Abstract
Strongly anharmonic phonon properties of CuCl are investigated with a combination of first-principles simulations and inelastic neutron scattering measurements. An unusual quasiparticle peak in the spectral functions emerges with increasing temperature, in both simulations and measurements, emanating from exceptionally strong coupling between conventional phonon modes. Associated with this strong anharmonicity, the lattice thermal conductivity (κ) of CuCl is extremely low and exhibits anomalous, non-monotonic pressure dependence. We show how this behavior arises from nested phonon dispersions augmenting the phase space available for anharmonic three-phonon scattering processes, and contrast this mechanism with common arguments based on negative Grüneisen parameters. These results demonstrate the importance of considering the topology of phonon dispersions toward understanding phonon scattering processes and designing new ultralow thermal conductivity materials for technological applications.
