Orbital order out of spin disorder: how to measure the orbital gap

The interplay between spin and orbital degrees of freedom in the Mott-Hubba rd insulator is studied by considering an orbitally degenerate superexchang e model. We argue that orbital order and the orbital excitation gap in this model are generated through the order-from-disorder mechanism known previo usly from frustrated spin models. We propose that the orbital gap should sh ow up indirectly in the dynamical spin structure factor; it can therefore b e measured using the conventional inelastic neutron scattering method.