Electronic origin of the unusual thermal properties of copper-based semiconductors: The s-d coupling-induced large phonon anharmonicity

Copper (Cu)-based materials (such as cuprates, Cu chalcogenides, and Cu halides) often exhibit unusual properties such as superconductivity, ultralow thermal conductivity, and superionicity. However, the electronic origin of these unusual behaviors remains elusive. In this study, we demonstrate that the high-lying occupied 3d orbital of Cu causes a strong s-d coupling with its unoccupied 4s state when local symmetry is reduced. This leads to strong phonon anharmonicity and is responsible for these intriguing properties. For example, during thermal transport, symmetry-controlled s-d coupling can substantially lower the lattice potential barrier, thereby enhancing the anharmonicity and scattering between phonons and ultimately significantly reducing lattice thermal conductivity. We confirmed this understanding with Raman spectra measurements, which demonstrated a remarkable red shift in the phonon vibrational frequency with an increase in the temperature of Cu-based semiconductors. Our study shows that the cause of phonon anharmonicity is related to the fundamental electronic structures, which can also explain other unusual physical properties of the Cu compounds.