Anomalous phonon softening in the topological insulator Sn-doped Bi 1.1 Sb 0.9 Te 2 S

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 α and β , respectively. While the evolution of the β mode with temperature can be well described by the expected anharmonic decay process, the α mode shifts to lower frequencies with decreasing temperature. Such an anomalous softening of the α 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.