  We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally
high fluorine concentration by using a CsCl flux method. Comparing to
conventional flux methods, this method can introduce about double the amount of
fluorine into the oxygen site. The obtained single crystal shows the highest
superconducting transition temperature (Tc = 57.5 K) in single crystalline iron
pnictides. In addition, the residual resistivity ratio (RRR) is almost three
times as large as that of previously reported single crystals. This suggests
that our single crystals are suitable for investigation of the intrinsic
superconducting properties since they have few defects and impurities. Using
both the Werthamer-Helfand-Hohenberg model and the effective mass model, we
demonstrated that a higher fluorine concentration suppresses the anisotropic
superconductivity of SmFeAsO1-xFx.
