Thermal conductivity of the n = 1-5 and 10 members of the (SrTiO₃)_n SrO Ruddlesden-Popper superlattices

Unlike many superlattice structures, Ruddlesden-Popper phases have atomically abrupt interfaces useful for interrogating how periodic atomic layers affect thermal properties. Here, we measure the thermal conductivity in thin films of the n = 1-5 and 10 members of the (SrTiO3)nSrO Ruddlesden-Popper superlattices grown by molecular-beam epitaxy and compare the results to a single crystal of the n = 1 Ruddlesden-Popper SrLaAlO4. The thermal conductivity cross-plane to the superlattice layering (k33) is measured using time-domain thermoreflectance as a function of temperature and the results are compared to first-principles calculations. The thermal conductivity of this homologous series decreases with increasing interface density. Characterization by x-ray diffraction and scanning transmission electron microscopy confirms that these samples have a Ruddlesden-Popper superlattice structure.