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

Abstract

We report on theoretical studies on ferromagnetic superconductors, URhGe, and UCoGe and identifies the pairing state as a non-unitary spin-triplet one, analogous to superfluid $^3$He-A phase. A recently found superconductor UTe$_2$ with almost ferromagnet is analyzed by the same manner. Through investigating their peculiar upper critical field $H_{\rm c2}$ 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 $H_{\rm c2}$ in URhGe, an S-shaped in UCoGe and an L-shaped $H_{\rm c2}$ in UTe$_2$ observed under the field direction parallel to the magnetic hard axis in orthorhombic crystals in common. The identification with double chiral form: ${\bf d}(k)=(\hat{b}+i\hat{c})(k_b+ik_c)$ 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 $H_{\rm c2}$ for the off-axis direction fields associated with first order meta-magnetic transition, and (3) Weyl point nodes oriented along the $a$-axis. All three compounds are found to be topologically rich solid-state materials worth further investigation.