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

Abstract

A ${}^{59}$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.

5 pages, 4 figures

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Cited by in corpus (5)

• Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by ${}^{59}$Co NMR
• Pairing interaction in superconducting UCoGe tunable by magnetic field
• Coexistent spin-triplet superconducting and ferromagnetic phases induced by the Hund's rule coupling and electronic correlations II: Effect of applied magnetic field
• On the size of superconducting islands on the density-wave background in organic metals
• Superconductivity at the Pressure-Induced Ferromagnetic Critical Region in UCoGe