Large upper critical field of superconductivity in the single crystal U₆Co

We grew single crystals of U₆Co by the self-flux method and measured the magnetic susceptibility, resistivity, and specific heat. The magnetic susceptibility shows very small anisotropy and weak temperature dependence, indicating small spin-susceptibility. Superconductivity was clearly observed in the resistivity, susceptibility, and specific heat at T_c ∼ 2.3 K. The upper critical field was remarkably large, 7.9 and 6.6 T for H ∥ [001] and [110], respectively, in the tetragonal structure, indicating that the ellipsoidal Fermi surface is slightly suppressed along the [001] direction according to the effective mass model. The specific heat shows a large jump at T_c with ΔC/γT_c = 1.58, and the field dependence of the specific heat at low temperatures shows an almost linear increase. These experimental results are well explained by the BCS model in the dirty limit condition. U₆Co is most likely a conventional s-wave superconductor with a full superconducting gap.