O. A. Usov

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.

Ceramic materials for use in microwave electronics

A study is reported of the structure and electrical properties of BaLn2Ti4O12 compositions (BLT), where Ln=La, Nd, or Sm, with calcium, strontium, or lead substituted for barium, and bismuth for the lanthanide. The BLT compound is shown to be characterized by a broad isomorphicity region, at the edge of which form solid solutions with the highest dielectric permittivity reached thus far, up to about 170. A number of thermally stable ceramic materials with a high dielectric permittivity of 80 to 120, which permit construction of microwave dielectric resonators with Q factors of up to 600 for ɛ=120 and up to 3000 for ɛ=80 at f=4 GHz, have been developed based on BLT solid solutions.

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.

Titanium K-edge absorption structure in Ti1−xNbxO2

The fine structure in the titanium x-ray K-edge absorption has been measured in Ti1−xNbxO2 mixed dioxides (x=0–0.1) with rutile structure in a laboratory-type spectrometer by total electron quantum-yield measurement. The position of the XANES lines is shown to be in good agreement with classical x-ray absorption spectra obtained in transmission. The structure and main features of the XANES spectra, including the effects of impurities and manyelectron excitations, are discussed. It is suggested that the intensity of the B peak characteristic of the titanium K edge depends on the Nb concentration and correlates with the charge state of titanium ions.

Surface morphology and Raman spectroscopy of thin layers of antimony and bismuth chalcogenides

The phonon spectra in thin layers of bismuth telluride and solid solutions of Bi2–xSbxTe3–ySey of different composition, belonging to three-dimensional topological insulators, have been investigated by micro-Raman spectroscopy, and the morphology of an interlayer van der Waals (0001) surface in them has been studied by semicontact atomic force microscopy at room temperature. The analysis of the Raman spectra and the intensity ratio of active and inactive longitudinal optical modes depending on the composition, morphology of the interlayer surface, and thickness of the layers enabled the estimation of the effect of topological surface states of Dirac fermions, associated with the strengthening of the electron–phonon interaction as a result of resonance Raman scattering, and the identification of the compositions, in which the contribution of topological surface states becomes dominant.

Optical studies of a two-dimensional photonic crystal with the InAs/InGaAs quantum-dot structure as an active region

A two-dimensional semiconductor photonic crystal with a hexagonal lattice of submicrometer holes is produced by etching the GaAs/AlGaAs planar structure containing the InAs/InGaAs quantum dots in the waveguide layer. By analyzing the reflectance spectra at variable angles of incidence and polarizations of light, the photonic band structure is determined. The Fano resonance peaks revealed in the reflectance spectra at the TM (TE) polarization along the Γ-K (Γ-M) symmetry direction are due to the resonance interaction of optically active photonic bands with the incident light. The band structure of the radiation leakage modes is investigated by studying the angular dependence of the photoluminescence intensity. A threefold increase in the photoluminescence intensity revealed at the resonance frequency of the photonic crystal is attributed to the Purcell effect.

Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi2Te3 heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.

On the morphology of the interlayer surface and micro-Raman spectra of layered films in topological insulators based on bismuth telluride

Resonant micro-Raman spectra and the morphology of the interlayer Van der Waals surface are studied for layered thin films of n-Bi2Te3 and solid solutions based on Bi2Te3. It is found that the composition, thickness, surface morphology, and the method of obtaining films affect the relative intensity of Raman phonons, which are sensitive to the topological surface states of Dirac fermions.

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi0.5Sb1.5Te3 solid solution

It is shown that the Seebeck coefficient α, the power factor α2σ, and the density-of-states effective mass m/m0 in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α2σ, and m/m0 is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.

Cathodoluminescence studies of C60 fullerene-based films and nanostructures

The objects of investigation in this work are submicron Cm fullerene-based films and fullerene nanostructures fabricated by electron-beam lithography. The set of techniques for studying the fullerenes is extended due to attraction of the spectral cathodoluminescence (CL) to either of the initial fullerene films or submicron net structures with period of about 0.9 μm, wall height 0.6 μm, and width about 0.4 μm.The maxima of the CL spectra of the initial C60 films are in good agreement with the energy-band structure of the amorphous fullerene film. In turn, the CL spectrum of the fullerene net structures possesses both peculiarities of the fullerene energy-band structure and it clearly exhibits the oscillations of the spectrum with a period as large as 0.08 eV. Such periodical fullerence net structures possess the properties of an optical resonator on the system “fullerene net (n = 2.3)-air (n = 1).”