Observation of second sound in graphite over 200 K

• Published in: Nature communications Vol. 13; no. 1; p. 285
• Main Authors: Ding, Zhiwei, Chen, Ke, Song, Bai, Shin, Jungwoo, Maznev, Alexei A., Nelson, Keith A., Chen, Gang
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12.01.2022; Nature Publishing Group; Nature Portfolio
• Subjects: 119/118; 140/125; 639/301/1034; 639/766/119; 639/766/25; Boltzmann transport equation; First principles; Graphite; High temperature; Humanities and Social Sciences; Isotope effect; Low temperature; multidisciplinary; Phonons; Room temperature; Science; Science (multidisciplinary); Sound; Sound propagation; Wave propagation; Wave velocity; Zero sound
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-27907-z

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. For over half a century, temperature wave was deemed exotic and mattered only at extremely low temperatures. Here, the authors reported direct observation of second sound, signature of the temperature wave, at a record-high temperature of over 200 K.