Temperature in a Peierls-Boltzmann Treatment of Nonlocal Phonon Heat Transport

In nonmagnetic insulators, phonons are the carriers of heat. If heat enters in a region and temperature is measured at a point within phonon mean free paths of the heated region, ballistic propagation causes a nonlocal relation between local temperature and heat insertion. This paper focusses on the solution of the exact Peierls-Boltzmann equation (PBE), the relaxation time approximation (RTA), and the definition of local temperature needed in both cases. The concept of a non-local thermal susceptibility (analogous to charge susceptibility) is defined. A formal solution is obtained for heating with a single Fourier component $P(vec{r},t)=P_0 exp(ivec{k}cdotvec{r}-iomega t)$, where $P$ is the local rate of heating). The results are illustrated by Debye model calculations in RTA for a three-dimensional periodic system where heat is added and removed with $P(vec{r},t)=P(x)$ from isolated evenly spaced segments with period $L$ in $x$. The ratio $L/ell_{rm min}$ is varied from 6 to $infty$, where $ell_{rm min}$ is the minimum mean free path. The Debye phonons are assumed to scatter anharmonically with mean free paths varying as $ell_{rm min}(q_D/q)^2$ where $q_D$ is the Debye wavevector. The results illustrate the expected local (diffusive) response for $ell_{rm min}ll L$, and a diffusive to ballistic crossover as $ell_{rm min}$ increases toward the scale $L$. The results also illustrate the confusing problem of temperature definition. This confusion is not present in the exact treatment but is fundamental in RTA.