Unusual Upper Critical Field of the Ferromagnetic Superconductor UCoGe

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 parallela,b and shows a strong anisotropy B(c2)(a) approximately B(c2)(b)>>B(c2)(c). This provides 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)=0.07micro(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.

Ferromagnetic order in U(Rh, Co)Ge

“…In Fig.4a Magnetization measurements on single-crystalline samples show URhGe [12] and UCoGe [22] are uniaxial ferromagnets with the ordered moment pointing along the c axis. The size of the ordered moments is 0.35-0.4 µ B /U-atom [1,12] and 0.07 µ B /U-atom, respectively, which is approximately twice as large as m 0 of the polycrystalline samples in agreement with an uniaxial anisotropy.…”

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

“…Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis revealed remarkable enigmatic behavior 7,8 : superconductivity survives far beyond the Pauli-limiting field along the a and b axis, whereas H c2 along the magnetic easy axis (c axis) is as small as 0.5 T. Since the NMR linewidth along the c axis is so broad, and H c2 along the c axis is so small that we could not detect the 59 Co NMR signal for H||c in the SC state, we focus on the 59 Co Knight-shift measurements for H||a and b.…”

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

“…In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9]. The observed characteristic H c2 behavior is one of mysterious features of SC UCoGe and its origin can be related to the mechanism of the superconductivity.Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-type FM ones along the c axis (longitudinal FM spin fluctuations) [10].…”

“…Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-type FM ones along the c axis (longitudinal FM spin fluctuations) [10].…”

“…Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing. In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9].…”