Nature of spin excitations in two-dimensional Mott insulators: Undoped cuprates and other materials

We investigate the excitation spectrum of a two-dimensional resonating vale nce bond (RVB) state. Treating the pi -flux phase with antiferromagnetic co rrelations as a variational ground state, we recover the long wavelength ma gnon as an "RVB exciton." However, this excitation does,lot exhaust the ent ire spectral weight and the high-energy spectrum is dominated by fermionic excitations. The latter can be observed directly by inelastic neutron scatt ering, and we predict their characteristic energy scales along different hi gh symmetry directions, in the magnetic Brillouin zone. We also interpret e xperimental results on two magnon Raman scattering and midinfrared absorpti on within this scenario.