Spin-wave dispersion in half-doped La3/2Sr1/2NiO4

Recent neutron scattering measurements reveal spin and charge ordering in the half-doped nickelate, La3/2Sr1/2NiO4. Many of the features of the magnetic excitations have been explained in terms of the spin waves of diagonal stripes with a magnetic coupling structure which is fourfold symmetric. However, an optical mode dispersing away from the , point was not captured by this theory. We show here that this apparent optical mode is a natural consequence of stripe twinning in a diagonal stripe pattern with a magnetic coupling structure which is twofold symmetric, i.e., one possessing the same spatial rotational symmetry as the ground state.