Observation of second sound in graphite at temperatures above 100 K

Graphite gets a second sound Between the two extremes of ballistic and diffusive lattice thermal transport is the potential for an exotic wave-like state known as second sound. Huberman et al. used fast, transient thermal grating measurements to show the existence of second sound in graphite between 85 and 125 kelvin (see the Perspective by Shi). Previous observations of second sound have been rare, confined to isotopically pure materials at very low temperatures. The observation of second sound in graphite is likely due to its layered nature, suggesting that this thermal transport mode may be accessible in other two-dimensional materials. Science, this issue p. 375; see also p. 332 Second sound was observed in highly oriented pyrolytic graphite. Wavelike thermal transport in solids, referred to as second sound, is an exotic phenomenon previously limited to a handful of materials at low temperatures. The rare occurrence of this effect restricted its scientific and practical importance. We directly observed second sound in graphite at temperatures above 100 kelvins by using time-resolved optical measurements of thermal transport on the micrometer-length scale. Our experimental results are in qualitative agreement with ab initio calculations that predict wavelike phonon hydrodynamics. We believe that these results potentially indicate an important role of second sound in microscale transient heat transport in two-dimensional and layered materials in a wide temperature range.