Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

The gap structure of a novel uranium-based superconductor UTe2, situated in the vicinity of ferromagnetic quantum criticality, has been investigated via specific-heat C(T,H,ω) measurements in various field orientations. Its angular ω(φ,θ) variation shows a characteristic shoulder anomaly with a local minimum in Hâ1a at moderate fields rotated within the ab and ac planes. Based on theoretical calculations, these features can be attributed to the presence of point nodes in the superconducting gap along the a direction. Under the field orientation along the easy-magnetization a axis, an unusual temperature dependence of the upper critical field at low fields together with a convex downward curvature in C(H) were observed. These anomalous behaviors can be explained on the basis of a nonunitary triplet state model with equal-spin pairing whose Tc is tuned by the magnetization along the a axis. From these results, the gap symmetry of UTe2 is most likely described by a vector order parameter of d(k)=(b+ic)(kb+ikc).