Critical dynamics of singlet and triplet excitations in strongly frustrated spin systems

Data for the two-dimensional J₁--J₂ model shows that the critical points for singlet and triplet excitations near J₂/J₁≈0.38 are very close together, or coincident. We propose that this is a more general result: in strongly frustrated spin systems the critical dynamics of singlet and triplet excitations are closely related. An effective field theory for such a critical point, or points, is developed, and its implications discussed. The analysis suggests the existence of gapless or almost gapless singlet excitations in the magnetically disordered phase.