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

TL;DR

This study investigates the anisotropic enhancement of the lower critical field in ultraclean UTe2 crystals, revealing unusual low-temperature behavior likely influenced by ferromagnetic fluctuations, advancing understanding of its exotic superconductivity.

Contribution

It provides the first thermodynamic measurements of critical fields in ultraclean UTe2 crystals and links anisotropic $H_{c1}$ enhancements to magnetic fluctuations.

Findings

Anomalously enhanced $H_{c1}$ along $b$- and $c$-axes at low temperatures

Unusual low-temperature upturns in $H_{c1}$ values

Strong Ising-like ferromagnetic fluctuations influence vortex line energy

Abstract

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 $H_{\rm c}$, the lower critical field $H_{\rm c1}$, and the upper critical field $H_{\rm c2}$ at low fields of these high-quality single crystals. From the comparison of the anisotropies in $H_{\rm c1}$ and $H_{\rm c2}$, we find that the experimental $H_{\rm c1}$ 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 $H_{\rm c1}$.