Low-Temperature Specific Heat and Pair-Breaking in Superconducting Zr₆FeSb₂

The superconducting properties of Zr 6 FeSb 2 were studied using low temperature specific heat measurements on polycrystalline samples prepared by arc-melting followed by annealing. The samples were nearly phase-pure Zr 6 FeSb 2 with secondary phases constituting less than 2% as proved by the electron-probe microanalysis (EPMA). The specific heat exhibited a broad superconducting transition characterized by a peak at 0.75 K and a large residual electronic specific heat coefficient γ res = 39 mJ/K 2 mol, which is approximately 57% of the normal-state coefficient γ N . The large γ res is unlikely to originate from the small amount of non-superconducting secondary phases. Instead, we attribute γ res to significant pair-breaking effects caused by paramagnetic moments.