SHARP PHOTOEMISSION SPECTRA IN THE QUANTUM ANTIFERROMAGNET

The low energy photoemission spectra in quantum antiferromagnets are s tudied by using several approximation-free calculations and rigorous t heorems. The important and measurable property found is that the hole eigenstates with momenta differing by the antiferromagnetic wave vecto r Q are equivalent and degenerate in energy. However, the correspondin g eigenstates differ by the presence or the absence of a well-defined quasiparticle corresponding to a singular. zero-energy, magnon, carryi ng spin one and momentum Q. This difference between the two eigenstate s affects dramatically the spectral weight as a function of the scatte red momentum, since a sharp effect at the surface of the magnetic Bril louin zone is predicted, in apparent agreement with recent experimenta l data.