  Recent discovery of oxypnictide superconductor LaFeAs(O,F) (LFAO-F) with the
critical temperature (Tc) of 26 K and succeeding revelation of much increased
Tc upon substitution of La for other rare earth elements (such as Sm, leading
to ~43 K) and application of pressure for LFAO-F (~ 43 K) has triggered broad
interest in the mechanism yielding relatively high Tc in this new class of
compounds. While they share a feature with high-Tc cuprates that
superconductivity occurs upon carrier doping to pristine compound which
exhibits magnetism, they also resemble the heavy-fermion compounds in the sense
that superconductivity appears in the vicinity of magnetic phase. Investigation
of electronic states near the boundary between these two phases might provide
some useful information on the mechanism of superconductivity, as it has been
proved to be the case in many exotic superconductors. Here we show by muon
experiment in the LFAO-F compound that a macroscopic phase separation into
superconducting and spin glass-like magnetic phases occurs at x=0.06 that is
near the phase boundary, where both the magnetism and superconductivity develop
simultaneously below a common Tc ~ 18 K. This accordance strongly suggests
intimate relationship between magnetism and superconductivity typically found
in heavy-fermion systems near the quantum critical point.
