Phase Locking in the Heisenberg Helimagnet

The commensurability energy ΔE is calculated for a Heisenberg helimagnet whose wavelength is three lattice constants at zero temperature with a small but nonzero uniform field applied in the plane of polarization of the spins. It is shown that ΔE=0 for classical spins but ΔE≠0 for quantum spins when spin‐wave interactions are considered.