NMR studies of single crystals of the topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> at low temperatures

A powder sample and single-crystal plates of the topological insulator Bi₂Te₃ have been investigated using the ¹²⁵Te NMR method at room temperature and at low temperatures in the range from 12.5 to 16.5 K. The NMR spectra of the single-crystal plates have been studied in the orientation where the crystallographic axis c is directed parallel or perpendicular to the magnetic field. The spectra have been obtained by means of recording spin-echo signals and plotting their envelopes. It has been shown that the NMR spectra for the bismuth telluride powder and plates with the orientation c ⊥ B consist of two lines, which are presumably attributed to tellurium nuclei in two crystallographic positions in the bulk of the sample. The position and shape of the lines are determined by the chemical shift and the Knight shift. For the orientation of the plates c || B, the spectrum contains an additional component in the high-frequency region, which cannot appear due to the angular dependence of the line shifts caused by tellurium nuclei in the bulk of the topological insulator. At a low temperature, the additional line dominates in the spectrum.     

Ab initio study of 2DEG at the surface of topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

By means of ab initio DFT calculation, we analyze the mechanism that drives the formation and evolution of the 2D electron gas (2DEG) states at the surface of Bi₂Te₃ topological insulator (TI). As it has been proved earlier it is due to an expansion of the van der Waals (vdW) spacing produced by intercalation of adsorbates. We will show that the effect of this expansion, in this particular surface, leads to several intriguing phenomena. On one hand we observe a different dispersion of the Dirac cone with respect to the ideal surface and the formation of Parabolic Bands (PB) below the conduction band and M-shaped bands in the valence band, the latters have been observed recently in photoemission experiments. On the other hand the expansion of the vdW gaps changes the symmetry of the orbitals forming the Dirac cone and therefore producing modifications in the local spin texture. The localization of these new 2DEG-states and the relocalization of the Dirac cone will be studied as well.     

Optical functions of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the range of plasmon excitation and interband transitions

The most narrow sub-Doppler frequency resonances in the linear absorption of monochromatic radiation that propagates in the normal direction through a cell containing a layer of rarefied gas medium with a thickness smaller than or on the order of the wavelength of this radiation are theoretically studied. The calculation is performed using as an example a three-dimensional gas cell shaped like a rectangular parallelepiped. It is shown that the width and amplitude of considered sub-Doppler resonances (in the vicinity of centers of rather weak quantum transitions) significantly depend on the transit relaxation of atomic particles, which is determined by their transit times through the irradiated region of the cell both in longitudinal and in transverse directions. The restrictions of the approximation of the planar one-dimensional cell that was previously used in such calculations are determined. Possible applications of linear absorption resonances in ultrathin (nanometer) gas cells as references for optical frequency standards are discussed.     

Specific features of the electronic,spin, and atomic structures of a topological insulator Bi<Subscript>2</Subscript>Te<Subscript>2.4</Subscript>Se<Subscript>0.6</Subscript>

The specific features of the electronic and spin structures of a triple topological insulator Bi₂Te_2.4Se_0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi₂Te_2.4Se_0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi₂Te_2.4Se_0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 Å. Stability of the Dirac cone of the Bi₂Te_2.4Se_0.6 compound to deposition of a Pt monolayer on the surface is shown.     

Quantum interference effects in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> topological insulator nanowires with variable cross-section lengths

Stimulated by the recent realization of three dimensional topological insulator nanowireinterferometers, a theoretical analysis of quantum interference effects on the low energyspectrum of Bi₂Se₃ nanowires is presented. The electronic properties areanalyzed in nanowires with circular, square and rectangular cross-sections starting from acontinuum three dimensional model with particular emphasis on magnetic and geometricaleffects. The theoretical study is based on numerically exact diagonalizations of thediscretized model for all the geometries. In the case of the cylindrical wire, anapproximate analytical solution of the continuum model is also discussed. Although amagnetic field corresponding to half quantum flux is expected to close the band gapinduced by Berry phase, in all the studied geometries with finite area cross-sections, thegap closes for magnetic fields typically larger than those expected. Furthermore,unexpectedly, due to geometrical quantum interference effects, for a rectangular wire witha sufficiently large aspect ratio and smaller side ranging from 50 Å and 100 Å, the gapcloses for a specific finite area cross-section without the application of a magneticfield.     

