Upper critical field enhancements of TMB HPCVD magnesium diboride

The $H_{c2}$ of four well textured carbon-doped MgB$_2$ films grown by HPCVD from tri-methyl boron (TMB) at flow rates from 2.5 to 10 sccm was measured in fields up to 45T. $H_{c2}$ derived from low- current, four-point magnetoresistance clearly increases with increasing TMB flow rate. TMB appears to be a more uniform dopant than the (C$_6$H$_7$)$_2$Mg used earlier. These earlier films exhibited $H_{c2}^{\parallel}(0)$ up to 70 T but also were imperfectly connected ($\rho(50\mbox{ K})\sim200-800$~$\mu\Omega$cm) due to excess amorphous C-rich phases observed between the MgB$_2$ grains. In strong contrast, $\rho(50\mbox{ K})$ was only $10-20$~$\mu\Omega$cm for the TMB films. When first measured, the linearly extrapolated $H_{c2}^{\parallel}(0)$ reached $\sim$40 T for the film with the highest TMB flow rate, but after about 3 months of aging, this value rose to $\sim50$~T. The angular dependence of $H_{c2} $ for this sample was measured up to 45~T yielding $H_{c2}^{\parallel}(4.2\mbox{ K}) = 45.8$~T and showing the good Ginzburg-Landau scaling with an $H_{c2}$ anisotropy of 2.88 at 4.2 K. These results are discussed in terms of the theory of dirty two-gap superconductors as a part of an in-depth study of the effect of ternary doping of magnesium diboride.