Anisotropic field-induced changes in the superconducting order parameter of UT e2

Abstract

UTe2 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 UTe2 single crystal, with a single anomaly at a high superconducting transition temperature Tc=2.1 K and a low residual Sommerfeld coefficient γ0. Applying magnetic fields up to 12 T along the crystallographic axes a, b, and c, we observe highly anisotropic behavior in γ0(H). Below 4 T, γ0(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 γ0(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 dB2u+iɛdB1u or dB2u+iɛdAu. Our findings support B2u as the primary superconducting order parameter in UTe2.