Dr Zhun-Yong Ong from the Institute of High Performance Computing Singapore will be giving a Materials Modelling Seminar.  All are welcome to attend.

Abstract:
The diffusion of heat across the boundary between two insulating crystalline solids is controlled by its Kapitsa (thermal) resistance, which is determined at the microscopic level by the reflection and transmission of quantised lattice vibrations (i.e. phonons), and depends strongly on the crystallographic microstructure of the interface. However, our conceptualisation of phonon scattering by the interface relies heavily on analogies from wave optics and acoustics, and remains limited by the lack of computationally efficient methods  or quantifying the transmission and reflection of individual phonons, constraining our ability to analyse the theoretical connection between phonon scattering and interfacial microstructure. 

In this talk, I discuss how these difficulties can be overcome by extending the Atomistic Green's Function method that is commonly used to study ballistic phonon transport. I first show how the Kapitsa resistance phenomenon can be treated as a scattering (S-matrix)  roblem within the familiar conceptual framework of conventional quantum  mechanics. This approach allows us to employ existing theoretical machinery, originally developed for studying quantum transport in open systems, as the basis for our extension of the Atomistic Green's Function (AGF) method.Our extension of the AGF method enables the precise calculation of transition amplitudes between phonon channels (i.e. the individual elements of the S-matrix) and yields insights into the dependence of the transmission and reflection  oefficients on interfacial microstructure as well as phonon frequency, momentum and polarisation. Other possible applications of our extended AGF approach include the lattice defect and edge scattering of phonons. To illustrate the utility of this method, we present some  imulation results and analysis, obtained using inputs from ab initio calculations, for the MoS2/WS2 interface and isotopically non-uniform graphene. The concepts and numerical techniques developed in our approach may be potentially useful for analogous scattering  roblems in other areas such as tight-binding models, photonics and acoustics.