Imaging GHz surface acoustic wave modes in electrostricted LaAlO$_3$/SrTiO$_3$ heterostructures

TL;DR

This study demonstrates GHz surface acoustic wave modes in LaAlO$_3$/SrTiO$_3$ heterostructures at room temperature, visualizes their waveforms with high resolution, and explores their potential for quantum device applications.

Contribution

It provides the first detailed observation and imaging of GHz SAWs in STO-based heterostructures, revealing low-loss modes and a shear horizontal type for quantum transport.

Findings

SAW modes up to 2.2 GHz with low propagation loss

Atomic Acoustic Force Microscopy visualizes SAW waveforms at sub-micron resolution

Identifies shear horizontal mode suitable for quantum device coupling

Abstract

The LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface hosts a gate-tunable superconducting two-dimensional electron gas (2DEG) which can be programmed to create quantum devices such as ballistic electron waveguides and quantum dots. To fully exploit this platform for quantum transport, a key requirement is the ability to shuttle single electrons, electron pairs, and other exotic states between spatially separated devices with precision. Surface acoustic waves (SAWs), which travel along the surface of a solid, offer a powerful route to achieve this through their moving electrical potential that captures and transfers electrons. %acoustoelectric coupling. In particular, SAWs in the GHz regime enable fast, controlled transport of individual quantum particles. Although this approach is well-explored in GaAs-based 2DEG, SAW generation in STO remains largely unexplored due to the lack of intrinsic piezoelectricity at room temperature. Here, we investigate room-temperature SAWs in LAO/STO and observe SAW modes up to 2.2 GHz with very low propagation loss of the order $10^{-3}$ dB per wavelength. To directly visualize these modes, we employ Atomic Acoustic Force Microscopy (AAFM), achieving sub-micron resolution imaging of the SAW wave forms, providing insight into the electrostriction-induced SAW generation mechanism. Our measurements indicate a shear horizontal-type mode, which provides the ability to couple to in-plane degrees of freedom for future acoustoelectric and quantum device applications. This work studies the fundamentals of SAW excitation and propagation on STO, a widely used and commercially available substrate, enabling straightforward coupling of SAWs to a broad range of materials that can be grown or transferred onto STO.