Stability of the (0001) surface of the Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> topological insulator

The inertness of the cleaved (0001) surface of a Bi₂Se₃ single crystal to oxidation has been demonstrated using X-ray photoelectron spectroscopy, as well as atomic-force and scanning tunneling microscopy and spectroscopy. No intrinsic bismuth and selenium oxides are formed on the surface after a month of storage in air. Atomically flat surfaces with macroscopic sizes (∼1 cm²) and rms roughness less than 0.1 nm have been prepared, and (1 × 1)-(0001) Bi₂Se₃ atomic structure has been resolved. The tunneling conductance measurements have shown that the energy dependence of the surface density of states is quasilinear in the band gap of Bi₂Se₃.     

Thermal sensors based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> and (Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>)<Subscript>70</Subscript>(Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>)<Subscript>30</Subscript> thin films

Thin films of Sb₂Te₃ and (Sb₂Te₃)₇₀(Bi₂Te₃)₃₀ alloy and have been deposited on precleaned glass substrate by thermal evaporation technique in a vacuum of 2?×?10^?6 Torr. The structural study was carried out by X-ray diffractometer, which shows that the films are polycrystalline in nature. The grain size, microstrain and dislocation density were determined. The Seebeck coefficient was determined as the ratio of the potential difference across the films to the temperature difference. The power factor for the (Sb₂Te₃)₇₀ (Bi₂Te₃)₃₀ and (Sb₂Te₃) is found to be 19.602 and 1.066 of the film of thickness 1,500 Å, respectively. The Van der-Pauw technique was used to measure the Hall coefficient at room temperature. The carrier concentration was calculated and the results were discussed.     

Magnetic Resonance Study of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Crystals of 3D topological insulators, bismuth telluride Bi₂Te₃, doped with manganese were studied using electron spin resonance (ESR) spectroscopy together with the SQUID magnetometry, transport measurements, and X-ray characterization. The obtained ESR data, such as the temperature and the angular dependence of the resonance field, reveal the specific critical behavior and confirm the ferromagnetic ordering of Mn spins even at modest doping. In addition to the studies of the critical behavior of diluted ferromagnet Bi_2?x Mn _x Te_3, we also discuss the effects of the limited solubility of Mn ions giving rise to microscopic inclusions of the spurious magnetic phases which were revealed using ESR technique.     

Heat capacity study of relaxor PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> in a wide temperature range

Heat capacity of the PbMg_1/3Nb_2/3O₃ compound is measured using the methods of adiabatic and differential scanning calorimetry in the temperature range 80–750 K. Two blurred anomalies on the C _p (T) dependence are observed in wide temperature intervals of 200–400 K and 500–700 K. The results of studies are discussed together with data on the structure and phonon spectrum in the framework of spherical random bond-random field model.     

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Structural and thermoelectric properties in (Sb<Subscript>1-x</Subscript>Bi<Subscript>x</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> thin films

We have investigated the structural and thermoelectric properties of (Sb_1-xBi_x)₂Te₃ thin films on CdTe(111)B. Analysis of X-ray diffraction patterns (–2 scans and rocking curves) of the films shows that they are of high quality and that they are well aligned with their (00.1) axis normal to the substrates. Measurements of the temperature-dependent thermoelectric power, resistivity, and Hall coefficient of the films were performed with respect to the binary composition, x. For the samples in the range 0.2<x<0.3, the room-temperature thermopower values were in the range 159–184 V/K, the room-temperature carrier concentrations were 3.93–5.13×10¹⁹ cm^-3, and the room-temperature mobilities were 24.6–64.0 cm²V^-1s^-1. PACS 72.20.Pa; 72.80.Jc; 73.6l.Le     

