  We report on a new electromagnetic phenomenon that emerges in Mott
insulators, i.e., materials that do not conduct electricity because of strong
electronic Coulomb repulsion. The phenomenon manifests as antiferromagnetic
ordering due to orbital electric currents which are spontaneously generated
from the coupling between spin currents and an external homogenous magnetic
field. This novel spin-charge current effect provides the mechanism to detect
the so far elusive spin currents by means of unpolarized neutron scattering,
nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism
by solving a half-filled Hubbard model on a frustrated ladder, a simple but
nontrivial case of strongly interacting electrons.
