Clean Epitaxial MgB}$_{2}$\textbf{ Films Fabricated by \textit{Ex Situ} Annealing of CVD-Grown B Films in Mg Vapor

Epitaxial MgB$_{2 }$films have been successfully fabricated by \textit{ex} situ annealing of B films, grown by chemical vapor deposition (CVD), in Mg vapour. The films show a sharp superconducting transition T$_{c}$ of about 40 K, a low residual resistivity of less than 2 $\mu \Omega $cm, and a high residual resistivity ratio RRR of about 10. At self field, the value of critical current density$ J_{c}$ for a 3 $\mu $m thick film is 1.7$\times $10$^{6}$ Acm$^{-2}$ at 5 K and 1.2$\times $10$^{6}$ Acm$^{-2}$ at 20 K. The high $T_{c}$, low residual resistivity, high \textit{RRR} and high $J_{c}$ indicate the cleanness and good connectivity of the films. The results demonstrate that the \textit{ex situ} deposition method can produce clean MgB$_{2}$ films with superior superconducting properties, which is significant for applications such as MgB$_{2}$ superconducting cavities and coated conductor wires and tapes.