Measurements were made of the temperature dependence of the upper critical field, HC2(T), of C-doped MgB2. HC2(T) did not follow the well-known Werthamer–Helfand–Hohenberg result for a 1-gap dirty superconductor but could be described well by the result of a recent theoretical calculation for a 2-gap dirty superconductor. HC2(0) of the C-doped material was determined to be between 29 and 38T; substantially higher than that of pure MgB2 (15–23T).

Enhancement of the Upper Critical Field of MgB2 by Carbon-Doping. X.Huang, W.Mickelson, B.C.Regan, A.Zettl: Solid State Communications, 2005, 136[5], 278-82