Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150816

150816 - Elucidating Phonon Coherence Behaviors and Thermal Transport in Superlattices by Wave-Packet Simulation 

Thermal transport in metamaterials featuring a secondary periodicity is strongly influenced by phonon coherence, resulting in unique thermophysical properties. Notably, in superlattices (SLs), structures composed of alternating layers of two or more materials, a minimum lattice thermal conductivity (κ_L) emerges as the period length is reduced to a few atomic layers before increasing with further reduction in layer thickness. If phonons within the system behaved purely as particles, it is expected that κ_L would monotonically decrease with period length since phonon-interface scattering is intensified. This behavior is not observed as strong interference of scattered phonons manifest as wave-like modes that propagate through the interfaces coherently without scattering, thus raising κ_L. The minimum κ_L at a critical layer thickness is attributed to a transition between these two phonon transport regimes: interface scattering and coherent wave transport. The existence of two transport regimes is well explained by the presence of two distinct types of phonon modes that contribute to κ_L . The first type represents phonon modes belonging to the constituent base materials of the SL, e.g., the phonon modes of bulk silicon (Si) and germanium (Ge) within a Si/Ge SL. The second type includes phonon modes aligning with the secondary periodicity of the SL, following the dispersion relations of the SL rather than those of the base materials. The former is referred to as incoherent phonons, while the latter is denoted as coherent phonons. The character of these two phonon types in SLs, particularly coherent phonons, has mostly been indirectly analyzed through macroscopic coefficients such as κ_L and/or thermal boundary conductance in the existing literature. In this work, we leverage the atomistic wave-packet method to directly simulate the transport of phonons with well-defined polarization and wavelength. A most unique aspect of the wave-packet method is the ability to specify the spatial coherence length, defined as the real-space width of a phonon wave-packet that serves as a measure of the spatial correlations of atomic motion within the phonon wave. The magnitude of coherence length and its comparison to the length scale of a metamaterial’s microstructure strongly dictates the interference states and therefore characterizes the phonon transport regime. Consequently, the wave-packet method is a powerful tool to study phonon coherence behaviors through direct simulation of interference. Here we conduct two analyses of transmission of both incoherent and coherent phonons in periodic and aperiodic SLs. Through direct dissection of the wave dynamics, we elucidate how thermal transport in these metamaterials is subject to specific phonon coherence behaviors. Our results support existing analyses and experiments as well as unveil new physics, possibly guiding innovation in design of thermal devices incorporating SL structures.

Presenting Author: Theodore Maranets University of Nevada, Reno

Presenting Author Biography: Theodore is a PhD candidate at the University of Nevada, Reno having joined the Mechanical Engineering department in Fall 2021. His research focuses on elucidating the wave behaviors of atomic vibrations, known as phonons, and their impact on heat conduction through atomistic modeling techniques.

Authors:

Theodore Maranets University of Nevada, Reno
Yan Wang University of Nevada, Reno