Anomalous phonon softening in the topological insulator Sn-doped Bi1.1Sb0.9Te2S

Abstract

Investigations into the phonon behavior provide important information on interactions between different excitations in quantum materials. We perform a detailed study of the phonon behavior in the topological insulator Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S using infrared spectroscopy. We observe two IR-active phonon modes at about 64 and 165 cm$^{-1}$, which are labeled with $\\alpha$ and $\\beta$, respectively. While the evolution of the $\\beta$ mode with temperature can be well described by the expected anharmonic decay process, the $\\alpha$ mode shifts to lower frequencies with decreasing temperature. Such an anomalous softening of the $\\alpha$ mode may arise from the charged-phonon effect due to the coupling between this mode and the topological surface states in Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S.