  Materials with ultralow thermal conductivity are of great interest for
efficient energy conversion and thermal barrier coating. Copper-based
semiconductors such as copper chalcogenides and copper halides are known to
possess extreme low thermal conductivity, whereas the fundamental origin of the
low thermal conductivity observed in the copper-based materials remains
elusive. Here, we reveal that s-d coupling induced giant phonon anharmonicity
is the fundamental mechanism responsible for the ultralow thermal conductivity
of copper compounds. The symmetry controlled strong coupling of high-lying
occupied copper 3d orbital with the unoccupied 4s state under thermal vibration
remarkably lowers the lattice potential barrier, which enhances anharmonic
scattering between phonons. This understanding is confirmed by
temperature-dependent Raman spectra measurements. Our study offers an insight
at atomic level connecting electronic structures with phonon vibration modes,
and thus sheds light on materials properties that rely on electron-phonon
coupling, such as thermoelectricity and superconductivity.
