Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe

A Co-59 nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure P-c, and the NQR spectrum just below P-c showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature T-curie, suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above P-c, but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around P-c. Nuclear spin-lattice relaxation rate 1/T-1 showed the FM critical fluctuations around P-c, which persist above P-c and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. 1/T-1 sharply decreased in the superconducting (SC) state above P-c with a single component, in contrast to the two-component 1/T-1 in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe.