Experimental Observation of Thermal Waves in Thin Graphite

Phonon transport regimes are classified based on the relative occurrence of phonon scattering events. In bulks, heat is generally transferred in the diffusive mode through momentum-destroying resistive scattering. As mean free paths of phonons become larger than or comparable to the characteristic length with decreasing sample size and temperature, the phonon transport regime transitions into non-Fourier transport modes such as ballistic or hydrodynamic phonon transport, where the dominant phonon scattering mode is boundary or normal scattering, respectively. Particularly, the condition required for hydrodynamic phonon transport is rarely satisfied for most bulk materials. Recent calculations have demonstrated that graphitic materials are promising candidates for near-room temperature hydrodynamic thermal transport due to their large anharmonicity and high Debye temperature, which result in exceptional momentum-conserving normal scatterings. To support this, experimental attempts have been conducted, but they are still limited by sample preparation and technical measurement issues. In this study, we examined the signature phenomenon of phonon hydrodynamics, <i>i.e.</i>, second sound, in graphite using an optical pump-probe experiment, which was configured with spatially offset beams to characterize basal-plane thermal wave propagations. Additionally, mechanically exfoliated thin graphite flakes with various thicknesses were measured to characterize the thickness effect on phonon hydrodynamic transport. These experimental results were analyzed by solving the viscous heat equations, which describe wave-like phonon hydrodynamic behavior, using first-principle calculation results