Lattice thermal transport beyond the quasiparticle approximation: Nontrivial spectral competition between three- and four-phonon interactions

The authors develop here a first-principles framework beyond the quasiparticle approximation (BQPA) incorporating three- and four-phonon interactions. By applying this to MgO, PbTe, and AgCl, they reveal a nontrivial spectral competition where three-phonon softening is counteracted by four-phonon hardening. This cancellation brings BQPA thermal conductivity predictions into unexpectedly close agreement with standard quasiparticle results, highlighting the necessity of treating both full spectral functions and higher-order anharmonicity on equal footing for accurate modeling.