The curious case of cuprous chloride: Giant thermal resistance and anharmonic quasiparticle spectra driven by dispersion nesting

Strongly anharmonic phonon properties of CuCl are investigated with inelastic neutron-scattering measurements and first-principles simulations. An unusual quasiparticle spectral peak emerges in the phonon density of states 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 of CuCl is extremely low and exhibits anomalous, nonmonotonic pressure dependence. We show how this behavior arises from the structure of the phonon dispersions augmenting the phase space available for anharmonic three-phonon scattering processes, and contrast this mechanism with common arguments based on negative Gr\"uneisen parameters. These results demonstrate the importance of considering intrinsic phonon-dispersion structure toward understanding scattering processes and designing new ultralow thermal conductivity materials.