M. P. Volkov

Pseudogap and its temperature dependence in YBCO from the data of resistance measurements

The temperature dependence of the excess conductivity Δσ for Δσ = A(1 − T/T*)exp(Δ*/T) (YBCO) epitaxial films is analyzed. The excess conductivity is determined from the difference between the normal resistance extrapolated to the low-temperature range and the measured resistance. It is demonstrated that the temperature dependence of the excess conductivity is adequately described by the relationship Δσ = A(1 − T/T*)exp(Δ*/T). The pseudogap width and its temperature dependence are calculated under the assumption that the temperature behavior of the excess conductivity is associated with the formation of the pseudogap at temperatures well above the critical temperature Tc of superconductivity. The results obtained are compared with the available experimental and theoretical data. The crossover to fluctuation conductivity near the critical temperature Tc is discussed.

Surface states of charge carriers in epitaxial films of the topological insulator Bi2Te3

The galvanomagnetic properties of p-type bismuth telluride heteroepitaxial films grown by the hot wall epitaxy method on oriented muscovite mica substrates have been investigated. Quantum oscillations of the magnetoresistance associated with surface electronic states in three-dimensional topological insulators have been studied in strong magnetic fields ranging from 6 to 14 T at low temperatures. The cyclotron effective mass, charge carrier mobility, and parameters of the Fermi surface have been determined based on the results of analyzing the magnetoresistance oscillations. The dependences of the cross-sectional area of the Fermi surface S(kF), the wave vector kF, and the surface concentration of charge carriers ns on the frequency of magnetoresistance oscillations in p-type Bi2Te3 heteroepitaxial films have been obtained. The experimentally observed shift of the Landau level index is consistent with the value of the Berry phase, which is characteristic of topological surface states of Dirac fermions in the films. The properties of topological surface states of charge carriers in p-type Bi2Te3 films obtained by analyzing the magnetoresistance oscillations significantly expand fields of practical application and stimulate the investigation of transport properties of chalcogenide films.

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge Systems at Centrifugation

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge systems at centrifugation is studied. Of them, the Al-Si system is the least prone to sedimentation. In the others, sedimentation considerably changes the structure of the alloys at the bottom of the ingots compared with their top. At certain concentrations of the constituents, the number of crystallites in the lower part of the ingot is larger than in the upper part and the crystallites at the bottom are coarser than at the top. The Si: Ge atomic ratio in the Al-Si-Ge system changes by a factor of 2–12 against the initial ratio (1: 1) when the (Si + Ge) concentration changes as a result of centrifugation. Also, this ratio changes over the crystal surface (in the samples not subjected to centrifugation, this ratio remains unchanged over the surface). Crystallites in the Al-Si-Ge system are covered by Ge.

Structure and mechanical properties of Al-Si(Ge) alloys upon melt centrifugation and quenching

The structures of eutectic binary Al-12.7 at. % Si and Al-29.7 at. % Ge alloys and a ternary Al-10 at. % Si-10 at. % Ge alloy produced by quenching levitated melts or through solidification either in the presence or in the absence of a centrifugal acceleration of 7g are studied. Centrifugation is found to cause an increase in the silicon content in the Al-Si alloy in the direction opposite to the direction of centrifugal acceleration and an increase in the germanium content in the Al-Ge alloy in the direction of centrifugal acceleration. These differences are explained by the fact that the densities of silicon and germanium clusters and solidification centers differ from the liquid-phase density at temperatures of solidification. The related changes in the values of the Young’s modulus and in the stress-strain curves can be due to sedimentation-induced changes in the composition of samples cut from the middle part of an ingot. The processes of decomposition and recovery are shown to have a substantial effect on the elastic moduli of these alloys.

Thermoelectric and galvanomagnetic properties of bismuth chalcogenide nanostructured heteroepitaxial films

Hot wall technique was used to grow block single crystal films of Bi2Te3 and solid solutions of Bi0.5Sb1.5Te3 on mica (muscovite) substrates. X-ray diffraction studies demonstrated that the crystalline c-axis in the films was normal to the substrate plane. Seebeck coefficient, electrical conductivity and magnetoresistivity tensor components were measured at various orientations of magnetic and electric fields in the temperature interval 77–300 K. Scattering mechanism of charge carriers in the films was studied using temperature dependences of the degeneracy parameter and the Seebeck coefficient in terms of a many-valley model of energy spectrum. Obtained results have shown that the effective scattering parameter is considerably different from the value specific for an acoustic scattering of charge carriers in the weakly degenerate films due to an additional scattering of charge carriers on interface and interctystallite boundaries. These features of charge carrier scattering are supposed to affect electronic transport in the films and enhance figure of merit.

