Transport Anisotropy due to Spiral Spin Order in Underdoped Cuprates

I will discuss the in-plane transport anisotropy in the spin-glass phase of La$_{2-x}$Sr$_x$CuO$_4$ within a theoretical scenario where the physics is purely spin driven (no charge order is present), and a spiral spin density wave is formed in the ground state. Such an approach is well justified for the extended t--J model at low doping. In the low-temperature, variable-range hopping regime, the calculated anisotropy of 50--80 percent (depending on temperature) is in excellent agreement with experiment [1], demonstrating that charge ordering tendencies are not necessary to explain the observed transport anisotropy. This work is part of a series in which we show that the spiral approach provides a consistent description of the low-doping region. \newline [1] V.N. Kotov and O.P. Sushkov, Phys. Rev. B 72, 184519 (2005).