The unoccupied part of the band structure of topological insulators Bi 2 Te x Se 3−x (x = 0,2,3) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly dispersing surface states are found at the center of the surface Brillouin zone at energies about 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction-induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi 2 Se 3 and Bi 2 Te 2 Se. In Bi 2 Te 3 small structural variations can change the character of the local energy gap, depending on whether or not an unoccupied Dirac state exists. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state.

Polytypes (Ps) of monoclinic TlInS2 with lattice parameters c (c ≈ 15 Å, c ≈ 30 Å, c ≈ 60 Å, c ≈ 120 Å and c ≈ 240 Å) have been obtained by long time annealing in vacuum or in sulfur vapour (P c ≈ 240 Å) of TlInS2 crystal at different temperatures‐T. From the temperature dependence of dielectric constant ϵ (T) of pure Ps it is established, that well known ferroelectric phase transition (FPT) in TlInS2 takes place with single peaks of dielectric anomaly (DA) at slightly different temperatures for different Ps. Samples with pure P are unstable and are returning to mixed Ps state again after some time. The most unstable is the P with c ≈ 15 Å. It is shown that FPT and optic measurements can be used as Ps analysis methods in TlInS2 samples. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The phonon structure of the silver sulfide Ag 2 S was investigated, experimentally using Raman spectroscopy, and theoretically using the density-functional perturbation theory for the first time. Seven Raman-active modes were observed and identified at 23,39,42,44, 62, 65, and 243 cm À1 . Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. The phonon band structure and the relative Raman intensities were also investigated by ab initio calculations and compared with the experimental data. The temperature, laser power, and illumination time dependencies of frequency, linewidth, and intensity of the Raman-active modes are discussed. In the Raman spectra at higher frequencies 1300-1700 cm À1 , additional broad Raman modes observed in all samples at higher laser powers 8-10 mW were ascribed to luminescence from b-Ag 2 S. The phonon and Raman spectra of the b-Ag 2 S provide a useful insight into the b-Ag 2 S ! a-Ag 2 S phase transition. Finally, calculated infrared vibrational mode frequencies were compared with measured infrared mode frequencies.

No abstract

No abstract

Dependencies of the lattice parameters and the coefficient of thermal expansion of TlInTe2 crystals on temperature are studied at a wide range of temperatures by using XRD, dilatometer, differential thermal analysis, and impedance spectroscopy (IS) methods. The IS measurements of the dielectric function were done along the c-axis and normal to the (110)-plane. IS for E ⊥ (110)-plane was done for two values of the amplitudes, U0 = 0.04 V and 1 V, of ac-voltage with different frequencies; moreover, the measurements for E∥c were realized at U0 = 0.04 V only. IS measurements demonstrate a sequence of the phase transitions (PTs) to the ferroelectric and the ionic conductivity states, also to a structural phase transition. Tl+ ions are shown to shift in the direction of normal to c-axis in the ferroelectric PT, but they are activated along the c-axis in the ionic conductivity PT. The dielectric function in the ionic conductivity PT is shown to increase to a huge positive value and then to drop to a big negative value, reversing its sign. An explanation of this effect is provided as a competition of Drude and polarization contribution to the dielectric function under ac-electric field. Thermal effects of the phase transitions were determined.

We calculate the Josephson current between two one-dimensional (1D) nanowires oriented along x with proximity induced s-wave superconducting pairing and separated by a narrow dielectric barrier in the presence of both Rashba spin-orbit interaction (SOI) characterized by strength α and Zeeman fields (h alongẑ and B in the x − y plane). We formulate a general method for computing the Andreev bound states energy which allows us to obtain analytical expressions for the energy of these states in several asymptotic cases. We find that in the absence of the magnetic fields the energy gap between the Andreev bound states decreases with increasing Rashba SOI constant leading eventually to touching of the levels. In the absence of Rashba SOI, the Andreev bound states depend on the magnetic fields and display oscillatory behavior with orientational angle of B leading to magneto-Josephson effect. We also present analytic expressions for the dc Josephson current charting out their dependence on B, h, and α. We demonstrate the existence of finite spin-Josephson current in these junctions in the presence of external magnetic fields and provide analytic expressions for its dependence on α, B and h. Finally, we study the AC Josephson effect in the presence of the SOI (for |B| = h = 0) and an external radiation and show that the width of the resulting Shapiro steps in such a system can be tuned by varying α. We discuss experiments which can test our theoretical results.

No abstract