On possible deep subsurface states in topological insulators: The PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript> system

Theoretical studies of the bulk and surface electronic structures of PbBi₄Te₇ are presented. The PbBi₄Te₇ compound has a layered structure of five-layer (Bi₂Te₃) and seven-layer (PbBi₂Te₄) blocks alternating along the hexagonal axis. Analysis of the spin-orbit-induced inversion of the band gap edges indicates that this compound is a three-dimensional topological insulator. The topological properties of this compound are mainly determined by the PbBi₂Te₄ blocks. The Dirac cone is formed on the PbBi₄Te₇(0001) surface near the $ \bar \Gamma $ \bar \Gamma point for any block (either Bi₂Te₃ or PbBi₂Te₄) forming the surface. It is shown that the Dirac state can be localized not only on the surface but also deeply beneath it.     

Electrical properties of the high-pressure phases of gallium and indium tellurides

A study has been made of the resistance ρ, the thermopower S, and magnetoresistance MR of Ga₂Te₃ and α-In₂Te₃ single crystals at pressures P up to 25 GPa. It is found that the resistance ρ and |S| sharply decrease at ∼0–5 and 1.5–3 GPa, respectively. The semiconductor-metal phase transitions in the temperature range from 77 to 300 K are established from the sign reversal of the temperature coefficient of ρ to occur at P>4.4 and >1.9 GPa. The values S ≈+(10–20)μ V/K for the metallic phases with a Bi₂Te₃-type structure agree with those for liquid In₂Te₃ and Ga₂Te₃. Negative MR is revealed in In₂Te₃ at P≈1.9 GPa. No MR is observed in Ga₂Te₃ up to 25 GPa. The variation of the electronic structure of In₂Te₃ and Ga₂Te₃ under pressure is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1004–1008. Original Russian Text Copyright ? 2000 by Shchennikov, Savchenko, Popova.     

The elastic behaviour of tetrahedral materials with vacant sites

As part of a study of defect tetrahedral structure compounds, the elastic constants of single crystal specimens of Hg₃Ga₂□Te₆ and HgIn₂□Te₄ have been measured between 77 K and room temperature using the pulse superposition technique. These compounds are included in the series HgTe, Hg₅In₂□Te₈, Hg₃In₂□Te₆ and HgIn₂□Te₄ and HgTe, Hg₅Ga₂□Te₈, Hg₃Ga₂□Te₆ where there is a progressive increase in the concentration of vacant sites. While the other compounds studied are cubic, HgIn₂□Te₄ has a tetragonal structure with a $\text{c}\text{a}$ ratio of 2.0. The components of the elastic stiffness tensor of this material at 77 K are (in units of 10¹¹ dyne cm^?2) C₁₁ = 4.31 C₁₂ = 2.54 C₄₄ = 2.14 C₃₃ = 4.47 C₁₃ = 2.18 C₆₆ = 2.41. In a cubic material C₁₁ = C₃₃, C₄₄ = C₆₆ and C₁₂ = C₁₃: the elastic behaviour of this tetragonal compound closely resembles that of a cubic material, as might be anticipated from its structure. This similarity is further illustrated by reference to the symmetry of phase velocity and Young's modulus surfaces. Examination of the elastic constants and reduced elastic constants of these compounds shows a regular trend, the elastic stiffness decreases as the number of vacant sites increases. There is an approximately linear relationship between the reduced bulk modulus and the number of sited vacancies.     

Ternary compounds based on binary topological insulators as an efficient way for modifying the Dirac cone

The electronic structure of A^IVB^VI · A₂^VB₃^VI ternary compounds consisting of seven-layer atomic blocks separated by van der Waals gaps has been theoretically investigated. The YbBi(Sb)2Te₄ compounds have been considered, for which a similar atomic structure has been predicted. It has been shown that most compounds based on Group IV elements, as well as YbBi₂Te₄, are three-dimensional topological insulators. Calculations of the surface electronic structure of MBi₂Te₄, where M is a Group IV element or Yb, demonstrate the possibility of tuning the Dirac surface conduction state owing to the first element.     

