Spatially inhomogeneous superconductivity in UTe2

Abstract

Newly-discovered superconductor UTe2 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 UTe2 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 (Tc). Notably, samples with higher Tc 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 (constant pressure) in the pressure-temperature phase diagram but also provide a consistent scenario for the sample dependence of UTe2.

Quenched randomness, thermal fluctuations and reentrant superconductivity: application to UTe$_2$

“…In this work, we discussed the effect of inhomogeneity near a field-driven first-order transition. Inhomogeneity in UTe 2 has been observed in NMR 14 and specific heat experiments 13 . Typically, the recent specific heat measurement observes different signals from different parts of a sample, which directly illustrates the importance of inhomogeneity.…”

“…(1)Kerr effect where time-reversal symmetry is found to be broken; (2) split transitions in specific heat, which are strictly required for the chiral phase in the orthorhombic system. However, the split transitions are observed in some samples 13,15,16 ; ], while other samples show a single transition 8,13,[16][17][18] , which puts the existence of the chiral phase into question. The proposed chiral phase has an intrinsic magnetization along the c-axis, which is perpendicular to the field H y .…”

“…Indeed, predominantly incommensurate antiferromagnetic fluctuations have been observed in this system 10,11 . UTe 2 also shows multiple superconducting phases especially as a function of pressure 12,13 . Further evidence for multiple phases comes from a recent Kerr effect study suggesting the interesting possibility of spin-triplet superconductivity with broken time-reversal symmetry.…”

Investigating the limits of superconductivity in UTe2

“…The specific heat of each piece is measured individually (solid lines) and compared to the data for the entire crystal (symbols). This experiment, first performed in [54], revealed a substantial variation in the specific heat around the superconducting transition in samples displaying a split transition at T c1 = 1.67 K and T c2 = 1.46 K. As mentioned above, these samples were grown at much higher temperatures (i.e., hot end at 1060 • C and cold end at 1000 • C). Importantly, these results have been reproduced by an independent group in [51].…”

“…As shown in figure 2(b), a substantial spatial inhomogeneity is also observed in samples grown at lower temperatures that also display a split transition. In addition, the specific heat variation in sample s6b is more pronounced than those in [51,54] as it occurs even at temperatures away from the superconducting transition, and it affects both the normal state and the residual specific heat coefficients. Here, we again observe the trend of increased residual specific heat as the specific heat jump at T c decreases.…”