Unusual upper critical field of the ferromagnetic superconductor UCoGe

TL;DR

This study measures the upper critical field in UCoGe, revealing anisotropic, unconventional superconducting behavior with evidence for equal spin pairing and possible two-band superconductivity.

Contribution

It provides the first detailed measurements of $B_{c2}(T)$ in UCoGe, showing anisotropic critical fields and evidence for an axial gap symmetry with point nodes.

Findings

$B_{c2}(0)$ exceeds Pauli limit for certain directions

Strong anisotropy in critical fields along different axes

Possible two-band superconductivity indicated by curvature in $B_{c2}(T)$

Abstract

We report upper critical field $B_{c2}(T)$ measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. $B_{c2}(0)$ obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for $B \parallel a,b$ and shows a strong anisotropy $B_{c2}^{a} \simeq B_{c2}^{b} \gg B_{c2} ^{c}$. This provide evidence for an equal spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment $m_0 = 0.07 \mu_{B}$. An unusual curvature or kink is observed in the temperature variation of $B_{c2}$, which possibly indicates UCoGe is a two-band ferromagnetic superconductor.