Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe_{2}

The gap structure of a novel uranium-based superconductor UTe_{2}, 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 H∥a 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 T_{c} is tuned by the magnetization along the a axis. From these results, the gap symmetry of UTe_{2} is most likely described by a vector order parameter of d(k)=(b+ic)(k_{b}+ik_{c}).