Solid state synthesis of nonstoichiometric Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> composites as visible-light photocatalyst

Nonstoichiometric Bi₂WO₆ photocatalyst with the composition of Bi_2?+?x WO_6?+?1.5x (?0.25 ≤ x ≤ 1) wa synthesized by a facile solid state reaction method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-vis absorption spectrum. The Bi_2.5WO_6.75 photocatalyst showed excellent visible-light-driven photocatalytic performance; nearly 100 % of RhB (10 ppm, pH?=?3?~?4) was decomposed within 25 min, which demonstrated that nonstoichiometric semiconductors could be an efficient visible-light-driven photocatalyst.     

Weak antilocalization in thin films of the Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> solid solution

A technology has been developed for the preparation of thin films of the Bi₂Te_2.7Se_0.3 solid solution through the thermal evaporation in a vacuum using the “hot-wall” method. The high quality of the thin films thus prepared has been confirmed by the X-ray diffraction and Raman scattering data. The electron transport has been investigated over wide ranges of temperatures (1.4–300 K) and magnetic fields (up to 8 T). It has been assumed that the observed weak antilocalization is associated with the dominant contribution from the surface states of a topological insulator. The dephasing length has been estimated.     

Impact of Ultrathin Pb Films on the Topological Surface and Quantum-Well States of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulators

The effect of an ultrathin Pb film deposited on the surface of Bi₂Se₃ and Sb₂Te₃ compounds on the electronic state structure of topological insulators is studied experimentally by the angle-resolved photoemission spectroscopy (ARPES) technique. The following features are revealed: formation of two-dimensional quantum-well states in the near-surface region, an increase in the binding energy of the Dirac cone and the core levels, and a simultaneous electronic states intensity redistribution in the system in photoemission spectra. The results obtained show that topological states may coexist at the interface between studied materials and a superconductor, which seems to be promising for application in quantum computers.     

Preferential growth of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> films with a nanolayer structure: enhancement of thermoelectric properties induced by nanocrystal boundaries

Preferential growth of different crystal planes in layered Bi₂Te₃ thin films with each layer <40 nm has been achieved by a simple magnetron co-sputtering method. The preferential growth of (015) plane or (001) was achieved at special depositing conditions due to the more sufficient growth along the in-plane direction induced by the enhanced diffusion of atoms and lower deposition rate. The Bi₂Te₃ film with preferential growth of (001) plane possesses about two times higher electrical conductivity and Seebeck coefficient as compared to the film with preferential growth of (015) plane, due to the greatly enhanced carrier mobility. Furthermore, the thermal conductivity has been suppressed due to more phonon scattering at grain boundaries, compared with ordinary Bi₂Te₃ alloys and films.     

Thermal conductivity of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>: Sn and the effect of codoping by Pb and I atoms

The variation of the lattice thermal conductivity of Bi₂Te₃ induced either by alloying it with tin alone or by codoping the lattice with an acceptor or donor impurity was studied. The experimental data obtained at room and liquid nitrogen temperatures argue for the validity of the model of quasi-local impurity states associated with tin atoms.     

Optical properties of CdF<Subscript>2</Subscript> in a wide energy range

For either of the two reflection spectra of cadmium difluoride that are known from experiments, a complete set of the fundamental optical functions is calculated in the energy range 4–45 eV with the Kramers-Kronig relationships. The basic features of the optical spectra are established, and a hypothesis for their origin is suggested based on the known theoretical results for the band structure.     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

<Superscript>77</Superscript>Se Low-Temperature NMR in the Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystalline Topological Insulator

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.     

Electrophysical parameters of <Emphasis Type="Italic">c</Emphasis>-oriented Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> 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.