High-field properties of carbon-doped MgB₂ thin films by hybrid physical-chemical vapor deposition using different carbon sources

We have studied the high-field properties of carbon-doped MgB₂ thin films prepared by hybrid physical-chemical vapor deposition (HPCVD). Carbon doping was accomplished by adding carbon-containing gas, such as bis(methylcyclopentadienyl)magnesium and trimethylboron, into the hydrogen carrier gas during the deposition. In both cases, T_c drops slowly and residual resistivity increases considerably with carbon doping. Both the a and c lattice constants increase with carbon content in the films, a behavior different from that of bulk carbon-doped MgB₂ samples. The films heavily doped with trimethylboron show very high parallel H_c2 over 70T at low temperatures and a large temperature derivative near T_c. These behaviors are found to depend on the unique microstructure of the films, which consists of MgB₂ layers a few-nanometers thick separated by non-superconducting MgB₂C₂ layers. This leads to an increase in the parallel H_c2 by the geometrical effect, which is in addition to the significant enhancement of H_c2 due to changes in the scattering rates within and between the two bands present in films doped using both carbon sources. The high H_c2 and high-field J_c(H) values observed in this work are very promising for the application of MgB ₂ in high magnetic fields.