The role of interface phonon modes in the thermal conductivity of superlattices

In superlattices (SLs), wherein alternating atomic layers are separated by interfaces, conventional understanding suggests that thermal conductivity decreases as the interface concentration increases, since interfaces scatter phonons, thus hindering heat transfer. However, it has been found that, in SrTiO₃/CaTiO₃ superlattices, when the atomic-layer thickness approaches the unit-cell scale, interface phonon modes dominate the material's thermal properties. Consequently, the thermal conductivity decreases and then increases with rising interface concentration. In this work, we perform density-functional-theory (DFT) calculations on phonon properties of SrTiO₃/CaTiO₃ SLs with different layer thicknesses. We found isolated interface modes in thick-layer SLs and modes propagating through the material in thin-layer SLs. We further employ DFT-based machine-learning molecular dynamics (MD) simulations and calculate thermal conductivity as a function of SL layer thickness. The role of different types of phonon modes is analysed and discussed. This study illuminates the prospects of tailoring thermal transport in superlattices.