V. A. Danilov

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.

Electrical resistivity and magnetotransport in La0.67Ba0.33MnO3 films asymmetrically biaxially compressed by an NdGaO3(001) substrate

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 ρ2T4.5 and the coefficient ρ2 decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈−0.17 for μ0H = 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).

Response of the electric resistance of 40-nm-thick La0.67Ca0.33MnO3 films to an increase in the lattice mismatch between film and substrate

We have studied the structure and resistivity of 40-nm-thick La0.67Ca0.33MnO3 (LCMO) films coherently grown on (001)-oriented La0.29Sr0.71Al0.65Ta0.35O3 single crystal substrates bearing epitaxial interlayers of strontium titanate with a thickness of 7 or 70 nm. As the effective mismatch between the crystal lattice parameters of the film and substrate increases, the ρ value exhibits a sharp growth, while the maximum of the ρ(T) curve shifts by ∼40 K toward lower temperatures. At T < 150 K, the temperature dependence of the resistivity of the LCMO films obeys the relation ρ ∼ ρ1T4.5, where the coefficient ρ1 decreases with increasing applied magnetic field strength and with decreasing lattice mismatch between the manganite film and the substrate.

Interfacial capacitance in epitaxial heterostructures La0.67Ca0.33MnO3/SrTiO3/La0.67Ca0.33MnO

Epitaxial trilayer heterostructures of the type La0.67Ca0.33MnO3/SrTiO3/La0.67Ca0.33MnO3 were grown by laser ablation on (001)[(LaAlO3)0.3+(Sr2AlTaO6)0.7] substrates. The real part of the dielectric permittivity ε and the loss factor tan δ of a 1100-nm-thick SrTiO3 interlayer were studied in the temperature interval T=4.2–300 K in a nonbiased state and at a bias voltage of ±2.5 V applied to the manganite electrodes. Using the temperature dependence ε(T) measured for the SrTiO3 layer grown between the manganite electrodes, we have estimated the capacitance of La0.67Ca0.33MnO3/SrTiO3 interfaces (C1≈2 μF/cm2) related to the electric field penetrating from the interlayer into La0.67Ca0.33MnO3.

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.

Anisotropic magnetoresistance of partially relaxed SrRuO3 films

The SrRuO3 films (50 nm thick) grown by laser evaporation on (001)(LaAlO3)0.3 + (Sr2AlTaO6)0.7 substrates were under partially relaxed biaxial compressive mechanical stresses. The films consisted of crystallites with lateral dimensions of 40–100 nm and a relative azimuthal misorientation of about 0.9°. Ferromagnetic ordering of spins in the SrRuO3 films was manifested by a change in the slope of the temperature dependence of their electrical resistivity ρ at T ≈ 155 K. For a magnetic field H parallel to the measuring current, the maximum values (∼7.5%) of the magnetoresistance MR = [ρ(μ0H = 5 T) − ρ(μ0H = 0)]/ρ(μ0H = 0) were observed at temperatures of about 100 K. At T = 95 K (μ0H = 5 T), the anisotropic magnetoresistance of the films was 8% and increased by a factor of approximately 1.5 with decreasing temperature to 4.2 K.

Response of the capacitance of a planar La0.67Ca0.33MnO3/SrTiO3/La0.67Ca0.33MnO3 heterostructure to an electric field

Manganite film electrodes were integrated with a spacer layer of strontium titanate to produce an epitaxial La0.67Ca0.33MnO3/(1000 nm)SrTiO3/La0.67Ca0.33MnO3 (LCMO/STO/LCMO) heterostructure by laser ablation. At T = 300 K, the mechanical stresses in the STO layer relaxed to a considerable extent, while the LCMO electrodes were found to be under biaxial lateral tensile strain, with the lattice unit cell of the top electrode distorted considerably stronger (a∥/a⊥ ≈ 1.026) than that of the bottom electrode (≈1.008) (a∥ and a⊥ are the unit cell parameters in the substrate plane and along the normal to its surface, respectively). The reciprocal of the capacitance C of the plane-parallel LCMO/STO/LCMO film capacitors thus formed increased almost linearly with increasing temperature T in the range 50–250 K. At T < 100 K, the capacitance C decreased by approximately 50% in an electric field E = 40 kV/cm. After the electric field E was varied as 0 → + 100 kV/cm → 0, the capacitance C decreased by approximately 3% and the maximum in the C(E, T > 200 K) dependence shifted by approximately 9 kV/cm with respect to the point E = 0.

Electrophysical parameters of c-oriented Bi2Te3 films with a low concentration of antistructural defects

Epitaxial c-oriented Bi2Te3 films 1.2 μm in thickness are grown by the hot wall method for a low supersaturation of the vapor phase over the surface of mica substrates. The hexagonal unit cell parameters a = 4.386 Å and c = 30.452 Å of the grown films almost coincide with the corresponding parameters of stoichiometric bulk Bi2Te3 crystals. At T = 100 K, the Hall concentration of electrons in the films is on the order of 8 × 1018 cm−3, while the highest values of the thermoelectric coefficient (α ≈ 280 μV K−1) are observed at temperatures on the order of 260 K. Under impurity conduction conditions, conductivity σ of the films increases upon cooling in inverse proportion to the squared temperature. In the temperature range 100–200 K, thermoelectric power parameter α2σ of Bi2Te3 films has values of 80–90 μW cm−1 K−2.

Response of the electrical resistance of La0.67Ca0.33MnO3(40 nm) films mechanically compressed by the substrate in the course of their formation to electric and magnetic fields

The structure and electrical resistance of La0.67Ca0.33MnO3(40 nm) epitaxial films grown quasicoherently on the surface of LaAlO3(001) substrates are investigated. Compressive mechanical stresses that are active in the substrate plane during nucleation and growth encourage a decrease in the effective unit cell volume and an increase in the relative concentration of tetravalent manganese ions in the manganite layers. This leads to a decrease in the temperature of the maximum in the temperature dependence of the electrical resistivity of the films by approximately 90 K compared to the Curie temperature for the corresponding stoichiometric bulk crystals. It is found that, at T < 120 K and μ0H = 0 (where H is the magnetic field strength), the measuring current depends nonlinearly on the voltage Vb applied to the contacts. An increase in the applied voltage Vb and in the magnetic field strength H favors linearization of the current-voltage characteristics of the films.