Quantum effects in helimagnets: Disappearance of the soft mode at the wave vector of magnetic order

Usually the quantum nature of spins has only a limited effect on properties like the spin wave spectrum of a system and the S → ∞ limit (classical vectorial model) is often considered. On the contrary, we show that in helimagnets the quantum corrections can have dramatic effects. In absence of magnetic anisotropy the spin wave spectrum in the S → ∞ limit vanishes at the wave vectors k=±Q, Q being the wave vector characterizing the helical order. We find that this soft mode is destroyed by quantum effects and also the magnitude of Q can be modified significantly. Moreover, even any structure in the spectrum for k∼±Q is very easily eliminated for Hamiltonian parameters of physical interest. The model we consider which we call ANNNH model, is the quantum analogous of the well-known ANNNI model.