Paramagnetic Effects of j-electron Superconductivity and Application to UTe2

The putative spin-triplet superconductor UTe2 has an Ising-like magnetic anisotropy, indicating the Cooper pair spins parallel to the easy-axis. However, the upper critical field for the hard-axis direction is much larger than that for the easyaxis, implying the strong suppression of the Pauli depairing effect even for the hard-axis direction. This observation is consistent with the recent NMR measurements which show the tiny decrease of the Knight shift below the transition temperature T-c for hard-axis magnetic fields. We clarify that these behaviors are naturally understood as a result of multiorbital f-electron bands with total angular momentum j = 5/2. Taking into account spin-orbit couplings for an orthorhombic structure, we demonstrate that for hard-axis magnetic fields, the decrease of the spin susceptibility below T-c and the Pauli depairing effect are strongly suppressed.

Automated summary

The putative spin-triplet superconductor UTe2 exhibits an Ising-like magnetic anisotropy, with the upper critical field for the hard-axis direction significantly larger than that for the easy-axis, indicating strong suppression of the Pauli depairing effect. This behavior is a result of multiorbital f-electron bands with total angular momentum j = 5/2, where spin-orbit couplings for an orthorhombic structure lead to a decrease in spin susceptibility and strong suppression of the Pauli depairing effect in hard-axis magnetic fields below the transition temperature Tc.