Static magnetic correlations near the insulating-superconducting phase boundary in La_2-xSr_xCuO₄

An elastic neutron-scattering study has been performed on several single crystals of La_2-xSr_xCuO₄ for x near a lower critical concentration x_c for superconductivity. In the insulating spin-glass phase (x=0.04 and 0.053), the previously reported one-dimensional spin modulation along the orthorhombic b axis is confirmed. Just inside the superconducting phase (x=0.06), however, two pairs of incommensurate magnetic peaks are additionally observed corresponding to the spin modulation parallel to the tetragonal axes. These two types of spin modulations with similar incommensurabilities coexist near the boundary. The peak width κ along the spin-modulation direction exhibits an anomalous maximum in the superconducting phase near x_c, where the incommensurability δ increases monotonically upon doping across the phase boundary. These results are discussed in connection with the doping-induced superconducting phase transition.