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The heat-carrying acoustic excitations of amorphous silicon are of interestbecause their mean free paths may approach micron scales at room temperature.Despite extensive investigation, the origin of the weak acoustic damping in theheat-carrying frequencies remains a topic of debate. Here, we reportmeasurements of the thermal conductivity mean free path accumulation functionin amorphous silicon thin films from 60 - 315 K using transient gratingspectroscopy. With additional picosecond acoustics measurements and consideringthe known frequency-dependencies of damping mechanisms in glasses, wereconstruct the mean free paths from $\sim 0.1-3$ THz. The mean free paths areindependent of temperature and exhibit a Rayleigh scattering trend over most ofthis frequency range. The observed trend is inconsistent with the predictionsof numerical studies based on normal mode analysis but agrees with diversemeasurements on other glasses. The micron-scale MFPs in amorphous Si arise fromthe absence of anharmonic or two-level system damping in the sub-THzfrequencies, leading to heat-carrying acoustic excitations withroom-temperature damping comparable to that of other glasses at cryogenictemperatures.

Origin of micron-scale propagation lengths of heat-carrying acoustic excitations in amorphous silicon