Role of crystal structure and junction morphology on interface thermal conductance

TL;DR

This paper investigates how crystal structure and junction morphology influence interface thermal conductance, highlighting the role of mode spectrum and disorder in conductance enhancement or reduction.

Contribution

It introduces a mode-based framework to analyze how different interface morphologies affect thermal conductance, emphasizing the impact of mode spectrum expansion and voids in conserving modes.

Findings

Mixed interfaces can significantly enhance conductance when voids in conserving modes appear.

Weak interlayer coupling leads to rigid band shifts and mode spectrum expansion.

Interfacial alloy scattering can counteract conductance gains from mode expansion.

Abstract

We argue that the relative thermal conductance between interfaces with different morphologies is controlled by crystal structure through $M_{min}/M_c > 1$, the ratio between the {\it minimum mode} count on either side $M_{min}$, and the {\it conserving modes} $M_c$ that preserve phonon momentum transverse to the interface. Junctions with an added homogenous layer, "uniform", and "abrupt" junctions are limited to $M_c$ while junctions with interfacial disorder, "mixed", exploit the expansion of mode spectrum to $M_{min}$. In our studies with cubic crystals, the largest enhancement of conductance from "abrupt" to "mixed" interfaces seems to be correlated with the emergence of voids in the conserving modes, where $M_c = 0$. Such voids typically arise when the interlayer coupling is weakly dispersive, making the bands shift rigidly with momentum. Interfacial mixing also increases alloy scattering, which reduces conductance in opposition with the mode spectrum expansion. Thus the conductance across a "mixed' junction does not always increase relative to that at a "uniform" interface.