Abstract
Understanding elementary excitations and their couplings in condensed matter systems is critical to
develop better energy-conversion devices. In thermoelectric materials, the heat-to-electricity
conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles
responsible for macroscopic thermal transport. The material with the current record for
thermoelectric conversion efficiency, SnSe, achieves an ultra-low thermal conductivity, but the
mechanism enabling this strong phonon scattering remains largely unknown. Using inelastic
neutron scattering measurements and first-principles simulations, we mapped the four-dimensional
phonon dispersion surfaces of SnSe, and revealed the origin of ionic-potential anharmonicity
responsible for the unique properties of SnSe. We show that the giant phonon scattering arises
from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice
instability. The present results provide a microscopic picture connecting electronic structure and
phonon anharmonicity in SnSe, and offers precious insights on how electron-phonon and phononphonon
interactions may lead to the realization of ultra-low thermal conductivity.
