Laboratoire de Physique Théorique
Toulouse - UMR 5152

I present an overview of experimental data on underdoped La_2-xSr_xCuO_4 and the theoretical explanation of these results based on the spiral solution of the extended t-J model. Specifically I consider the following effects :

1)Incommensurate spin structures have been observed in the Neel phase (x < 0.02), in the Spin Glass phase (0.02 < x < 0.055), and in the superconducting phase (0.055 < x). We explain the structures, calculate incommensurabilities, and in the cases of the Neel and Spin Glass phases calculate the entire profile of the neutron scattering lines.

2)We explain the mechanism of pinning of the incommensurate spin structure to the orthorhombic b-direction in the Neel and in the Spin Glass phases.

3)We explain the mechanism of 45^o rotation of the spin structure exactly at the point of the insulator-superconductor transition (x=0.055) and demonstrate that the rotation is enforced by the Pauli principle.

4)We explain the mechanism for the anisotropy of dc conductivity in the insulating phase (x<0.055). The theoretical limit for the low temperature anisotropy is 60-70% in a good agreement with the experimental data.