Magnons as pseudo-Goldstones in La-based cuprates: the effects of doping

In the Néel phase of La-based cuprates, magnons are pseudo-Goldstones as the rotational symmetry of the spin system is explicitly broken by the small anisotropies. Respecting the approximate SO(3) symmetry we calculate the doping and temperature dependence of the magnon gaps in La2-xSrxCuO4 (LSCO) within the framework of the anisotropic linear σ-model coupled to dipoles representing holes. We show that the temperature behaviour of the reduced magnon gaps depends weakly on the hole concentration and that the relative in-plane gap reduction with doping is insensitive to anisotropies of the parent compound. The obtained magnon gaps agree with experiments on LSCO, in particular the strong in-plane gap reduction with doping is reproduced.