Local spin spirals in the Néel phase of La_2−xSr_xCuO₄

Experimental observations of lightly doped La_2−xSr_xCuO₄, x<0.02, revealed remarkable magnetic properties such as the incommensurate noncollinear ordering (additional to the Néel ordering) and a tremendous doping dependence of the uniform longitudinal susceptibility. We show that the spiral solution of the t-t^′-t^″-J model obtained by taking into account the Coulomb trapping of holes by Sr ions describes these puzzling data perfectly well. Our solution firstly explains why the incommensurate structure is directed along the orthorhombic b axis, and secondly allows a numerical calculation of the positions and shapes of the incommensurate neutron scattering peaks. Thirdly, we calculate the doping dependence of the spin-wave gap, and lastly, we study the longitudinal magnetic susceptibility and show that its doping dependence is due to the noncollinearity of the spin spiral.