PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Our study thus reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.
*This research was primarily supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement No. DMR-1720595. F.G.G.H. acknowledges financial support from the Brasil@Rice Collaborative Grant, the São Paulo Research Foundation (FAPESP) Grants No. 2015/16191-5 and No. 2018/06142-5, and Grant No. 307737/2020-9 of the National Council for Scientific and Technological Development (CNPq). M. R-V. was supported by LANL LDRD Program and by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, Condensed Matter Theory Program. G. A. F. acknowledges additional support from NSF DMR-1949701 and NSF DMR-2114825. J. T. and I. K. acknowledge the support from the Japan Society for the Promotion of Science (JSPS) (KAKENHI No. 20H05662).
