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1.
NMR studies of 125Te in the topological insulator bismuth telluride Bi2Te3 in a wide temperature range from room temperature to 12.5 K are performed. The pulsed NMR spectrometer Bruker Avance 400 is applied. The NMR spectra are obtained for the powder from Bi2Te3 single crystal and monocrystalline plates with the orientations c || B and cB. At room temperature, the spectra consist of two lines related to two nonequivalent positions of tellurium nuclei Te1 and Te2. The parameters of the NMR frequency shift tensor are found from the powder spectrum. The temperature dependences of the spectra for the powder and plates with the orientation c ⊥ B agree with each other. The line shift with decreasing temperature is explained by the reduction of the Knight shift. The thermal activation energy of charge carriers is estimated. The spectra for the plates with the orientation c || B demonstrate peculiar behavior below 91 K. The spin-lattice relaxation time for the powder and monocrystalline plates with both orientations at room temperature is measured.  相似文献   

2.
77Se nuclear magnetic resonance (NMR) measurements in the Bi2Se3 topological insulator single crystal were carried out at temperatures 15.8, 88, and 293 K. Bismuth selenide single crystalline plate was studied in the orientation when the crystallographic c-axis was parallel to the external magnetic field B0. We observed two component NMR spectra at the three temperatures. It was shown that the NMR spectrum almost did not move with decreasing temperature and the density of charge carriers did not follow the thermal activation law.  相似文献   

3.
The magnetic properties of LiCu2O2 single-crystal samples without twinning are investigated using electron spin resonance and nuclear magnetic resonance spectroscopy. The experimental results obtained are described in terms of the model of a planar spiral antiferromagnet for the orientation of the magnetic field Hb or Hc and the model of a collinear spin-modulated antiferromagnet for the orientation of the static magnetic field Ha.  相似文献   

4.
The effect of an external magnetic field on permittivity has been studied in a Cu3B2O6 single crystal with a layered structure in the direction perpendicular to layers (bc-planes). It has been found that the appreciable magnetodielectric effect in the temperature range below the Néel temperature (≈10 K) takes place only at one magnetic field orientation H and one crystallographic direction, i.e., H || b. Such “selectivity” of the magnetodielectric effect correlates with the anisotropic behavior of magnetic properties of the crystal.  相似文献   

5.
Anisotropy of the magnetic properties of Sm0.55Sr0.45MnO3 single crystals has been studied. A significant increase in the antiferromagnetic component of magnetization in the case of orientation of an external magnetic field H close to the c axis has been found. Magnetization for a field lying in the ab plane seems typical of a ferromagnet. Anisotropy of susceptibility reaches 2.2 in weak fields and nearly vanishes at H > 1 T.  相似文献   

6.
Reactions between superdislocations involved in deformation in the basal, prismatic, and type-I and II pyramidal planes in single-crystal Ti3Al are considered. The types of dislocation interactions are established that result in the formation dislocation barriers (microcrack nuclei). The force and energy conditions for microcracks to arise are found. The interaction between a and 2c + a superdislocations results in microcracks with the plane of opening lying in basal and pyramidal planes; the interaction of 2c + a superdislocations in different pyramidal planes results in the formation of microcracks in prismatic and pyramidal planes; and the interaction of a superdislocations in basal and/or pyramidal planes does not cause the formation of dislocation barriers. The types of microcracks are classified in terms of the orientation of deformation axes of single crystals, and the regions of the stereographic triangle are determined characterized by a preferential type of crack opening.  相似文献   

7.
The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr2RuO4 single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρa) and that along the c axis (ρc), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (Ha and Hc), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρa(T) and ρc(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (~10m e , where m e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρa and ρc. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρa, c(T) ∝ 1/T type.  相似文献   

8.
Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS3) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization Ec is described by the Urbach rule. For the crystal in the polarization Eb, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ0, related to the electron-phonon coupling constant, and the energy ?ωph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS3 are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS3, as well as the topological disorder of glassy PbGeS3, have been estimated.  相似文献   

