We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of over 50 T. A clear signature of superconducting gap opening below $T_c$ was observed in the far-infrared reflectance spectra, with $2\Delta /{\mathrm{kT}}_c\approx 3.5–4.2$. Furthermore, we show that the new superconductor has electron-type conducting carriers with a rather low-carrier density.

• Received 2 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.057007