Analyses of thermal diffusivity data on complex insulators and on strongly correlated electron systems hosted in similar complex crystal structures suggest that quantum chaos is a good description for thermalization processes in these systems, particularly in the high-temperature regime where the many phonon bands and their interactions dominate the thermal transport. Here we observe that for these systems diffusive thermal transport is controlled by a universal Planckian timescale [Formula: see text] and a unique velocity [Formula: see text] Specifically, [Formula: see text] for complex insulators, and [Formula: see text] in the presence of strongly correlated itinerant electrons ([Formula: see text] and [Formula: see text] are the phonon and electron velocities, respectively). For the complex correlated electron systems we further show that charge diffusivity, while also reaching the Planckian relaxation bound, is largely dominated by the Fermi velocity of the electrons, hence suggesting that it is only the thermal (energy) diffusivity that describes chaos diffusivity.
Keywords: phonons; quantum chaos; thermal diffusivity; thermalization.