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.
Categories: cond-mat.supr-con