UGe2: A ferromagnetic spin-triplet superconductor

The identification of a spin-triplet superfluid phase in ${}^3\mathrm{He}$ naturally led to more general theoretical predictions that spin triplet superconductivity might occur near to a ferromagnetic instability in some metals. The recent discovery of superconductivity near a ferromagnetic quantum critical point in ${\mathrm{UGe}}_2$ now calls for these predictions to be reexamined experimentally. In this light it initially appears surprising that superconductivity in ${\mathrm{UGe}}_2$ has only been detected in the ferromagnetic phase and not also at pressures above the critical pressure for the suppression of ferromagnetism. In this paper we provide evidence that the superconductivity is indeed a bulk property. We also observe the evolution with pressure of the magnetic order by neutron scattering and find that the ferromagnetic component of the order is still present at a pressure and temperature where superconductivity is found. In resistivity measurements we identify an additional transition within the ferromagnetic state. The characteristic temperature of this transition, $T_x,$ decreases with pressure and disappears at a pressure $P_x$ close to the pressure at which the superconductivity is strongest. Evidence is presented that this transition is also induced by a magnetic field at pressures just above $P_x.$ An observed unusual reentrant behavior of the superconductivity with field at a pressure of 13.5 kbar is then qualitatively explained. These results suggest that the transition at $P_x$ is intricately related to the appearance of superconductivity, which could explain why the superconductivity is apparently confined to the ferromagnetic phase.