Non--Ginsburg-Landau Critical Current Behavior in MgCNi3

We present transport critical current measurements on microfibers consisting of a 80-nm thick layer of polycrystalline MgCNi3 synthesized directly onto 7-micron diameter carbon fibers. Near the transition temperature, Tc, the critical current density, Jc, is well described by the power law form [1-(T/T_c)^2)]^2, with no crossover to the Ginsburg-Landau exponent 1.5. We extrapolate Jc(0) ~ 4 x 10^7 A/cm^2, which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential Jc(T,H) = Jc(T) exp(-H/Ho) over the entire field range of 0 to 9 T. The unconventional scaling behavior of the critical current appears rooted in an anomalous temperature dependence of the London penetration depth, suggesting an unusual superconducting ground state in MgCNi3.