Dynamics and lifetimes of resonant phonons in the quasiparticle and nonquasiparticle regimes

The dynamics and lifetimes of a phonon with model resonant interactions from the quasiparticle to nonquasiparticle regime were investigated employing Green's function and the Green-Kubo method. In the weak-coupling case, the dynamics of the resonant phonon are analogous to a damped harmonic oscillator and its lifetime t is in accordance with the standard phonon transport theory tau = 1/2 Gamma ( Gamma being the imaginary part of phonon self-energy). In the strong-coupling nonquasiparticle regime, however, the resonant phonon "propagates" in a complex form of wave packets and the actual phonon lifetime tGK as determined from the Green-Kubo formula significantly deviates from the standard tau = 1/2 Gamma relation. Taking the four-phonon resonant AgCrSe2 and threephonon resonant PbSe model systems as examples, the phonon nonquasiparticle dynamics and their lifetimes in real materials are further investigated by first-principle calculations. Substantial discrepancies between tGK and t are found for the strongly resonant phonons at high-symmetrical points in both materials. Meanwhile, the lifetime tGK of the phonons that are not subjected to resonant phonon interactions almost recovers to the conventional theoretical result tau despite the non-Lorentzian spectral feature of these phonons. It is suggested that omega tau(GK), instead of the conventional omega tau, can be applied as a criterion to distinguish phonon quasiparticles and nonquasiparticles in addition to the phonon spectral functions.