Evidence of Ferroelectric Distortions in Topological Crystalline Insulators via Transverse Thermoelectric Measurements

The transverse thermoelectric (Nernst) effect is a powerful probe for studying the electronic and structural properties of materials. In this study, we employ transverse thermoelectric measurements to investigate the ferroelectric distortion in the topological crystalline insulator (TCI) Pb0.60Sn0.40Te, a compound derived from PbTe and SnTe, known for their exceptional thermoelectric performance and distinct ferroelectric properties. By leveraging Nernst measurements, we provide direct evidence of ferroelectric distortion in this TCI, corroborated by Shubnikov-de Haas quantum oscillations that confirm the presence of two topologically nontrivial Fermi pockets. Density functional theory calculations show that these pockets originate from the L and T points in the Brillouin zone of the distorted structure within the TCI phase. Raman spectroscopy further identifies a structural phase transition below 50 K, consistent with the quantum oscillation observations. This observation is further substantiated by temperature-dependent synchrotron X-ray pair distribution function analysis and transmission electron microscopy, which confirm the local off-centering of cations at low temperature. These findings underscore the potential of transverse thermoelectric measurements in unveiling ferroelectric distortions and their role in modulating topological quantum states, opening new directions for research into the synergy between ferroelectricity and topological phases.