Super-Suppression of Long Phonon Mean-Free-Paths in Nano-engineered Si due to Heat Current Anticorrelations

TL;DR

This study demonstrates that specific nanoscale pore arrangements in silicon can drastically reduce thermal conductivity by super-suppressing long phonon mean-free-paths through heat current anticorrelations, enhancing thermoelectric efficiency.

Contribution

The paper introduces a novel understanding of heat current anticorrelations in nanostructured silicon, revealing elastic backscattering effects that significantly shorten phonon mean-free-paths beyond geometric expectations.

Findings

Long-wavelength phonons are strongly backscattered by pore arrangements.

Heat current anticorrelations lead to super-suppression of phonon mean-free-paths.

Ray tracing signatures can diagnose correlated scattering in nanostructures.

Abstract

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations, a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in numerical Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This super-suppression of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems.