Examining normal modes as fundamental heat carriers in amorphous solids: the case of amorphous silicon

Abstract

Normal mode decomposition of atomic vibrations has been used to provide microscopic understanding of thermal transport in amorphous solids for decades. In normal mode methods, it is naturally assumed that atoms vibrate around their equilibrium positions and that individual normal modes are the fundamental vibrational excitations transporting heat. With the abundance of predictions from normal mode methods and experimental measurements now available, we carefully analyze these calculations in amorphous silicon, a model amorphous solid. We find a number of discrepancies, suggesting that treating individual normal modes as fundamental heat carriers may not be accurate in amorphous solids. Further, our classical and ab-initio molecular dynamics simulations of amorphous silicon demonstrate a large degree of atomic diffusion, especially at high temperatures, leading to the conclusion that thermal transport in amorphous solids could be better described starting from the perspective of liquid dynamics rather than from crystalline solids.

Strong phase correlation between diffusons governs heat conduction in amorphous silicon

“…The similarity in the lattice aperiodicity and atomic instability makes the collective excitation a reasonable description of thermal vibrations in amorphous materials as discussed before in Refs. [21,23,43]. The calculation method of DSF is presented in the Methodology section.…”

“…1 can also provide the information of wavevector for different vibrations but in the frame of collective excitations as proposed in Refs. [21,23,43]. By implementing the wave-packet simulation, we further study the detailed transport properties of propagons and diffusons.…”

Transport and relaxation of current-generated nonequilibrium phonons from nonlocal electronic measurements

“…4(c) shows that the average wavevector of the generated modes remains almost constant when the center wavevector of the incident wavepacket is varied by more than a factor of 5, consistent with our experimental observations. This result indicates a breakdown of the perturbative picture underlying the concept of quasiparticles [phonons] [45], enabling nonresonant scattering not constrained by the usual quasiparticle momentum and energy conservation laws.…”