Phonon hydrodynamic transport at elevated temperature

Abstract

For over half a century, phonon hydrodynamic transport was deemed exotic and mattered only at extremely low temperatures. In this work, by combining the theoretical and experimental approach, we successfully predict and confirm the existence of phonon hydrodynamic transport in graphite above 200 K. More specifically, we introduce a direction-dependent definition of normal and Umklapp scattering, which gives an improved description of mode-specific phonon dynamics. By extending the classical Fuchs-Sondheimer solution, we developed a first-principles framework to study phonon hydrodynamics under the size effect with mode-by-mode phonon scattering details. We unambiguously revealed the Poiseuille heat flow by studying the variation of heat flow as the graphite ribbon width and identified for the first time the existence of phonon Knudsen minimum – an unusual phenomenon unique to hydrodynamic regime – which can be observed up to 90 K. Using a sub-picosecond transient grating technique, we directly observed second sound in graphite at record-high temperatures of 200 K. With the enlarged grating-period window, we firstly reported the dispersion of thermal wave, whose velocity increases with decreasing grating period. Our experimental findings are well explained with the interplay among “three fluids”: ballistic, diffusive, and hydrodynamic phonons. We believe our study may stimulate further work into discovering more material systems possessing significant phonon hydrodynamic features, as well as new research into understanding and manipulating the phonon transport in the hydrodynamic scheme.