Observation of second sound in graphite at temperatures above 100 K

TL;DR

This study reports the first direct observation of second sound in graphite at temperatures above 100 K, indicating wavelike heat transport at near-room temperatures in layered materials.

Contribution

It provides experimental evidence of second sound in graphite at high temperatures, supported by ab initio calculations, expanding the understanding of thermal transport in layered materials.

Findings

Second sound observed in graphite above 100 K

Qualitative agreement with ab initio phonon hydrodynamics predictions

Implications for heat transport in 2D and layered materials

Abstract

Wavelike thermal transport in solids, referred to as second sound, has until now been an exotic phenomenon limited to a handful of materials at low temperatures. This has restricted interest in its occurrence and in its potential applications. Through time-resolved optical measurements of thermal transport on 5-20 {\mu}m length scales in graphite, we have made direct observations of second sound at temperatures above 100 K. The results are in qualitative agreement with ab initio calculations that predict wavelike phonon hydrodynamics on ~ 1-{\mu}m length scale up to almost room temperature. The results suggest an important role of second sound in microscale transient heat transport in two-dimensional and layered materials in a wide temperature range.