Novel Vistas in Thermal Transport: Relaxons, Frictions, Viscosities, and Coherences*

We examine the emergence of correlated transport phenomena in systems described by the linearized Boltzmann transport equation (LBTE), with examples then centered around heat transport. First, we show how phonons, traditionally considered the carriers of heat, are only a high-temperature approximation to the exact carriers, that we term relaxons, showing how thermal conductivity can be mapped exaclty into a kinetic theory of the relaxon gas, and where the effect of boundaries can be understood in terms of frictions. Second, we show how the LBTE admits coherent propagating solutions with well defined wavevectors and lifetimes, of which second sound is a special case. Last, we discuss generalizations towards the Navier-Stokes continuum, and towards disordered materials. Work performed in collaboration with Andrea Cepellotti (UC Berkeley), Michele Simoncelli (EPFL), and Francesco Mauri (Sapienza U. of Rome).