Abstract
We present inelastic neutron scattering results of phonons in powders, with and 0.3. The sample is a topological crystalline insulator, and the sample is a superconductor with a bulk superconducting transition temperature 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 , there is an enhancement of these features for energies below twice the superconducting-gap energy. We further note that the superconductivity in occurs in samples with normal-state resistivities of order 10 , 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 appears to be driven by phonons, a proper understanding will require ideas beyond simple BCS theory.
- Received 12 April 2018
DOI:https://doi.org/10.1103/PhysRevB.97.220502
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