Search for: All records

Abstract As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices 1–9 . However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity 10,11 , are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries 12,13 , the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses. 

We report on the dielectric response of epitaxial BaSnO3 films grown on Nb-doped SrTiO3 (001) substrates using a hybrid molecular beam epitaxy approach. Metal-insulator-metal capacitors were fabricated to obtain frequency- and temperature-dependent dielectric constant and loss. Irrespective of film thickness and cation stoichiometry, the dielectric constant obtained from Ba1−xSn1−yO3 films remained largely unchanged at 15-17 and was independent of frequency and temperature. A loss tangent of ∼1 × 10−3 at 1 kHz < f < 100 kHz was obtained for stoichiometric films, which increased significantly with non-stoichiometry. Using density functional theory calculations, these results are discussed in the context of point defect complexes that can form during film synthesis.