Pairing interaction in superconducting UCoGe tunable by magnetic field

TL;DR

This study provides a semi-quantitative comparison between spin fluctuations and superconducting transition temperature in UCoGe, supporting the theory that spin fluctuations mediate unconventional superconductivity in this material.

Contribution

It offers the first semi-quantitative comparison linking NMR-derived spin fluctuations with $T_{SC}$ in UCoGe, confirming the spin-fluctuation-mediated pairing mechanism.

Findings

Superconducting transition temperature $T_{SC}$ correlates with ferromagnetic fluctuations.

External magnetic fields can tune $T_{SC}$ via changes in spin fluctuations.

Theoretical formalism explains pressure and field effects on $T_{SC}$ in UCoGe.

Abstract

The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semi-quantitative comparison between the superconducting-transition temperature ($T_{\rm SC}$) and spin fluctuations derived from the nuclear magnetic resonance (NMR) experiment on the ferromagnetic (FM) superconductor UCoGe, in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of $T_{\rm SC}$ by applied fields, as well as the pressure variation of $T_{\rm SC}$ around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.