Nonunitary triplet superconductivity tuned by field-controlled magnetization: URhGe, UCoGe, and UTe_2

摘要

We report on a theoretical study on ferromagnetic superconductors URhGe and UCoGe and identify the pairing state as a nonunitary spin-triplet one with time-reversal symmetry broken, analogous to the superfluid ~3He-A phase. A recently found superconductor UTe_2 with almost ferromagnetism is analyzed by the same manner. Through investigating their peculiar upper critical field H_(c2) shapes, it is shown that the pairing symmetry realized in all three compounds can be tuned by their magnetization curves under applied fields. This leads to the reentrant H_(c2) in URhGe, an S-shaped in UCoGe and an L-shaped H_(c2) in UTe_2 observed for the magnetic hard b-axis direction. The identification with double chiral form d(k) = (b + ic)(k_b + ik_c) in UTe_2 naturally enables us to understand (1) multiple phases with A_1, A_2, and A_0 phases observed under pressure, (2) the enhanced reentrant H_(c2) for the off-axis direction associated with first-order metamagnetic transition, and (3) Weyl point nodes oriented along the magnetic easy a axis. All three compounds are found to be topologically rich solid-state materials worth further investigation.