  The microscopic details of flux line lattice state studied by muon spin
rotation is reported in an electron-doped high-$T_{\rm c}$ cuprate
superconductor, Sr$_{1-x}$La$_{x}$CuO$_{2}$ (SLCO, $x=0.10$--0.15). A clear
sign of phase separation between magnetic and non-magnetic phases is observed,
where the effective magnetic penetration depth [$\lambda\equiv\lambda(T,H)$] is
determined selectively for the latter phase. The extremely small value of
$\lambda(0,0)$ %versus $T_{\rm c}$ and corresponding large superfluid density
($n_s \propto \lambda^{-2}$) is consistent with presence of a large Fermi
surface with carrier density of $1+x$, which suggests the breakdown of the
"doped Mott insulator" even at the "optimal doping" in SLCO. Moreover, a
relatively weak anisotropy in the superconducting order parameter is suggested
by the field dependence of $\lambda(0,H)$. These observations strongly suggest
that the superconductivity in SLCO is of a different class from hole-doped
cuprates.
