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

Abstract

A $^{59}\mathrm{Co}$ 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 ${T}_{\text{Curie}}$, 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.