Phonon relaxation times of germanium determined by molecular dynamics at 1000 K

Abstract

Molecular dynamics simulations (MD) and the normal mode decomposition are used to determine the phonon relaxation times of acoustical and optical modes of germanium (Ge) at 1000 K and 1 atm. The relaxation times are calculated from the temporal decay of the autocorrelation function of the total energy of each normal mode in the [100] direction. Two sets of force field parameters are used to obtain the total energy of each phonon mode. We have found, under the assumption of an isotropic crystal, that the acoustic modes contribute about 90 % to the overall thermal conductivity (being the contribution of longitudinal acoustic modes 60 %), and that the behavior of the relaxation times of acoustic modes can be well represented by power functions with exponents close to 2. Both results are in agreement with previous estimations for silicon (Si) and Ge using MD and ab initio simulations, respectively. Lastly, we have found that only one parameter set is able to reproduce the experimental thermal conductivity at this temperature.