Spin waves in frustrated quantum spin systems

The author presents a general Green function formalism to study the spin wave excitations in frustrated spin systems which are periodically defined. He gives an expression for the spin wave spectrum omega k and self-consistent equations for the local order parameter. He finds that omega k is linear in k for long wavelengths except for the ferromagnetic ground state and that the classical gauge invariance is broken by quantum fluctuations. The author applies the formalism to the fully frustrated simple cubic lattice. The zero-point motion is found to be rather small and the gauge invariance is restored at a temperature T0 below the transition. Comparison with Monte Carlo results is made.