High-field properties of carbon-dopedMgB2 thin films by hybrid physical–chemical vapor deposition using different carbon sources

Abstract

We have studied the high-field properties of carbon-dopedMgB2 thin films prepared by hybrid physical–chemical vapor deposition (HPCVD). Carbon doping wasaccomplished by adding carbon-containing gas, such as bis(methylcyclopentadienyl)magnesiumand trimethylboron, into the hydrogen carrier gas during the deposition. In both cases,Tc drops slowly and residual resistivity increases considerably with carbon doping. Both thea andc latticeconstants increase with carbon content in the films, a behavior different from that of bulk carbon-dopedMgB2 samples. The films heavily doped with trimethylboron show very high parallelHc2 over 70 T at low temperatures and a large temperature derivative near Tc. These behaviors are found to depend on the unique microstructure of the films, which consists ofMgB2 layers a few-nanometers thick separated by non-superconductingMgB2C2 layers. This leads to an increase in the parallelHc2 by the geometrical effect, which is in addition to the significant enhancement ofHc2 due to changes in the scattering rates within and between the twobands present in films doped using both carbon sources. The highHc2 andhigh-field Jc(H) values observed in this work are very promising for the application ofMgB2 in high magnetic fields.