  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 wavevector $Q$ are equivalent and
degenerate in energy. However the corresponding eigenstates differs 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.
