Anisotropic enhancement of lower critical field in ultraclean crystals of spin-triplet superconductor candidate UTe2

Abstract

The paramagnetic spin-triplet superconductor candidate 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 Hc, the lower critical field Hc1, and the upper critical field Hc2 at low fields of these high-quality single crystals. From the comparison of the anisotropies in Hc1 and Hc2, we find that the experimental Hc1 values with the magnetic field along the 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 Hc1.