Title: Thermal boundary conductance and thermal conductivity strongly depend on nearby environment

Abstract: At the nanoscale, the thermal boundary conductance (TBC) and thermal conductivity are not intrinsic properties of interfaces or materials but depend on the nearby environment. In this study, we demonstrate how the TBC of an interface is affected by the existence of a second interface, as well as how the thermal conductivity of a material is affected by the nearby materials. Using Si and Ge modeled by classical molecular dynamics simulations, the following phenomena are discovered. (1) The existence of a nearby interface can significantly change the TBC of the original interface. For example, by adding an interface after Si/Ge, the TBC can be increased from 400 to 700 MW/m2K. This is because the nearby interface serves as a filter of phonon modes, which selectively allows particular modes to pass through and affect the TBC of the original interfaces. This impact will disappear at the diffusive limit when the distance between interfaces is much longer than the phonon mean free path so that phonon modes recover equilibrium statistics before arriving at the second interface. (2) The thermal conductivity of a material can be significantly changed by the existence of neighboring materials. For example, a standalone 30-nm-thick Si's thermal conductivity can be increased from 50 to 280 W/mK, a more than 4-fold increase, beating the bulk thermal conductivity of Si, after being sandwiched between two Ge slabs. This is because the Ge slabs on the two sides serve as filters that only allow low-frequency phonons to transport heat in Si, which carry more heat than optical phonons. This work opens a new area of successive interface thermal transport and is expected to be important for nanoscale thermal characterization and thermal management.