Correlation between superconductivity and ferromagnetism in UGe2

We measured the ac magnetic susceptibility of the ferromagnet UGe2 that exhibits superconductivity at high pressures within a ferromagnetic phase. It is known that the ferromagnetic phase is separated into two phases and thus there are two critical pressures, namely, P-X similar to 12kbar at which an unknown phase transition terminates and P-FM similar to 16 kbar at which a Curie temperature is suppressed to zero. We observed the following features in the vicinity of P-X: the discontinuity in the plot of superconducting transition temperature versus pressure, the maximum superconducting volume fraction in contrast to the small volume fraction near P-FM, and the markedly enhanced initial slope of the upper critical magnetic field (exceeding 200 kOe/K). These results indicate that the superconductivity directly correlates with P-X. We conclude therefore that it is the critical point P-X, rather than P-FM, that plays a crucial role in the appearance of the superconductivity.