Ultrasound Studies on the Unconventional Superconductor UTe₂

The unconventional spin-triplet superconductor UTe₂ has fascinated the condensed matter community since its discovery in 2019. Many intriguing properties like a giant upper critical field, unconventional magnetism, field-reinforced and reentrant superconductivity, and multiple superconducting phases under pressure have been discovered in the meantime. However, even the zero field and ambient pressure superconducting ground state is not yet understood. Investigations into this part of the phase diagram are complicated, since exact details vary significantly between different samples. For example, samples exist with a single superconducting transition at ambient pressure, but also with two successive transitions, similar to the experiments under hydrostatic pressure. We measure the full elastic tensor of both one and two transition samples using resonant ultrasound spectroscopy to study the symmetry of the superconducting order parameter, as well as to determine possible origins of this stark sample dependence. We find discontinuities at T_c in all compressional moduli, with an especially large jump in c₃₃, implying a particular sensitivity of superconductivity to deformations along the c axis.