Anisotropic Enhancement of Lower Critical Field in Ultraclean Crystals of Spin-Triplet Superconductor UTe2
Abstract
The paramagnetic spin-triplet superconductor UTe2 has attracted significant attention because of its exotic superconducting properties including an extremely high upper critical field and possible chiral superconducting states. Recently, ultraclean single crystals of UTe2 have become available, and thus measurements on these crystals are crucial to elucidate the intrinsic superconducting properties. Here, we report the thermodynamic critical field H c, the lower critical field H c1, and the upper critical field H c2 at low fields of these high-quality single crystals. From the comparison of the anisotropies in H c1 and H c2, we find that the experimental H c1 values with the magnetic field along b- and c-axes are anomalously enhanced, showing unusual low-temperature upturns. We propose an effect of the strong Ising-like ferromagnetic fluctuations on the vortex line energy as the origin of the anisotropic enhancement of H c1.