Abstract
We present a neutron-scattering study of phonons in single crystals of (Pb0.5Sn0.5)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 (≈4meV 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.
| Original language | English |
|---|---|
| Article number | 104511 |
| Journal | Physical Review B |
| Volume | 102 |
| Issue number | 10 |
| DOIs | |
| State | Published - Sep 2020 |
Funding
The work at Brookhaven National Laboratory was supported by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (DOE) under Contract No. DE-SC0012704. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. J.S.W. was supported by the National Natural Science Foundation of China under Grants No. 11822405 and No. 11674157, and the Natural Science Foundation of Jiangsu Province under Grant No. BK20180006. L.W. and J.Y. acknowledge the National Science Foundation for its support on this project with award No. 1915933.