Thermal-Response Functions computed from First Principles with Applications to Two-Dimensional Materials

Thermal response, or alternately thermal suscpetibility, is a useful way to describe the propagation of energy in response to an external heat pulse. The particular advantage of this approach is that there is no a priori assumption made as to whether transport is ballistic or diffusive. Here I present an approach to compute thermal response functions using input from Density-Functional Theory (DFT) calculations. Calculations are presented for two-dimensional materials at low temperatures. The results demonstrate the propagation of second sound in graphene, with insight into length and timescales where ballistic transport can be expected. Comparison is made to recent experimental results. Finally, the work is discussed in the context of phonon Boltzmann theory in the relaxation time approximation.