Upper Critical Field of Carbon-doped MgB$_{2}$ Thin Films by HPCVD Using TMB

We report $H_{c2}$ measurements in high magnetic field up to 60 T on carbon-doped MgB$_{2}$ thin films. About 100 nm thick carbon-doped MgB$_{2}$ films were fabricated by the Hybrid Physical-Chemical Vapor Deposition (HPCVD) method using Trimethylboron (TMB) as the doping source. For heavily doped MgB$_{2}$ films, high slope $-dH_{c2}^{//ab} /dT$ near T$_{c}$, as large as 8.3 T/K, was obtained for a sample with the T$_{c}$ of 32.3 K. The $H_{c2}$(T) in ab-plane has a downward curvature and is above 60 T at low temperatures. The c-direction $H_{c2}$ is close to 20 T. The $H_{c2}$ anisotropy increases with carbon-doping, as opposed to the reported results from other carbon-doping techniques. The TEM study shows the MgB$_{2}$ forms many nm thick layers sandwiched in the MgB$_{2}$C$_{2}$ matrix. Using two-gap dirty limit theory, the temperature dependences of $H_{c2}$ and abnormal $H_{c2}$ anisotropy in the heavily doped samples are explained by the presence of many very thin MgB$_{2}$ layers.