Spin-flop transitions and spin-wave gaps in La2CuO4

We study the spin-wave spectrum and the spin-flop transitions in La2CuO4 in a uniform magnetic field at zero temperature. Using the nonlinear sigma model, we show that a field applied along the orthorhombic b direction leads to a two-step rotation of the staggered magnetization, first in the bc plane and then in the ac plane, until the order parameter is completely aligned along the c axis. In contrast, for a perpendicular magnetic field, we find a conventional spin-flop transition induced by the competition between the field and the interlayer coupling. A comparison with recent measurements of the field dependence of the in-plane spin-wave gap shows a beautiful agreement between theory and experiments.