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

## TL;DR

This study investigates how pressure-induced ferromagnetic fluctuations influence superconductivity in UCoGe, revealing critical fluctuations near a quantum phase transition and their role in the superconducting state.

## Contribution

It provides experimental evidence of ferromagnetic quantum critical fluctuations persisting into the paramagnetic phase and their enhancement of superconductivity in UCoGe.

## Key findings

- FM phase vanishes at critical pressure with phase separation observed.
- Critical FM fluctuations persist above the critical pressure.
- Superconductivity remains almost unchanged across the FM quantum phase transition.

## 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 $T_{\textrm{Curie}}$, 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.