PARTICLE-HOLE BOUND-STATES IN MOTT-HUBBARD INSULATORS

I present results that indicate the existence of strongly bound particle-hole excitations below the edge of the upper Hubbard band in a single-band, large-U Hubbard model in two dimensions, at half-filling. The appearance of these exciton states is due to correlation effects between a vacancy and a doubly occupied site induced by the antiferromagnetic background, in contrast to the conventional mechanism of exciton formation via long-range Coulomb attraction. I argue that in the insulating parent compounds of the high-temperature superconducting cuprates these correlation effects are of the same order of magnitude as particle-hole attraction due to longer-range Coulomb interactions.