Signatures of coherent phonon transport in frequency dependent lattice thermal conductivity

TL;DR

This paper demonstrates that frequency-dependent lattice thermal conductivity, $ppa( u)$, reveals signatures of coherent phonon transport, especially in complex materials, and provides experimental pathways to verify these effects.

Contribution

It introduces a method to identify coherent phonon transport via peaks in $ppa( u)$ and applies it to CuCl, challenging traditional interpretations of thermal conductivity in complex materials.

Findings

Peaks in $ppa( u)$ indicate coherent transport signatures.

Application to CuCl reveals clear evidence of coherence.

Reevaluation of thermoreflectance data in anharmonic materials.

Abstract

Thermal transport in highly anharmonic, amorphous, or alloyed materials often deviates from the predictions of conventional phonon-based models. First-principles approaches have introduced a coherent contribution to account for these deviations and to explain ultra-low lattice thermal conductivity, but direct experimental evidence for this mechanism remains elusive. Here, we propose that the frequency-dependent lattice thermal conductivity, $\kappa(\nu)$, provides a direct signature of coherent transport. Specifically, we show that peaks in $\kappa(\nu)$ arise from the frequency nesting of modes with identical wave vectors. Applying this approach to CuCl, we identify clear signatures of coherent transport in its dynamical lattice thermal conductivity. We revisit the interpretation of thermoreflectance experiments and argue that the conventional understanding breaks down in strongly anharmonic crystals, alloys, and amorphous materials. Finally, we discuss experimental pathways to measure $\kappa(\nu)$, offering a new route to verify coherent contributions in thermal transport.