Surface spin-polarized currents generated in topological insulators by circularly polarized synchrotron radiation and their photoelectron spectroscopy indication

A new method for generating spin-polarized currents in topological insulators has been proposed and investigated. The method is associated with the spin-dependent asymmetry of the generation of holes at the Fermi level for branches of topological surface states with the opposite spin orientation under the circularly polarized synchrotron radiation. The result of the generation of holes is the formation of compensating spin-polarized currents, the value of which is determined by the concentration of the generated holes and depends on the specific features of the electronic and spin structures of the system. The indicator of the formed spin-polarized current can be a shift of the Fermi edge in the photoelectron spectra upon photoexcitation by synchrotron radiation with the opposite circular polarization. The topological insulators with different stoichiometric compositions (Bi_1.5Sb_0.5Te_1.8Se_1.2 and PbBi₂Se₂Te₂) have been investigated. It has been found that there is a correlation in the shifts and generated spin-polarized currents with the specific features of the electronic spin structure. Investigations of the graphene/Pt(111) system have demonstrated the possibility of using this method for other systems with a spin-polarized electronic structure.     

Three- and two-dimensional topological insulators in Pb2Sb2Te5, Pb2Bi2Te5, and Pb2Bi2Se5 layered compounds

The electronic structure of ternary compounds Pb₂Sb₂Te₅, Pb₂Bi₂Te₅, and Pb₂Bi₂Se₅, which have a layered structure that consists of nine-layer atomic blocks separated by van der Waals gaps, has been theoretically studied. It has been shown that all studied compounds are three-dimensional topological insulators. The possibility of the existence of a two-dimensional topological insulator has been found in ultrathin (0001) Pb₂Sb₂Te₅ and Pb₂Bi₂Te₅ films. Oscillations of the ℤ₂ topological invariant with an increase in the film thickness have been observed in the latter compound.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.     

Electronic band structure of In2S3 and CdIn2S4 semiconductor spinels from the data of x-ray spectroscopy and theoretical calculations

The electronic band structure of the chalcogenide spinels In₂S₃ and CdIn₂S₄ has been studied using the FEFF8 program. It is shown that the valence band top is formed by the S p states mixed with the In s and In p states for In₂S₃ or with the Cd s, Cd p, In s, and In p states for CdIn₂S₄. Compared to In₂S₃, the presence of Cd atoms in the nearest environment of S atoms in CdIn₂S₄ does not considerably affect the electronic band structure. In CdIn₂S₄ the Cd 4d states, as well as the In 4d states, form a narrow localized band shifted deep into the valence band. The theoretical results are in good agreement with the experimental x-ray photoelectron and x-ray spectra.     

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

The thermoelectric power of linear chain synthetic metals Nb₃X₄ (with X=S, Se and Te) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb₃S₄ and Nb₃Se₄ we have found no special anomaly of the thermopower except for a little higher magnitude.With respect to Nb₃Te₄ anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.     

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(k _F), the wave vector k _F, and the surface concentration of charge carriers n _s 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.     

Inversion of conductivity type in Bi2Te3−xSx crystals

Transport parameters and optical properties of Bi₂Te_3?xS_x single-crystals with x=0–0.18 were studied. With increasing sulphur content the concentration of free current carriers decreases up to x=0.12, due to the interaction of S^x_Te defects with antisite defects Bi^′_Te, and then the p-type conductivity changes to the n-type. The optical gap of Bi₂Te_3?xS_x crystals increases with increasing S content. The obtained results led to the preparation of Bi₂Te₃-Bi₂Te_3?xS_xp–n junction by the heat treatment of p-type Bi₂Te₃ in S vapours.     

Electron band structure of α-ZnIn2S4 and related polytypes

Pseudopotential calculations have been carried out for the α, β and γ polytypic forms of the layer semiconductor ZnIn₂S₄, respectively, corresponding to space groups C⁵_3v, C¹_3v and D³_3d. The required form factors are consistent with those used in our previous calculations for ZnS and CdIn₂S₄. The band structure of the α phase, the only one up to now for which optical data are available, compares quite satisfyingly with very recent photoemission and reflectivity experimental data. The computed band structures of the β and γ phases are very alike; on the contrary, interesting differences exist between these structures and the α phase which could easily be verified by experimental investigations.     

Solid immiscibility and liquid structure in the Ga2(SexTe1−x)3 alloy system

Differential thermal analysis and X-ray diffraction studies of the system Ga₂(Se_xTe_1?x)₃ are reported and correlated with measurements of the electrical conductivity in the liquid state. The experiments reveal that Ga₂Te₃ and Ga₂Se₃ do not form a continuous series of pseudo-binary solid solutions but exhibit solid state immiscibility in the range 0.50 ? x ? 0.90. The composition dependence of the conductivity of the liquid alloys exhibits structure in the range of the solid state phase separation. Such “memory” in the liquid of a solid phase separation has not been reported previously and suggests the importance of concentration fluctuations in determining the electronic properties of the liquid alloys.     

