Unusual phonon density of states and response to the superconducting transition in the In-doped topological crystalline insulator Pb0.5Sn0.5Te

Abstract

We present inelastic neutron scattering results of phonons in (Pb0.5Sn0.5)1−xInxTe powders, with x=0 and 0.3. The x=0 sample is a topological crystalline insulator, and the x=0.3 sample is a superconductor with a bulk superconducting transition temperature Tc of 4.7 K. In both samples, we observe unexpected van Hove singularities in the phonon density of states at energies of 1–2.5 meV, suggestive of local modes. On cooling the superconducting sample through Tc, there is an enhancement of these features for energies below twice the superconducting-gap energy. We further note that the superconductivity in (Pb0.5Sn0.5)1−xInxTe occurs in samples with normal-state resistivities of order 10 mΩcm, indicative of bad-metal behavior. Calculations based on density functional theory suggest that the superconductivity is easily explainable in terms of electron-phonon coupling; however, they completely miss the low-frequency modes and do not explain the large resistivity. While the bulk superconducting state of (Pb0.5Sn0.5)0.7In0.3Te appears to be driven by phonons, a proper understanding will require ideas beyond simple BCS theory.

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