Magnetoresistance of La0.67Ca0.33MnO3 films the coherent growth of which is disturbed by mechanical stress relaxation

Because of a large (m = 1.8%) lattice mismatch between La0.67Ca0.33MnO3 and LaAlO3, manganite films grown on a lanthanum aluminate substrate experience biaxial mechanical compression stresses. Strong adhesion to the substrate causes a substantial tetragonal distortion (γ ≈ 1.04) of the unit cell in a 20-nm-thick layer of the manganite film coherently grown on (001)LaAlO3, while in the remaining part (≈75%) of the manganite film, stresses partially relax. The stress relaxation decreases γ and increases the effective volume of the unit cell of the La0.67Ca0.33MnO3 film. The relaxed part of the La0.67Ca0.33MnO3 film consists of crystallites 50–200 nm across azimuthally misoriented by approximately 0.3°. The temperature dependences of the resistivity and negative magnetoresistance of the manganite films exhibit maxima at 240 and 215 K, respectively. At temperatures below 50 K, the dependence of the resistivity on the magnetic induction taken with the induction varying from 0 to 14 T and vice versa becomes hysteresis.

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi0.93Sb0.07 alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi0.93Sb0.07 semiconductor alloys with electron concentrations in the range 1 × 1016 cm−3 < n < 2 × 1018 cm−3. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħωc/2 ≥ EF, the electrical conductivity increases with an increase in the magnetic field: σ22(H) ∼ Hk. A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ22(H) ≤ σ21(H).

Short-term synthesis of poly- and single-crystalline iron monochalcogenides

Poly- and single-crystalline samples of the Fe(Se,Te)1 − δ system have been synthesized for short times by annealing bulk iron in chalcogen vapors and by reacting powdered iron with chalcogen melts. The lattice unit cell parameters and critical temperatures (Tc) of the superconducting transition in the synthesized compounds agree with published data for the materials obtained by long-term synthesis using other methods. For Fe(Se0.8Te0.2)0.82 block crystals, the temperature dependence of the second critical field in the vicinity of Tc is linear with a slope of −4.9 T/K.

Redistribution of electrons between ellipsoids in a magnetic field in the quantum limit range in n-Bi-Sb alloys

The magnetoresistivities ρ22(H) and ρ32(H) and the Hall coefficient R32.1 for single-crystal samples of the n-Bi0.93Sb0.07 semiconducting alloy have been measured at low temperatures in magnetic fields up to H=14 T at H∥C2. The samples with three electron concentrations n1=1.25 × 1016 cms-3, n2=3.5×1016 cms-3, and n3=1.6×1017 cms-3 have been studied. The strong anisotropy of the electron spectrum of the alloys has made it possible to observe quantum oscillations of the magnetoresistivity ρ22(H) at H∥C2 for electrons of the secondary ellipsoids with the transition to the quantum limit in high magnetic fields. However, in the same magnetic fields, the quantization condition for electrons of the main ellipsoid is not satisfied. An increase in the energy of electrons of the secondary ellipsoids in the magnetic fields of the quantum limit leads to their migration to the main ellipsoid. After the complete migration, the Fermi energy for the alloy samples with the electron concentrations n1, n2, and n3 increases from 7.0 to 11.3 meV, from 11.0 to 17.1 meV, and from 20.2 to 30.6 meV, respectively. After the migration, the magnetoresistivity for electrons of the main ellipsoid increases with an increase in the magnetic field and the specific features in the behavior of the kinetic coefficients are observed in the vicinity of the magnetic field H=10 T. Therefore, the electronic topological transition from the three-valley electron spectrum to the single-valley electron spectrum occurs in the Bi0.93Sb0.07 single crystals for H∥C2 at low temperatures in the range of magnetic fields of the quantum limit.

Beats of quantum oscillations of the hall coefficient and resistivity in semiconductor alloys n-Bi0.93Sb0.07 at a small deviation of the magnetic field direction from the trigonal axis

Quantum oscillations of the resistivity ρ22 and Hall coefficient R12.3 in the semiconductor alloy n-Bi0.93Sb0.07 have been studied at H ‖ C3 and j ‖ C1 in magnetic fields to 14 T and at temperatures of 1.5, 4.5, 10, and 20 K. At temperatures of 1.5 and 4.5 K, beats of quantum oscillations of ρ22 and R12.3 due to a small deviation of the magnetic field H from the crystallographic C3 axis have been observed. To determine the oscillation period Δi, cyclotron mass mci, cyclotron frequency ωci, and extreme section Sextri, experimentally measured quantum oscillation beats have been compared with the model beats of oscillations of three harmonic functions, two of which have close frequencies. The deviation of the parameters Δi, mci, and Sextri from the same parameters when the magnetic field H exactly coincides with the trigonal C3 axis has made it possible to estimate the magnetic field H deflection angle from the trigonal C3 axis, which is ∼1°.