Spin-Triplet Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe: Theoretical Aspect

TL;DR

This paper presents a theoretical study demonstrating that ferromagnetic fluctuations can induce spin-triplet superconductivity in UCoGe, with calculations matching experimental observations of the upper critical magnetic field.

Contribution

The work provides the first theoretical evidence that ferromagnetic fluctuations serve as the pairing mechanism for superconductivity in UCoGe.

Findings

Calculated upper critical field $H_{c2}$ agrees with experimental data.

Ferromagnetic fluctuations can induce spin-triplet pairing.

First demonstration of FM fluctuations as pairing glue in a superconductor.

Abstract

Identification of pairing mechanisms leading to the unconventional superconductivity realized in copper-oxide, heavy-fermions, and organic compounds is one of the most challenging issues in condensed-matter physics. Clear evidence for an electron-phonon mechanism in conventional superconductors is seen by the isotope effect on the superconducting transition temperatures $T_{\rm SC}$, since isotopic substitution varies the phonon frequency without affecting the electronic states. In unconventional superconductors, magnetic fluctuations have been proposed to mediate superconductivity, and considerable efforts have been made to unravel relationships between normal-state magnetic fluctuations and superconductivity. Here, we show that characteristic experimental results on the ferromagnetic (FM) superconductor UCoGe ($T_{\rm Curie} \sim 2.5 $ K and $T_{\rm SC} \sim 0.6$ K) can be understood consistently within a scenario of the spin-triplet superconductivity induced by FM spin fluctuations. Temperature and angle dependencies of the upper critical magnetic field of the superconductivity ($H_{c2}$) are calculated on the basis of the above scenario by solving the Eliashberg equation. Calculated $H_{c2}$ well agrees with the characteristic experimental results observed in UCoGe. This is a first example that FM fluctuations are shown to be a pairing glue of superconductivity.