Infrared- and Raman-Spectroscopy Measurements of a Transition in the Crystal Structure and a Closing of the Energy Gap of BiTeI under Pressure

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15 GPa. The gap feature in the optical conductivity vanishes above $p\sim 9\mathrm{GPa}$ and does not reappear up to at least 15 GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9 GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring.

Figure 1

(a) The reflectance and transmission, and (b) the real part of the complex conductivity ${\sigma }_1(\omega )$. The left inset: blow-up of ${\sigma }_1(\omega )$. The right inset: sketch of band structure near the $A$ point with the chemical potential in dashed line. The dashed vertical lines mark the energy range where the high-pressure experiments were perfomed.

Figure 2

(a) Reflectivity shown for different pressures (legend in panel b). Inset: Close up in the low pressure range. (b) Real part of the optical conductivity, ${\sigma }_1(\omega )$. (c) Transmission through a $\sim 5\mu \mathrm{m}$ thick sample. (d) The pressure dependence of ${\sigma }_1(\omega )$ at different energies. (e) and (f) The loss function, $-\mathrm{Im}(1/\widehat{\epsilon }(\omega ))$. Inset shows a sketch of the sample environment within the diamond anvil pressure cell. In all panels, the data taken between 0.22 and 0.33 eV are not shown due to strong diamond absorption.

Figure 3

Raman spectra from 0 to 15 GPa. (a) and (b), parallel polarization. (c) and (d), crossed polarization.