Anisotropic Enhancement of Lower Critical Field in Ultraclean Crystals of Spin-Triplet Superconductor UTe2

The paramagnetic spin-triplet superconductor UTe${}_2$ 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 UTe${}_2$ have become available, and thus measurements on these crystals are crucial to elucidate the intrinsic superconducting properties. Here, we report the thermodynamic critical field $H_{\mathrm{c}}$, the lower critical field $H_{\mathrm{c}1}$, and the upper critical field $H_{\mathrm{c}2}$ at low fields of these high-quality single crystals. From the comparison of the anisotropies in $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$, we find that the experimental $H_{\mathrm{c}1}$ 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 $H_{\mathrm{c}1}$.