Flux line lattice state versus magnetism in electron-doped cuprate superconductor Sr_{1-x}La_{x}CuO_{2}

The microscopic details of flux line lattice state studied by muon spin rotation is reported in an electron-doped high-$T_{\mathrm{c}}$ 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 $T_{\mathrm{c}}$ 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.