Electron-phonon coupling and superconductivity in the doped topological crystalline insulator (Pb0.5Sn0.5)1−xInxTe

• Published in: Physical review. B 2020-09, Vol.102 (10), p.1
• Main Authors: Sapkota, A, Li, Y, Winn, B L, Podlesnyak, A, Xu, Guangyong, Xu, Zhijun, Ran, Kejing, Chen, Tong, Sun, Jian, Wen, Jinsheng, Wu, Lihua, Yang, Jihui, Li, Qiang, Gu, G D, Tranquada, J M
• Format: Article
• Language: English
• Subjects: Atomic properties; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Coupling; Diffusion; Displacements (lattice); Doped crystals; Elastic scattering; electron-phonon coupling; inelastic neutron scattering; lattice dynamics; Lattice vibration; Lead; Low temperature; Phonons; Room temperature; Single crystals; Structural stability; Superconductivity; Thermal conductivity; Tin
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.102.104511

We present a neutron-scattering study of phonons in single crystals of (Pb 0.5 Sn 0.5)1−xInxTe with x = 0 (metallic, but nonsuperconducting) and x = 0.2 (nonmetallic normal state, but superconducting). We map the phonon dispersions (more completely for x = 0) and find general consistency with theoretical calculations, except for the transverse and longitudinal optical (LO) modes at the Brillouin-zone center. At low temperature, both modes are strongly damped but sit at a finite energy (≈ 4 meV in both samples), shifting to higher energy at room temperature. These modes are soft due to a proximate structural instability driven by the sensitivity of Pb-Te and Sn-Te p -orbital hybridization to off-center displacements of the metal atoms. The impact of the soft optical modes on the low-energy acoustic modes is inferred from the low thermal conductivity, especially at low temperature. Given that the strongest electron-phonon coupling is predicted for the LO mode, which should be similar for both studied compositions, it is intriguing that only the In-doped crystal is superconducting. In addition, we observe elastic diffuse (Huang) scattering that is qualitatively explained by the difference in Pb-Te and Sn-Te bond lengths within the lattice of randomly distributed Pb and Sn sites. We also confirm the presence of anomalous diffuse low-energy atomic vibrations that we speculatively attribute to local fluctuations of individual Pb atoms between off-center sites.