Science https://doi.org/gfw6bm (2019)
In non-metallic solids, heat is carried primarily by phonons, the quasiparticles that describe the lattice vibrations. Phonons scatter with each other, and the rate of this process determines how heat transfers across the system — ballistic or diffusive. Now, Samuel Huberman and co-workers have showed that, in an intermediate regime between these two limiting cases, heat can also be transferred like a wave, similar to the propagation of pressure waves in air, a phenomenon referred to as second sound.
The wave-like transfer of heat was observed in graphite. Two short laser pulses crossed at the surface of the sample created a heat source. The subsequent thermal expansion caused a transient modulation of the surface displacement, which can be monitored by a third laser beam. While at room temperature the modulation decayed monotonically, at lower temperature an oscillated damping was observed. The frequency of the wave-like dynamics exhibited a linear dependence on the wave vector, leading to the second-sound velocity extracted from the data.
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Li, Y. A heat wave. Nat. Phys. 15, 422 (2019). https://doi.org/10.1038/s41567-019-0529-1