Direct Measurement of Room Temperature Non-diffusive Thermal Transport   Over Micron Distances in a Silicon Membrane
  The "textbook" phonon mean free path (MFP) of heat carrying phonons in silicon at room temperature is ~40 nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer MFPs. We present a simple experiment demonstrating that room temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ~1-10 {\mu}m. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum. 
Jeremy A. Johnson, A. A. Maznev, John Cuffe, Jeffrey K. Eliason,   Austin J. Minnich, Timothy Kehoe, Clivia M. Sotomayor Tor...
cond-mat.mtrl-sci cond-mat.mes-hall