How phonon coherence develops and contributes to heat conduction in periodic and aperiodic superlattices

TL;DR

This study explores how phonon coherence affects heat conduction in periodic and aperiodic superlattices, revealing that coherence length limits phonon transmission and influences thermal conductivity behavior.

Contribution

It demonstrates that coherent phonons in aperiodic superlattices are limited in spatial extension, leading to different heat conduction mechanisms compared to periodic superlattices.

Findings

Coherent phonons in aperiodic SLs are spatially limited.

Aperiodic SLs cause phonons to behave as non-propagative modes.

Periodic SLs enable ballistic phonon transport.

Abstract

This work investigates the impact of device length on thermal conductivity in periodic and aperiodic superlattices (SLs). While it is well known that thermal conductivity in aperiodic SLs exhibits a weaker dependence on device length compared to periodic SLs, existing literature attributes this behavior to the scattering of coherent phonons by aperiodically arranged interfaces. Through atomistic wave-packet simulations, we show that coherent phonons in aperiodic SLs have spatial extensions limited to a certain number of SL layers, which prevents transmission if the extension is shorter than the device length. Specifically, the disordered interface spacing in aperiodic SLs causes coherent phonons to behave as non-propagative vibrational modes, resulting in diffuse energy transmission. In periodic SLs, however, coherent phonons can propagate across the entire structure, enabling high transmission. The difference between ballistic transport in periodic SLs and diffuse transport in aperiodic SLs is captured in the length-dependence of phonon transmission. These findings provide new insights into phonon coherence and its implications for heat conduction in superlattices, with potential applications in the thermal design of nanostructures.