9.
The dynamics of the crystal lattice of RFe3(BO3)4 (R = Pr, Nd, Sm, Gd, Tb, Dy, and Ho) compounds in the high-symmetry R32 phase has been calculated. Significant changes in spectra of compounds with various rare-earth ions have been obtained only near the edge Λ point of the Brillouin zone (qΛ = 1/3(?2b1 + b2 + b3, where b1, b2, and b3 are the reciprocal lattice vectors) for acoustic oscillation branches. A decrease in the frequency of an acoustic mode at the point Λ has been revealed in all studied compounds. This frequency depends on the type of rare-earth ion and decreases from a compound with Pr to a compound with Ho down to imaginary values. Such a behavior of the frequency of the unstable acoustic mode is in good agreement with experimental data on the dependence of the temperature of the R32 → P3121 structural phase transition on the type of rare-earth ion in ferroborates.  相似文献   

10.
The transmission electron microscopy was used to examine the dislocation structure of intermetallic Ti3Al after deformation at temperatures T = 1073–1273 K. It is established that its microstructure contains mobile 2c + a and superdislocations. Possible models describing the destruction of barriers associated with 2c + a superdislocations in pyramidal planes are discussed using the results of computer simulation of the superdislocation core structure in Ti3Al.  相似文献   

11.
Optical absorption of MnGa2S4 single crystals is studied at two light polarizations (E ||C and EC). The polarization splitting of the absorption edge points to a splitting of the valence band of MnGa2S4. A contribution to the crystal-field splitting is made by two factors, namely, by a difference in the pseudopotential of cationic sublattice atoms and by tetragonal compression of the lattice along the C axis. A scheme of optical transitions in MnGa2S4 in the Brillouin zone center is suggested, according to which the optical transitions Г3 + Г4 → Г1 occur in the polarization EC, and the Г2 → Г1 transitions occur in the polarization E || C.  相似文献   

12.
In the absorption spectra of the hexagonal single-crystal manganite HoMnO3 in the paramagnetic ferroelectric state, lines near 1.1 and 2.0 μm were observed associated with the transitions 5 I 85 I 6 and 5 I 85 I 7, respectively, within the 4f 10 configuration of the Ho3+ ion. At T = 80 K, to the 5 I 85 I 7 transition corresponds one band at 1.9 μm for both polarizations Ec and Ec. As the temperature increases from 80 to 293 K, a low-energy band with a peak at 2.04 μm for Ec and a peak at 2.07 μm for Ec arises associated with transitions from an excited Stark level of the ground 5 I 8 multiplet to the Stark levels of the 5 I 7 multiplet and with an increase in the population of the initial Stark level, the energy of which is ~100 K.  相似文献   

13.
The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe3(BO3)4 single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial (lc), while, in a strong magnetic field Hc (H cr ~ 43 kOe at T = 4.2 K), a Fe3+ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostriction, and electric polarization. The threshold field values determined in the temperature interval 2–32 K are used to plot an H-T phase diagram. The contribution of the Pr3+ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.  相似文献   

14.
The magnetic and magnetodielectric properties of Ho0.5Nd0.5Fe3(BO3)4 ferroborate with the competing Ho–Fe and Nd–Fe exchange couplings have been experimentally and theoretically investigated. Step anomalies in the magnetization curves at the spin-reorientation transition induced by the magnetic field Bc have been found. The spontaneous spin-reorientation transition temperature TSR ≈ 8 K has been refined. The measured magnetic properties and observed features are interpreted using a single theoretical approach based on the molecular field approximation and calculations within the crystal field model of the rare-earth ion. Interpretation of the experimental data includes determination of the crystal field parameters for Ho3+ and Nd3+ ions in Ho0.5Nd0.5Fe3(BO3)4 and parameters of the Ho–Fe and Nd–Fe exchange couplings.  相似文献   

15.
The atom-vacancy ordering of cubic vanadium monoxide VO1.29, which has basis cubic structure B1 and structural vacancies in the metal sublattice, has been studied using the x-ray diffraction method. It has been shown that the formation of the tetragonal (space group I41/amd) ordered phase V52O64 of cubic vanadium monoxide VOy proceeds as a first-order phase transition through the disorder-order channel including 22 nonequivalent superstructure vectors of four stars {k 10}, {k 4}, {k 3}, and {k 2}. The distribution function of the vanadium atoms in the V52O64 tetragonal superstructure has been calculated.  相似文献   

