronzon ТЕМПЕРАТУРНЫЕ ЗАВИСИМОСТИ КОНСТАНТ МАГНИТНОЙ АНИЗОТРОПИИ МОНОКРИСТАЛЛИЧЕСКИХ ВКЛЮЧЕНИЙ MnSb В МАТРИЦЕ InSb Редакция журнала Известия РАН. Серия физическая, т. 88, в. 2, 2024 (year - 2024)

2. Dotdaev A. S., Rodionov Ya. I., Kugel K. I., and Aronzon B. A. Effects of anisotropy on the high-field magnetoresistance of Weyl semimetals PHYSICAL REVIEW B, vol. 108, P. 165125(10) 2023 (year - 2023) https://doi.org/10.1103/PhysRevB.108.165125

3. E. T. Kulatov, Yu. A. Uspenskii, K. I. Kugel Non-trivial evolution of the Dirac point in Cd3As2 doped by chromium Journal of Alloys and Compounds, - (year - 2024)

4. Rodionov Ya. I., Kugel K. I., and Aronzon B. A. Quantum magnetoresistance of Weyl semimetals with strong Coulomb disorder PHYSICAL REVIEW B, vol. 107, P. 155120 (13) 2023 (year - 2023) https://doi.org/10.1103/PhysRevB.107.155120

5. V.S. Zakhvalinskii, E.A. Pilyuk, A.V. Kochura, L.A. Morgun, A.B. Davydov, Aung Zaw Htet, V.V. Rodionov, B.A. Aronzon Влияние ориентации игольчатых включений NiSb на температурную зависимость сопротивления в монокристаллах Cd0.95Ni0.05Sb Физика Твердого Тела, 65, вып.3, стр. 397 – 403, 2023 (year - 2023) https://doi.org/10.21883/FTT.2023.03.54736.548

6. Aronzon B.A., Mekhiya A.B., Oveshnikov L.N., Davydov A.B., Prudkoglyad V.A., Rodionov Ya.I., Kugel K.I., Selivanov Yu.G. LINEAR MAGNETORESISTANCE IN (Bi1-xEux)2Se3 TOPOLOGICAL INSULATOR Сборник тезисов SISM, Самарканд, Сборник тезисов (year - 2023)

7. Aronzon B.A., Zaxvalinskii V.S., Pilyuk E.A., Kochura A.V., Morgun L.A., Davydov A.B., Aung Zaw Htet, Rodionov V.V. Природа сверхпроводимости в монокристаллах Cd0.95Ni0.05Sb XXVII Российская конференция по физике полупроводников (Нижний Новгород, 2023), Тезисы докладов и доклад (2023 г.), Сборник тезисов 1, 229 (year - 2023)

Annotation of the results obtained in 2021
We theoretically considered main effects introduced by manganese on the band structure and magnetic properties of the Cd3As2: Mn system. Nine different configurations of the Mn pair substituting cadmium atoms in the Cd3As2 cell were considered. Our results suggest the dominant antiferromagnetic sign of the interaction in the Mn-Mn pair and the presence of a tendency towards clustering of impurity atoms, which are consistent with the data of early experimental magnetic studies of this system. The key effect of Mn doping is the opening of a small energy gap at the Dirac point, while the middle part of the spectrum remains practically unchanged. Taking into account the inhomogeneous distribution of manganese atoms in real crystals, the average gap is about 40 meV. Magnetotransport studies of Cd3As2: Mn films at helium temperatures revealed a transition from weak antilocalization to localization upon increasing the manganese content, which implies a transition to the trivial phase and is consistent with the predicted gap opening. An estimate based on the calculated data shows that in the studied samples, the effect related to the gap opening is observed when the gap magnitude becomes close to the characteristic Fermi energy of charge carriers. Therefore, the presence of this gap may not be detected in systems with a sufficiently high Fermi level (high electron density). An analysis of the Cd3As2: Mn polycrystals properties showed that the introduction of manganese does not have a significant effect on the microstructure of crystals; however, it leads to an increase of disorder level in the system, in particular, of the charged defects concentration. Hall effect studies show the absence of anomalous contribution, which is consistent with the antiferromagnetic nature of the interaction in the Mn-Mn pair and indicates an insignificant role of MnAs inclusions (in the considered composition range) on transport in these systems. The presence of a strong linear magnetoresistance at low temperatures in the studied samples suggests the preservation of the Dirac semimetal phase for electrons with high Fermi energies, which agrees with the results of a theoretical analysis of the Cd3As2: Mn system. We obtained data of preliminary studies of Cd3As2: Mn crystals synthesized from the vapor phase, which showed that these samples are characterized by noticeably lower electron density values (as compared to polycrystals synthesized from a melt). We plan to use this feature in further studies to visualize the phase diagram of the Cd3As2: Mn system by plotting the dependence of the carrier effective mass on the manganese content and on the carrier Fermi energy. We carried out preliminary studies of the Cd3As2: Cr crystals, in which chromium atoms should interact with each other in a ferromagnetic manner (according to preliminary results of theoretical calculations). Using a model system, we considered the influence of the magnetic component on the properties of systems with a Dirac spectrum by analyzing the data for multilayer (Bi – FeNi)N films with nanometer thicknesses of layers. It was shown that a decrease in the thickness of the bismuth layers leads to a significant increase in their conductivity, in agreement with the theoretically predicted transition to the topological insulator state, while the morphology and magnetic properties of the FeNi layers have a significant effect on the conductivity of the Bi layers. We studied Cd3As2 films obtained using different substrates and different deposition rates. For the series of films obtained with a high deposition rate, we showed that an increase in the crystallinity of the system leads to a characteristic change in the morphology of the film surface. In this case, the obtained Raman spectra turn out to be insensitive to the variation of crystalline order in the samples under study. The appearance of the Cd3As2 crystalline phase is also accompanied by the appearance of a characteristic weak antilocalization effect, revealed by the magnetotransport studies at helium temperatures. We studied electron transport at subkelvin temperatures in Cd3As2 films grown on different substrates with low deposition rates. The obtained results confirm the presence of a superconducting transition, in particular, for samples synthesized on a strontium titanate substrate. In addition, we studied the high-field magnetotransport in these films, which revealed the presence of Shubnikov-de Haas oscillations. We plan to complete the analysis of obtained data in 2022, and to elaborate a quantitative picture of the effect of the crystallinity of the films on their properties, as well as to reveal the correlations of the parameters of the superconducting state with other parameters of the systems under consideration. The results obtained in this year were published in two papers in the Q1 journals and another article was prepared, which is currently under review in the Journal of Physics: Condensed Matter (Q1). We plan to prepare at least two papers to be submitted in 2022, based on the analysis of the data obtained in this year.

