Observation of THz range propagating elastic waves in amorphous materials using inelastic X-ray scattering.

The thermal atomic vibrations of amorphous solids can be distinguished by whether they propagate as elastic waves or do not propagate due to lack of atomic periodicity. In a-Si, prior works concluded that non-propagating waves are the dominant contributors to heat transport, with propagating waves being restricted to frequencies less than a few THz and scattered by anharmonicity. Our recent numerical study of a-Si supports a qualitatively different picture in which propagating elastic waves dominate the thermal conduction and are scattered by local fluctuations of elastic modulus rather than anharmonicity. Here, we present transient grating spectroscopy (TG) thermal conductivity measurements and dynamic structure factor measurements of a-Si and a-SiN with inelastic X-ray scattering (IXS). We explicitly demonstrate that propagating elastic waves exist up to around 10 THz despite the lack of periodicity in a-Si and are scattered by elastic fluctuations rather than anharmonicity in amorphous materials.