Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

The absence of simple examples of superconductivity adjoining itinerant-ele ctron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductiv ity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we rep ort the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the cla ssic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to a rise from the same electrons that produce band magnetism. In this case, sup erconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-single t pairing normally associated with nearly antiferromagnetic metals.