Spatially inhomogeneous superconductivity in UTe2

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$.
Comment: 6 pages, 4 figures, includes supplemental information, changed conclusion on the origin of double-transition feature observed in some UTe2 samples