The upper critical field in doped MgCNi3

The upper critical field (Hc2) in the doped and undoped intermetallic superconductor MgCNi3 is studied. In MgCNi3-xMx (M = Ru, Fe), Hc2 decreases monotonically as x increases. In the case of Fe doping, Hc2 changes most dramatically: from 10.7 T at x = 0 to 7.9 T at x = 0.015, which might be caused by the fact that the 3d electrons in Fe break superconducting Cooper-pairs. Although, doping carbon on the boron site in Mg(B1-xCx)2 has a substantial effect on the upper critical field, here we report that for MgC1-xBxNi3, boron substitution slightly increases Hc2 for the lowest B doping level, x = 0.04, only. For x > 0.04 the upper critical field decreases slowly as x increases. The initial slope of Hc2(T) in MgC0.96B0.04Ni3 is nearly the same as that of pure MgCNi3