BiTeCl and BiTeBr: A comparative high-pressure optical study

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 potential Weyl phase in BiTeBr at $5–6$ GPa, followed by the onset of a structural phase transition above 7 GPa.

Figure 1

(a) The schematic band structure of BiTeBr around the $A$ point. (b) The ambient pressure optical transmission for a 6.5 $\mu \mathrm{m}$ thin flake of BiTeBr.

Figure 2

Raman spectra taken at high pressure for (a) BiTeCl and (b) BiTeBr. The circles mark the Raman modes discussed in the text. The pressure-independent feature at 100 ${\mathrm{cm}}^{-1}$ is an experimental artifact.

Figure 3

High-pressure transmission spectra for BiTeCl [top, (a)–(c)] and BiTeBr [bottom, (d)–(f)]. (a) and (d) show the raw transmission data for a series of pressures. Thin lines show data at intermediate pressures. The gray bands denote photon energies where the data is unavailable due to the strong light absorption by the diamond anvils. (b) and (e) are color plots of the transmission data. Open circles show the position of $\beta$ and $\gamma$ extracted from the transmission curves at each pressure. Error bars are given by the symbol size, unless indicated otherwise. In the color plot the maximum value of transmission was normalized at each pressure. (c) and (f) show $log(1/T)$, which in a certain limit may be regarded as an approximate absorption coefficient.

Figure 4

(a) The DFT band structure of BiTeBr in the $HAL$ direction around the $A$ point plotted at ambient pressure $(p=0)$, at critical pressure $p_c=4$ GPa, and at $p>p_c$. (b) The pressure dependence of the parameters $\beta$ and $\gamma$ determined from the DFT band structure for the AL direction, with chemical potential at the bottom of the conduction band.