Static magnetic correlations near the insulating-superconducting phase boundary in La$_{2-x}$Sr$_{x}$CuO$_{4}$

A systematic neutron scattering study has been performed on several single crystals of La$_{2-x}$Sr$_{x}$CuO$_{4}$ for {\it x} near the lower critical concentration {\it x$_{c}$} for superconductivity. In the insulating spin-glass phase ({\it x} = 0.04 and 0.053), a single pair of incommensurate elastic magnetic peaks is observed at reciprocal lattice positions that are centered diagonally about ($\pi$,$\pi$) (unit square lattice notation) with respect to the tetragonal crystallographic axes in the ({\it h},{\it k},0) zone, corresponding to a one-dimensional spin modulation along the orthorhombic {\it b}-axis. Just inside the superconducting phase ({\it x} = 0.06), the incommensurate peaks at these diagonal positions broaden anisotropically, while two pairs of incommensurate magnetic peaks appear simultaneously at reciprocal lattice positions that are also centered on ($\pi$,$\pi$), but which run parallel to the tetragonal axes, identical to those previously reported in superconducting samples. Further into the superconducting phase ({\it x} = 0.07), the peak intensities at these parallel positions dominate those at the diagonal positions. The peak-width $\kappa$ along the spin-modulation direction exhibits an anomalous maximum in the superconducting phase near {\it x$_{c}$}, whereas the incommensurability parameter $\delta$ varies continuously across the phase boundary. These results are discussed in connection with the doping-induced superconducting phase transition.