Mapping the Phase Diagram of UTe₂ with Ultrasound

UTe₂ is a candidate spin triplet superconductor known to host at least three distinct superconducting phases as a function of applied hydrostatic pressure and magnetic field. It is still unclear if these phases are related, and if (and how) they influence one another. Ultrasound is a sensitive probe of the superconducting state in UTe₂ due to the large coupling between the superconducting order parameter and strain, and has been used to constrain the order parameter at ambient pressure. We extend pulse echo ultrasound in UTe₂ to hydrostatic pressures up to 2 GPa and observe clear signatures of transitions between the different superconducting states. The thermodynamic (elastic modulus) and transport (sound attenuation) quantities we measure provide insights into the order parameter symmetry and interactions between the different superconducting phases.