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Phys Rev Lett. 2013 Jan 11;110(2):025901. Epub 2013 Jan 8.

Direct measurement of room-temperature nondiffusive thermal transport over micron distances in a silicon membrane.

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Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. jeremy.johnson@psi.ch

Abstract

The "textbook" phonon mean free path 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 mean free paths. 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  μm. 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.

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