Paramagnetic effects of j-electron superconductivity and application to UTe2

Abstract

The putative spin-triplet superconductor UTe 2 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 easy-axis, 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 multi-orbital 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.

Anisotropic response of spin susceptibility in the superconducting state of UTe$_2$ probed with $^{125}$Te-NMR measurement

“…2 c) is defined as ∆K i (T ) ≡ K i (T )− K n,i with the normal-state Knight shift denoted as K n,i . The decrease in ∆K b (T ) observed at 1 T is ∼ 0.1%, which is consistent with the previous result [1] and the recent theoretical calculation based on the spin-triplet state [23]. Further reductions at a low temperature, related to the multi-gap properties, were not observed in the present sample as well as in the previous sample.…”

Non-unitary triplet superconductivity tuned by field-controlled magnetization --URhGe, UCoGe and UTe$_2$--

“…UTe 2 has been investigated experimentally [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and theoretically [29][30][31][32][33][34][35][36][37] . Simultaneously renewed interest on the former three compounds are developing.…”