Phonon Hydrodynamics in Graphene and Graphite

In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (<20 K). Recently, we predict based on first principles calculations that hydrodynamic phonon transport can occur in suspended graphene at significantly higher temperatures and wider temperature ranges compared to bulk materials. Furthermore, we identified graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (~100 K). The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp² hybrid bonding and weak van der Waals interlayer interactions. The hydrodynamic transport is manifested through drift motion of phonons, phonon Poiseuille flow, and second sound. Recent experiments demonstrating second sound up to 100K will be discussed.