Anisotropic field-induced changes in the superconducting order parameter of UTe_{2}

Abstract

UTe_{2} is a newly discovered unconventional superconductor, where electron Cooper pairs combine into a spin-triplet ground state. We study the specific heat C(H,T) of a high-quality UTe_{2} single crystal, with a single anomaly at a high superconducting transition temperature T_{c}=2.1 K and a low residual Sommerfeld coefficient γ_{0}. Applying magnetic fields up to 12 T along the crystallographic axes a, b, and c, we observe highly anisotropic behavior in γ_{0}(H). Below 4 T, γ_{0}(H)≈γ0+λi√H (i=a,b,c), which is expected for an unconventional superconductor with nodes on the Fermi surface away from the direction of the magnetic field. At 4 T (a and b axes) and 5.5 T (c axis), distinctive changes in the behavior of γ_{0}(H) suggest that a stable low-field ground state begins to evolve above 4 T. The data analysis indicates that the nodes are located near the a–b plane, with possible order parameters d_{B2u}+iɛd_{B1u} or d_{B2u}+iɛd_{Au}. Our findings support B_{2u} as the primary superconducting order parameter in UTe_{2}.