Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, Hc2(T), of carbon-doped MgB2. Hc2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. Hc2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB2 (15-23 T).