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Register a new userOptical properties of BiTeBr and BiTeCl
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We present a comparative study of the optical properties - reflectance, transmission and optical conductivity - and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $beta$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.
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We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation $beta$ between two Rashba-split conduction bands, and the absorption $gamma$ across the band gap. In the low pressure range, $p< 4$~GPa, for both compounds $beta$ is approximately constant with pressure and $gamma$ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6~GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a Weyl phase in BiTeBr at 5$-$6~GPa, followed by the onset of a structural phase transition at 7~GPa.
Within density functional theory, we study bulk band structure and surface states of BiTeBr. We consider both ordered and disordered phases which differ in atomic order in the Te-Br sublattice. On the basis of relativistic ab-initio calculations, we show that the ordered BiTeBr is energetically preferable as compared with the disordered one. We demonstrate that both Te- and Br-terminated surfaces of the ordered BiTeBr hold surface states with a giant spin-orbit splitting. The Te-terminated surface-state spin splitting has the Rashba-type behavior with the coupling parameter alpha_R ~ 2 eVAA.
We report measurements of Shubnikov-de Haas (SdH) oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, $F_{1}=65 pm 4$ Tesla and $F_{2}=156 pm 5$ Tesla. We also measured the infrared optical reflectance $left(cal R(omega)right)$ and Hall effect; we propose that the two frequencies correspond respectively to the inner and outer Fermi sheets of the Rashba spin-split bulk conduction band. The bulk carrier concentration was $n_{e}approx1times10^{19}$ cm$^{-3}$ and the effective masses $m_{1}^{*}=0.20 m_{0}$ for the inner and $m_{2}^{*}=0.27 m_{0}$ for the outer sheet. Surprisingly, despite its low effective mass, we found that the amplitude of $F_{2}$ is very rapidly suppressed with increasing temperature, being almost undetectable above $Tapprox4$ K.
We performed X-ray diffraction and electrical resistivity measurement up to pressures of 5 GPa and the first-principles calculations utilizing experimental structural parameters to investigate the pressure-induced topological phase transition in BiTeBr having a noncentrosymmetric layered structure (space group P3m1). The P3m1 structure remains stable up to pressures of 5 GPa; the ratio of lattice constants, c/a, has a minimum at pressures of 2.5 - 3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, according to the Wilson loop analysis, the topological nature of electronic states in noncentrosymmetric BiTeBr at 0 and 5 GPa are explicitly revealed to be trivial and non-trivial, respectively. These results strongly suggest that pressure-induced topological phase transition in BiTeBr occurs at the pressures of 2.9 GPa.
Optical and magneto-optical properties of ZnCoO films grown at low temperature by Atomic Layer Deposition are discussed. Strong wide band absorption, with onset at about 2.4 eV, is observed in ZnCoO in addition to Co-related intra-shell transitions. This absorption band is related to Co 2+ to 3+ photo-ionization transition. A strong photoluminescence (PL) quenching is observed, which we relate to Co recharging in ZnO lattice. Mechanisms of PL quenching are discussed.
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