16.
The La0.67Ba0.33MnO3(40 nm) films are quasi-coherently grown on an NdGaO3(001) substrate with an orthorhombic unit cell distortion of ~1.4%. The biaxial compressive stresses generated during nucleation and growth lead to a decrease in the unit cell volume of the grown layers. This, in turn, results in a decrease (by ~35 K) in the temperature of the maximum in the dependence of the electrical resistivity ρ of the layers on the temperature. For T < 150 K, the electrical resistivity ρ of the films increases in proportion to ρ2 T 4.5 and the coefficient ρ2 decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈?0.17 for μ0 H = 1 T) reaches a maximum at temperatures close to room temperature. The response of the electrical resistivity ρ of the La0.67Ba0.33MnO3(40 nm) films to the magnetic field depends on the crystallographic direction of the film orientation and the angle between H and I (where I is the electric current through the film).  相似文献   

17.
We study topological properties of phase transition points of two topologicallynon-trivial Z2 classes (D and DIII) in one dimension byassigning a Berry phase defined on closed circles around the gap closing points in theparameter space of momentum and a transition driving parameter. While the topologicalproperty of the Z2 system is generally characterized by aZ2topological invariant, we identify that it has a correspondence to the quantized Berryphase protected by the particle-hole symmetry, and then give a proper definition of Berryphase to the phase transition point. By applying our scheme to some specific models ofclass D and DIII, we demonstrate that the topological phase transition can be wellcharacterized by the Berry phase of the transition point, which reflects the change ofBerry phases of topologically different phases across the phase transition point.  相似文献   

18.
High-frequency losses in the strongly anisotropic layered superconductor Bi2Sr2CaCu2O8 are measured at 600 MHz under a magnetic field rocking about the ab plane. Anomalies in losses and hysteretic phenomena are found while performing periodic rocking, i.e., cycling the magnetic field component normal to the sample surface. Based on these observations, conclusions are drawn about the nature of magnetic-flux penetration into the superconductor. It is found that, in the range between 60 K and T c , the dynamics of magnetic-flux vortex lines normal to the ab plane in the presence of a constant magnetic field applied parallel to this plane is governed by the critical penetration field H c ⊥* and the surface barrier in the presence of thermally activated vortex motion (giant flux creep). The dependences of H c1 ⊥* and the characteristic field of the surface barrier on the magnitude of the parallel magnetic field are measured.  相似文献   

19.
The equation of the magnetization of a hexagonal crystal is derived for the first time for an arbitrary orientation of the external magnetic field relative to the crystallographic c axis. In order to clarify the magnetization mechanism for a real ensemble of small particles in the framework of the given problem, surface anisotropy (which is significant for nanosize objects) was taken into account along with crystalline magnetic anisotropy and anisotropy in the particle shape. Model computer experiments prove that the magnetization curves for nanocrystals oriented in a polar angle range of 65–90° exhibit an anomaly in the form of a jump, indicating a first-order spin-reorientation phase transition. This explains a larger steepness of the experimental curve reconstructed taking into account the interaction between particles as compared to the theoretical dependence obtained by Stoner and Wohlfarth [IEEE Trans. Magn. MAG 27 (4), 3469 (1991)]. An analysis of variation of the characteristic anisotropy surface and its cross section with increasing ratio |K2|/K1 of the crystalline magnetic anisotropy constants upon a transition from a macroscopic to a nanoscopic crystal shows that surface anisotropy leads to a change in the magnetic structure. As a result, an additional easy magnetization direction emerges in the basal plane apart from the easiest magnetization direction (along the c axis). The direction of hard magnetization emerges from the basal plane, the angle of its orientation relative to the c axis being a function of the ratio | K2|/K1.  相似文献   

20.
Structural neutron diffraction studies indicate that only one ordered phase arises after the disorder-order transition in nonstoichiometric cubic tantalum carbide TaCy. This phase arises in the composition range y = 0.79–0.89 due to long-term annealing with a decrease in temperature from 1600 to 300 K. It is incommensurate in the [1–11]B1 direction, but it is close to commensurate M6C5 structures (C2/m and P31 space groups) in mutual arrangement of atoms and vacancies in nonmetallic (1–11)B1 planes. The disorder-order transition channel that is associated with the formation of the incommensurate superstructure in TaCy carbide includes two arms k 5 (6) ≈ 0.473b2 and k 5 (5) = ?k 5 (6) of the {k5} star and arms of the {k4} and {k3} stars. The translation period of the incommensurate phase in the [1–11]B1 direction is 8.9–9.1 nm, which is larger than that in the commensurate phase M6C5 by a factor of about 18.  相似文献   

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