Sharp photoemission spectra in the quantum antiferromagnet

The low energy photoemission spectra in quantum antiferromagnets are studied by using several approximation-free calculations and rigorous theorems. The important and measurable property found is that the hole eigenstates with momenta differing by the antiferromagnetic wave vector Q are equivalent and degenerate in energy. However, the corresponding eigenstates differ by the presence or the absence of a well-defined quasiparticle corresponding to a singular. zero-energy, magnon, carrying spin one and momentum Q. This difference between the two eigenstates affects dramatically the spectral weight as a function of the scattered momentum, since a sharp effect at the surface of the magnetic Brillouin zone is predicted, in apparent agreement with recent experimental data.