Thermal insulating behavior in crystals at high frequencies

When solving heat-conduction problems with periodic temperature perturbations, the thermal conductivity is assumed to remain frequency independent. We, however, show by using the molecular dynamics technique and the fluctuation-dissipation theorem a decrease of the effective thermal conductivity of 2 orders of magnitude when the excitation frequency approaches or exceeds the reverse of the phonon mean relaxation time. Most of the dielectric and semiconductor materials have to be considered as strongly insulating in those conditions. The comparison between molecular-dynamics simulations performed in Si crystals and theoretical predictions reveals a clear agreement.