Second sound in graphite at temperatures above 100 K

April 17, 2019
  • In the new paper we report observations of the wave-like propagation of heat in HOPG graphite in the so-called second sound regime.  
  • Second sound in solids has long been considered an exotic phenomenon observed only in a few isotopically pure and defect-free single-crystal materials at very low temperatures (e.g. in solid 3He  between 0.42 - 0.58 K and in NaF between 11 - 14.5 K). 

Fig.1 In contrast to conventional diffusive heat spreading, in the second sound regime heat propagates as waves, in an apparent violation of the second law of thermodynamics. 

  • It has recently been predicted by our collaborators from Gang Chen's group that the temperature window for the occurence of second sound should be  much wider in graphene and graphite
  • We have observed second sound in HOPG graphite at temperatures up to ~120 K in a laser-induced transient grating experiment, in which two short laser pulses are crossed in the esample to create a spatially sinusoidal temperature profile, and diffraction of a cw probe beam is used to monitor the amplitude of this "thermal grating" vs time. 
  • While normally we observe an exponential thermal grating decay, in graphite around 100 K we see damped oscillations, with the signal dropping below zero, which indicates that maxima and minima of the thermal grating switch places. 

Fig. 2. Observation of second sound in graphite in the transient grating experiment. (a) Schematic illustration of the experiment. (b) Measurements at 300 K at different grating periods yield an exponential thermal grating decay. (c) At 85 K we see a damped oscillation with a negative phase, a hallmark of wave-like propagation. (d) Ab-initio simulations (no fitting parameters) of the thermal grating decay at 80K based on the Peierls-Boltzmann transport equation for phonons with a full three-phonon scatteribng matrix. 

  • The observation of  second sound at realtively high temperatures in a fairly conventional material (with isotopic disorder and polycrystalline) means that second sound should no longer be considered an exotic phenomenon with no practial implications. We anticipate that the high-temperature second sound will be obsrved in other layered and 2D materials. 

For more details, read the paper: S. Huberman et al., "Observation of second sound in graphite at temperatures above 100 K," Science DOI: 10.1126/science.aav3548