Spin-glass phase of cuprates

We investigate a phenomenological model for the spin-glass phase of La2-xSrxCuO4, in which it is assumed that holes doped into the CuO2 planes localize near their Sr dopant, where they cause a dipolar frustration of the antiferromagnetic environment. In absence of long-range antiferromagnetic order, the spin system can reduce frustration, and also its free energy, by forming a state with an ordered orientation of the dipole moments, which leads to the appearance of spiral spin correlations. To investigate this model, a nonlinear sigma model is used in which disorder is introduced via a randomly fluctuating gauge field. A renormalization-group study shows that the collinear fixed point of the model is destroyed through the disorder and that the disorder coupling leads to an additive renormalization of the order-parameter stiffness. Further, the stability of the spiral state against the formation of topological defects is investigated with the use of the replica trick. A critical disorder strength is found beyond which topological defects proliferate. Comparing our results with experimental data, it is found that for a hole density x>0.02, i.e., in the entire spin-glass regime, the disorder strength exceeds the critical threshold. In addition, some experiments are proposed in order to distinguish if the incommensurabilities observed in neutron-scattering experiments correspond to a diagonal stripe or a spiral phase.