Magnetism and superconductivity in La_{2- x}Sr_xCuO_4-y (invited)

We report the results of resistivity and magnetic susceptibility measurements of La_2-xSr_xCuO _4-y for 0≤x≤0.20. The various features (superconductivity, spin-density wave state, and localization) are consistent with a model of weakly correlated two-dimensional electrons in which van Hove singularities of the band structure near the Fermi level play a determinant role. We also emphasize that the electron localization observed near half-filling of the conduction band at x≅0.04 can be attributed to the effect of correlations boosted by electron Umklapp scattering in a nearly nested two-dimensional Fermi surface. We recall that similar effects have been observed in half-filled band organic conductors and superconductors.