Publications

1. Kovaleva N.N., Chvostova D., Pacherova O., Muratov A.V., Fekete L., Sherstnev I.A., Kugel K.I., Pudonin F.A., Dejneka A. Bismuth layer properties in the ultrathin Bi–FeNi multilayer films probed by spectroscopic ellipsometry Applied Physics Letters, Том 119, номер статьи 183101 (6 страниц) (year - 2021) https://doi.org/10.1063/5.0069691

2. Kulatov E.T., Uspenskii Yu.A., Oveshnikov L.N., Mekhiya A.B., Davydov A.B., Ril' A.I., Marenkin S.F., Aronzon B.A. Electronic, magnetic and magnetotransport properties of Mn-doped Dirac semimetal Cd3As2 Acta Materialia, Том 219, номер статьи 117249 (10 страниц) (year - 2021) https://doi.org/10.1016/j.actamat.2021.117249

Annotation of the results obtained in 2022
In this study, we analyzed the composition-dependent phenomena in (Cd1−xMnx )3As2 polycrystals with x = 0 – 0.08.We showed that the Mn-induced stabilization of minor α-phase in these systems occurs without substantial decrease in the characteristic crystallite sizes. The comparison of the estimated relevant length scales suggests that polycrystalline character of the studied samples plays only a minor role in electron transport, while the increase in defect density along with Mn content has more profound effect. We observed low-temperature linear magnetoresistance even for the samples with high Mn content. The decrease in the corresponding effect amplitude can be explained in terms of basic properties of the parent Cd3As2 compound. Therefore, we argue that Mn-doping qualitatively conserves characteristic features of the Dirac semimetal phase in the system with relatively high Fermi energies, in accordance with previous band calculations of the (Cd1−xMnx )3As2 compound. We study the effects a strong Coulomb disorder on the transverse magnetoresistance in Weyl semimetals at low temperatures. Using the diagrammatic technique and the Keldysh model to sum up the leading terms in the diagrammatic expansion, we find that the linear magnetoresistance exhibits a strong renormalization due to the long-range nature of the Coulomb interaction ~HlnH As disorder becomes even stronger (but still in the parametric range, where the Coulomb interaction can be treated as a long-range one), we find that the magnetoresistivity becomes quadratic in the magnetic field ~H^2. In this work we studied transport properties of eutectic InSb-MnSb crystals with oriented MnSb needle-like inclusions under a hydrostatic pressure up to P = 8 GPa and after its application. We show that the pressure-induced variation of the samples resistivity and transport parameters at room temperature can be interpreted in the terms of crystal phase transition of the InSb matrix. We argue that the irreversibility of this transition, deduced from transport measurements, may stem from the composite nature of the crystals studied. The resistivity of the samples subjected to high pressures increases upon cooling, which can be a consequence of the increased disorder accompanying the irreversible transition. Magnetotransport data for these samples suggest that charge carriers interact with a low-temperature magnetic phase. The latter can be formed by isolated Mn atoms in the InSb matrix. A composite Cd0.95Ni0.05Sb crystal consisting of CdSb host material and NiSb inclusions was grown by the modified Bridgman method. Using the scanning electron microscopy, X-ray phase analysis, energy-dispersive X-ray spectroscopy, and the studies of Raman scattering spectra, we demonstrated that the CdSb host material was a single crystal (having the orthorhombic Pbca structure) containing microcrystalline needle-like inclusions of the second (NiSb) phase (having the hexagonal structure of the NiAs P63/mmc type). It has been found that the