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

Abstract

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 $C(T,H,\hat{I}\text{c◯})$ measurements in various field orientations. Its angular $\hat{I}\text{c◯}(\ddot{I}•,\hat{I}¸)$ variation shows a characteristic shoulder anomaly with a local minimum in $H\parallel a$ at moderate fields rotated within the $ab$ and $ac$ planes. Based on the 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 $T_{\mathrm{c}}$ 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 $d(k)=(b+ic)(k_b+i{k}_c)$.

6 pages, 4 figures (main text) + 7 pages, 5 figures (Supplementary Material), accepted for publication in Phys. Rev. Research