Abstract
We investigate the influence of optical phonon coupling across interfaces comprised of different materials with varying crystallographic orientations on the overall thermal boundary conductance. We show that for interfaces formed between a fcc solid and a L10 solid (where L10 solids exhibit alternating atomic layers in certain orientations), coupling between acoustic phonons in the fcc crystal and optical phonons on the L10-side of the interface leads to a highly anisotropic thermal boundary conductance, where optical phonons can considerably enhance the conductance in a preferred crystallographic orientation of the layered solid. We attribute this in part to directionally dependent group velocities of optical phonons in the different crystallographic directions. For interfaces comprised of materials exhibiting diamond cubic crystal structures, higher conductances are observed for interfaces where there is a better overlap of acoustic phonons on either side of the interface, whereas, acoustic phonons directly coupling with high frequency optical phonons is shown to lower the overall conductance, especially at high temperatures where anharmonic interactions become important. Unique to the interfaces formed between the materials with diamond cubic crystal structures studied in this work, the presence of localized interfacial optical modes mediate thermal conductance across these interfaces.
Highlights
In most modern-day nanoscale devices, thermal transport is limited by the high densities of interfaces rather than the materials that comprise the device[1, 2]
We show that for interfaces formed between a fcc solid and a layered L10 solid, optical phonons from the L10 crystal preferentially couple to acoustic phonons of the monatomic crystal when the interface is comprised of the layered crystal oriented in the direction which accommodates for the higher group velocities of the optical modes
At higher temperatures where anharmonic interactions become important, acoustic phonons directly coupling with high frequency optical phonons is shown to lower the overall conductance for these structures
Summary
In most modern-day nanoscale devices, thermal transport is limited by the high densities of interfaces rather than the materials that comprise the device[1, 2]. Prior works have demonstrated the influence of extrinsic factors such as defect concentration and roughness around the interface[2, 6, 12,13,14,15,16], strength of cross-species interaction and chemistry around the interface[17,18,19,20,21,22] and the relative crystallographic orientations of the two materials comprising the interface[23,24,25] in dictating interfacial heat flow Despite these advances in understanding interfacial heat flow, relatively few studies have focused on understanding the role of optical phonons on thermal boundary conductance at solid-solid interfaces, even though optical phonons can make up to 90% of the available vibrational modes in some materials. The presence of localized optical modes at the interface mediate thermal conductance across these interfaces, which are unique to the diamond cubic crystal structures studied in this work
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