Crossover between SC states in an unconventional superconductor UCoGe driven by ferromagnetic spin fluctuations

TL;DR

This paper investigates the interplay between superconductivity and ferromagnetic spin fluctuations in UCoGe, revealing a crossover between different superconducting states driven by magnetic fluctuations near a quantum critical point.

Contribution

It uncovers the existence of a crossover between superconducting states in UCoGe influenced by ferromagnetic spin fluctuations, independent of long-range ferromagnetic order.

Findings

Superconductivity persists near the ferromagnetic quantum critical point.

Superconducting regime is enhanced by ferromagnetic spin fluctuations.

Superconductivity exists even without long-range ferromagnetic order.

Abstract

UCoGe is reported as a weak ferromagnetic (FM) two-band superconductor (SC) with the critical temperature, $T_{\mathrm{SC}}\sim$ 0.7 K and the Curie temperature, $T_{\mathrm{C}}\sim$ 3 K at ambient pressure. We report exotic attributes of the SC and FM state in moderate magnetic fields. The observed phenomena clearly demonstrate that the SC regime is superior to the paramagnetic state in the vicinity of $T_{\mathrm{SC}}$. Above $T_{\mathrm{SC}}$, the zero-field state is characterized by a regime with strong FM spin fluctuations, suggesting proximity of the FM quantum critical point (FM-QCP). In addition, we observed that the robust SC regime develops independently on the existence or lack of the long-range FM ordering.