Probing topological superconductors with next-generation ultrasound techniques

Ultrasound measurements have a long history of groundbreaking discoveries in condensed matter physics. For example, the observation of a drop in the sound attenuation at the superconducting transition in tin was a crucial step toward proving BCS theory. However, in recent years, ultrasound techniques—such as resonant ultrasound spectroscopy (RUS) and pulse-echo ultrasound—have faced limitations due to strict sample requirements. Challenges like small sample sizes, material brittleness, and hazards associated with handling certain compounds (e.g., uranium-based crystals) have become increasingly common in today’s quantum materials. Here, we present advances in both techniques that overcome these barriers. We have developed an improved RUS method capable of analyzing irregularly shaped samples, as well as high-frequency, thin-film ZnO transducers that extend pulse-echo ultrasound to thinner samples and a broader frequency range. Applying these improvements, we demonstrate that the superconducting order parameter in the topological superconductor candidate UTe2 has a single component, possibly of py-wave symmetry. Our results highlight ultrasound as a powerful tool in the search for topological superconductivity and underline its importance in quantum materials research.