Laser synthesis of thin layers of In4Se3, In4Te3 and modification of their structure and characteristics

Trends of structural modifications and phase composition occurring in In₄Se₃ thin films and In₄Se₃-In₄Te₃ epitaxial heterojunctions under laser irradiations have been investigated. Dynamics of the layer structure modification, depending on laser modes, i.e. pulse duration τ = 2-4 ms, irradiation intensity I₀ = 10-50 kW/cm², number of pulses N = 5-50, was studied by electron microscopy. An increase in laser influence promotes enlargement of the layer grains and transformation of their polycrystalline structure towards higher degree of stoichiometry. As a result of laser solid restructuring heterojunctions of In₄Se₃-In₄Te₃, being photosensitive within 1.0-2.0 μm and showing fast time of response, have been obtained. Laser modification of structure enables one to optimize electrical and optical properties of functional elements on the base of thin films and layers of In₄Se₃, In₄Te₃, widely used as infrared detectors and filters.     

Electronic transport properties of stuffed compositions of ferromagnetic copper chromium telluride: Cu1+xCr2Te4 (x=0-1)

We present here a detailed study of electronic transport properties of the metallic-ferromagnetic compounds Cu_1+xCr₂Te₄, having excess Cu atoms with x=0-1, from 2 to 400 K. The stuffing of the copper atoms in the parent structure reduces the ferromagnetic ordering temperature T_C from 325 to 156 K, while for the entire range the dependence of the electrical resistance and the thermopower with temperature and the anomalies in them on the magnetic ordering remain similar. All the compounds show a magnon-drag contribution in thermopower as a positive maximum around T_C/3, and a T² - dependence of resistivity at low temperatures. The increasing effects of the short range magnetic ordering in the paramagnetic resistivity are seen with the increase in the stuffing of atoms in these compounds. The transport properties are explained by the current carriers —the holes in a wide energy band dominated by the p-state of Te-atoms, which are scattered by the spindisorder in the paramagnetic phase and from the magnons in the ferromagnetic phase.     

Oxygen as promoter or poison in the catalytic dissociation of H2, C2H4, C2H2, and NH3 on Palladium

The dissociation rates of H₂, C₂H₄, C₂H₄, and NH₃ have been studied on oxygen covered Pd surfaces by measuring the water desorption rates during exposure to each of the molecules. These results are correlated with the hydrogen response of a Pd-MOS structure. The measurements show a trend (at 473 K) where oxygen blocks H₂ dissociation, blocks C₂H₄ dissociation only above a certain oxygen coverage, has no influence on C₂H₂ dissociation, and promotes NH₃ dissociation.     

First-principles study of antiferromagnetic cobalt spinels

We report a comparative study of the electronic structure and magnetic properties of two cobalt compounds Co₃O₄ and Co₃S₄, through first-principles Hubbard-U calculations. Our results indicate that Co₃O₄ and Co₃S₄ have similarities in crystal structure (normal spinel), magnetic order (antiferromagnetism), Co spin configuration (high spin Co²⁺ and low spin Co³⁺), and comparable band-gap energy. However, the U-dependence on electronic structure in two materials are different from each other. With a change in the applied U values, the band dispersion and the type of band gap are significantly changed in Co₃O₄, while the band-gap energy only is affected in Co₃S₄.     

Experimental and theoretical study of silicon-doped Sb2Te3 thin films: Structure and phase stability

The influence of Si in Sb₂Te₃ on structure and phase stability was studied by experiments and ab initio calculations. With the increase of Si content in Sb₂Te₃ samples, the crystallization temperature increases and the crystalline grain size decreases. The incorporation of Si atoms into Sb₂Te₃ lattice is energetically unfavorable and hence Si atoms most probably accumulated in the boundaries of Sb₂Te₃ grains.     

Thermoelectric properties and ferromagnetism of diluted magnetic semiconductors Sb<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>3</Subscript>

Thermoelectric properties of single crystals of a new dilute magnetic semiconductor p-Sb_{2 ? x} Cr _x Te₃ are studied in the temperature interval 7–300 K. The temperature dependences of the thermal conductivity are measured. The Seebeck coefficient S is found to increase upon doping with Cr. At low temperatures, a ferromagnetic phase with Curie temperature T _C ≈ 5.8 K exists for a Cr concentration x = 0.0215, its easy magnetization axis being parallel to the crystallographic axis C ₃. At T = 4.2 K, a negative magnetoresistance and anomalous Hall effect are observed; in strong magnetic fields, the Shubnikov-de Haas effect is manifested.