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)

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The percolation threshold (PT) temperatures Tc1,c2 of two non‐equivalent Ag+1,2 ions in Ag2S and Ag2Se lattices were determined from the temperature dependence of low‐frequency (25‐106Hz) impedance spectroscopy measurements. At PT temperatures Tc1≈200 K and Tc2≈ 334 K, an abrupt increase of the dielectric function ε1(ω) of Ag2Se crystal from a very large negative value, ε1 ≈‐109, to a very large positive value ε1≈ 106 was observed, which is assumed to result from the dipole relaxation. The strong peaks of the dielectric losses ε2(ω), tgδ(ω) and impedance Z in Ag2Se were also determined at Tc1,c2. Two significant peaks in ε1(ω), ε2(ω)and tgδ(ω) were observed at Tc1≈ 312‐320 K. and Tc2≈ 380‐430 K in the temperature dependences of these parameters. The measured parameters decreased significantly with increasing frequency, confirming the long relaxation times. The drastic decrease at T > 430 K of ε1from a very large positive value 109to a very large negative one ‐108 is in accordance with infrared (IR) ellipsometry measurements; this indicates that electrons of Ag+‐ ions are ionized because of the high dielectric function screening of lattice sites before the known β→α structural phase transition from monoclinic to a body‐centered cubic (bcc) structure. In addition, the activation energies of Ag+1,2 ions were estimated for Ag2S and Ag2Se crystals. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-dependent dielectric and conduction properties of the impurity-doped and gamma-irradiated samples of TlInS 2 semiconductor-ferroelectric with incommensurate phase are presented. As found, in both cases a stable relaxor state is emerging in the material with temperature provided that the central ion in InS 4 tetrahedron is replaced by an impurity atom such as Mn or Cr, or radiation dose exceeds 400 Mrad. Same as NMR-studies, the present work drives to a conclusion that In-displacements are among the components of the order parameter of the incommensurate phase transition. The origin of the non-activated conductivity observed in the relaxor state of TlInS 2 is assumed to be resonance tunneling executed by charge carriers from electron levels in the band gap through potential barriers created by incommensurate superstructure.