Anomalous phonons in Pb_0.5Sn_0.5Te

The (Pb_0.5Sn_0.5)_1-xIn_xTe system is of current interest because of its potential connection to topological superconductivity [1,2]. Recent inelastic neutron scattering (INS) measurements on powder samples of (Pb_0.5Sn_0.5)_1-xIn_xTe, x= 0 and 0.3, reported unexpected low-energy peaks (in the range of 1 to 2.5 meV) in the phonon density of states, which presently lack theoretical explanation [2]. To further determine the nature of these low-energy phonons, we recently performed an INS measurement on a Pb_0.5Sn_0.5Te (x = 0) crystal. The results reveal that dispersing longitudinal and transverse phonons are not the contributor. Instead, there exist diffusive phonons with spectral weight in the energy range of 1 to 2.5 meV and characteristics deviating from conventional behavior. This suggests that some localized non-dispersive modes are the source for the anomalous low-energy peaks in the density of states. However, a proper understandig of these diffusive excitations will require further study. [1] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011), [2] K. Ran et al., Phys. Rev. B 97, 220502(R) (2018).