Phonon transport in freestanding SrTiO3 down to the monolayer limit

Perovskites down to the monolayer limit have emerged and have attracted increased interest due to their two-dimensional nature with potentially novel physical properties. Here, we investigate the phonon transport in the oxide perovskite SrTiO3 with thicknesses from the monolayer limit to 10 nm by constructing an accurate first-principles machine-learning potential and combining it with the Boltzmann transport equation and homogeneous nonequilibrium molecular dynamics simulations. Compared to its bulk counterpart, the phonon dispersion relation of monolayer SrTiO3 is insensitive to temperature, and the calculated in-plane thermal conductivity of monolayer SrTiO3 is much larger than that of bulk SrTiO3, which mainly results from the unique out-of-plane atomic vibrations in monolayer SrTiO3. The thermal conductivity of SrTiO3 thin film first decreases and then approaches the bulk value as thickness increases from the monolayer limit to 10 nm. The hardening of the out-of-plane acoustic phonon branch and the transition of low-frequency optical phonons can explain the observed trend in thermal conductivity transitions. Our study demonstrates that monolayer SrTiO3 has a higher thermal conductivity than its bulk counterpart with covalent bonds at the first-principles level of accuracy, and dimension reduction has a weak inhibition on phonon transport in freestanding atomically smooth SrTiO3 thin films, which furthers the understanding of phonon transport in two-dimensional perovskite thin films.

Automated summary

In this study, the phonon transport in perovskite SrTiO3 with thicknesses ranging from the monolayer limit to 10 nm was investigated using a first-principles machine-learning potential combined with the Boltzmann transport equation and molecular dynamics simulations. It was found that the phonon dispersion relation of monolayer SrTiO3 is insensitive to temperature, and its thermal conductivity is higher than that of bulk SrTiO3 due to the unique atomic vibrations. The thermal conductivity of SrTiO3 thin film decreases initially and then approaches the bulk value with increasing thickness, which can be explained by the hardening of the out-of-plane acoustic phonon branch and the transition of low-frequency optical phonons.