Abstract
Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. We directly observe second sound in graphite at temperatures of over 200 K using a sub-picosecond transient grating technique. The experimentally determined dispersion relation of the thermal-wave velocity increases with decreasing grating period, consistent with first-principles-based solution of the Peierls-Boltzmann transport equation. Through simulation, we reveal this increase as a result of thermal zero sound—the thermal waves due to ballistic phonons. Our experimental findings are well explained with the interplay among three groups of phonons: ballistic, diffusive, and hydrodynamic phonons. Our ab initio calculations further predict a large isotope effect on the properties of thermal waves and the existence of second sound at room temperature in isotopically pure graphite.
Highlights
Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime
We report the observation of second sound at record-high temperatures of over 200 K via pulsed-laser-probed thermal grating (TTG) measurements at grating periods of around 2 μm
We show that the transport can be viewed as a mixture of three groups of phonons: hydrodynamic phonons experiencing strong normal scattering, contributing to second sound; ballistic phonons contributing to thermal zero sound; and resistive phonons contributing to diffusion along the temperature gradient
Summary
Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. Phonons can travel a distance longer than the conduction length scale without scattering, leading to an effective thermal conductivity value that diminishes as the length scale is reduced. This regime has been studied extensively owing to its importance in applications such as electronics thermal management and thermoelectric energy conversion[1,2,3,4]. Some of us recently reported observation of second sound above 100 K in graphite using the transient thermal grating (TTG) technique[21] in which the thermal transport length scale is imposed by the use of crossed excitation laser pulses that produce an optical interference pattern of alternating peaks and nulls at the sample, resulting in a sinusoidally varying temperature “grating” profile. The temporal resolution of our continuous-wave-laser-probed TTG system (~0.5 ns) imposed a lower limit of the grating period of about 6 μm, below which the kinetics could not be resolved
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