Relativistic and Anharmonic Contributions to Low Thermal Conductivity in Topological Material BiTe.

Understanding heat transport in topological materials containing heavy elements remains a challenge because relativistic effects and higher-order lattice anharmonicity may both be relevant to thermal transport. Here, BiTe is investigated as a representative system in which the effects of spin-orbit coupling (SOC) and higher-order phonon anharmonicity are examined separately. Angle-resolved photoemission spectroscopy and density functional theory confirm that SOC is essential to the topological electronic structure, while machine-learning moment-tensor potentials enable a quantitative assessment of its influence on phonon transport. Including SOC slightly increases the lattice thermal conductivity by weakening low-frequency phonon scattering. Temperature-dependent Raman spectroscopy reveals mode-dependent higher-order anharmonicity, and explicit transport calculations show that four-phonon scattering substantially suppresses thermal conductivity. These findings demonstrate that both SOC and higher-order phonon scattering should be considered for a reliable description of heat transport in BiTe and provide broader insight into thermal transport in topological materials containing heavy elements.