Emergent interface vibrational structure of oxide superlattices

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 superlattices1–9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries12,13, 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.

Emergence of distinct electronic states in epitaxially-fused PbSe quantum dot superlattices

“…As further validation, we conducted off-axis EELS measurements from the epi-SLs and isolated QDs. By collecting EELS many tens of milliradians away from the optic axis of the electron microscope, the contribution from the delocalized dipole scattering is minimized, facilitating the identification of highly-localized emergent properties 37 . The measurements shown above were performed on-axis to maximize the signal to noise, but Fig.…”

Imaging Vibrational Excitations in the Electron Microscope

“…The acoustic branch shows high dispersion when compared to the optical branch at the interface across the Brillouin zone (Γ−Σ−K−X), resulting in thermal transport across the interface. In parallel, other researchers measured the phonon modes in strontium titanate−calcium titanate superlattices 50 and silicon−germanium interfaces 51 to map the spatial extent of interfacial phonon modes. Recent work by Gadre et al 52 observed that abrupt Si−Ge interfaces impede the phonon propagation to a higher degree than gradual interfaces, thus explaining the decrease in the thermal conductivity at abrupt interfaces.…”

A Roadmap for Edge Computing Enabled Automated Multidimensional Transmission Electron Microscopy

“…It has helped tie together the structure-property relationships in materials systems as diverse as interfaces [12], superlattices [13], domain walls [14,15], grain boundaries [16], nanoparticles [17], and catalyst surfaces [18]. This has led to the discovery of novel applications such as phonon modes at polar vortices and two-dimensional electrical liquids at oxide interfaces [19][20][21]. In the physical sciences, these applications have been in fields as diverse as lithium-ion batteries, catalyst systems, and alloy designs to integrate electronic circuits [22][23][24][25][26].…”