QUANTUM-LATTICE FLUCTUATIONS AS A SOURCE OF FRUSTRATION IN THE ANTIFERROMAGNETIC HEISENBERG-MODEL ON A SQUARE LATTICE

A new approach has been developed to study the effect of quantum latti ce fluctuations as a source of frustration in the antiferromagnetic He isenberg model on a square lattice. By considering both the dispersion al and dispersionless phonons we show that when the spin-phonon coupli ng constant alpha(2)/2K or the phonon frequency omega(ph) increases th e sublattice magnetization [S-A(z)] decreases gradually. At some criti cal value (alpha(2)/2K)(c) or (omega(ph))(c), [S-A(z)] = 0 and the lon g-range order disappears. This can be attributed to the fact that the ground state fails to develop the antiferromagnetic long-range order b ecause of the quantum lattice fluctuations and some kind of quantum sp in liquid state might be energetically favorable.