Applicability of the phonon Boltzmann transport equation in the overdamped anharmonic regime

Determine whether the phonon Boltzmann transport equation can be used to model the lattice thermal conductivity in materials exhibiting strongly anharmonic, overdamped phonon dynamics with non-Lorentzian spectral functions, where phonon lifetimes and energies are not well-defined (e.g., in systems near displacive structural phase transitions).

Background

The paper highlights that in strongly anharmonic materials, phonon spectral functions can deviate markedly from Lorentzian lineshapes, producing overdamped dynamics, satellite peaks, or even complete damping without resolvable peaks. In such regimes, assigning well-defined phonon lifetimes or energies becomes problematic.

Since the Boltzmann transport equation (BTE) relies on a quasiparticle picture with well-defined phonon properties, its validity is questionable under these conditions. The authors therefore pose a direct question about the applicability of the BTE to model lattice thermal conductivity when the quasiparticle description breaks down.