Spiral Spin Order and Transport Anisotropy in Underdoped Cuprates

We discuss the spiral spin density wave model and its application to explain properties of underdoped La$_{2-x}$Sr$_x$CuO$_4$. We argue that the spiral picture is theoretically well justified in the context of the extended $t-J$ model, and then show that it can explain a number of observed features, such as the location and symmetry of the incommensurate peaks in elastic neutron scattering, as well as the in-plane resistivity anisotropy. A consistent description of the low doping region (below 10% or so) emerges from the spiral formulation, in which the holes show no tendency towards any type of charge order and the physics is purely spin driven.