anisotropy of the single-crystalline NiSb needles in the Cd0.95Ni0.05Sb composite samples (with the needles directed along and across the applied electric current) produces a pronounced effect on the electrical resistance and magnetoresistance of the crystal. The interface between a needle-like inclusion and the host material exhibit a transition to the superconducting state, which affects the electrical conductivity of the composite as a whole. The contributions to the effective magnetic anisotropy of the constants of the first - K1 and second - K2 orders are distinguished, which are confirmed by the data of electron microscopic, energy dispersive elemental, X-ray diffraction analyzes and vibrational microspectrometry arising from the structuring of crystallized InSb-MnSb eutectic compositions in the form of acicular single-crystal MnSb inclusions in an InSb matrix in fields up to 50 kOe with a temperature change from 5 to 350 K. The dependences K1(T) and K2(T) are obtained, and the signs of K1 < 0, K2 > 0 are determined from the calculated temperature dependences of the saturation magnetization in the studied range of magnetic fields. It is shown that the rivalry between the contributions of K1 and K2 to the effective magnetic anisotropy constant determines the magnetic ordering of the “easy plane” type; extrapolation of K1(T) to the region of high temperatures at 530 K predicts the appearance of a spin-reorientation transition to the magnetic anisotropy of the “easy axis” type. The results obtained in this year were published in two papers (one of them in the journal Q1) and articles, which are currently under review in journals PRB, JMMM and ФТТ. We plan to prepare at least two papers to be submitted in 2023 year.

Publications

1. Kochura А.V., Dzhamamedov R.G., Mekhiya A.B., Oveshnikov L.N., Arslanov T.R., Rodionov V.V., Alam M., Kuzmenko A.P., Davydov A.B., Aronzon B.A. The effect of high pressure on the electrical and transport properties of the InSb-MnSb magnetic eutectic composition AIP Advances, Том 12, номер статьи 035330 (5 страниц) (year - 2022) https://doi.org/10.1063/9.0000296

2. Oveshnikov L.N., Ril' A.I., Mekhiya A.B., Davydov A.B., Marenkin S.F., Aronzon B.A. Low-field linear magnetoresistance and transport parameters of (Cd1−xMnx)3As2 polycrystals The European Physical Journal Plus, Том 137, номер статьи 374 (11 страниц) (year - 2022) https://doi.org/10.1140/epjp/s13360-022-02560-7

3. Oveshnikov L.N., Davydov A.B., Mekhiya A.B., Ril' A.I., Marenkin S.F., Aronzon B.A. Линейное магнетосопротивление в поликристаллах дираковского полуметалла (Cd1-xMnx)3As2 XV Российская конференция по физике полупроводников (Нижний Новгород, 2022), Тезисы докладов, Тезисы докладов, стр. 406 (year - 2022)

4. Oveshnikov L.N., Davydov A.B., Morgun L.A., Suslov A.V., Ril' A.I., Zakhvalinsii V.S., Kochura A.V., Pudalov V.M., Aronzon B.A. Магнетотранспорт в сверхпроводящих плёнках Cd3As2 XV Российская конференция по физике полупроводников (Нижний Новгород, 2022), Тезисы докладов, Тезисы докладов, стр. 415 (year - 2022)

5. Oveshnikov L.N., Kulatov E.T., Uspenskii Yu.A., Ril' A.I., Mekhiya A.B., Davydov A.B., Marenkin S.F., Aronzon B.A. Влияние атомов Mn на свойства дираковского полуметалла Cd3As2 XXIV Уральская международная зимняя школа по физике полупроводников (Екатеринбург, 2022) Тезисы докладов, Сборник тезисов, стр. 229 (year - 2022)

6. Oveshnikov L.N., Ril' A.I., Streltsov D.R., Karateev I.A., Davydov A.B., Mekhiya A.B., Aronzon B.A. Микроструктура и магнетотранспорт плёнок Cd3As2 с различным уровнем кристалличности XIX Конференция «Сильно коррелированные электронные системы и квантовые критические явления» (Москва, 2022) Сборник тезисов, Сборник тезисов, стр. 111-112 (year - 2022)