Multiexciton molecules in the hexaborides

We investigate multiexciton bound states in a semiconducting phase of dival ent hexaborides. Due to three degenerate valleys in both the conduction and valence bands the binding energy of a 6-exciton molecule is greatly enhanc ed by the shell effect. The ground state energies of multiexciton molecules are calculated using the density functional formalism. We also show that c harged impurities stabilize multiexciton complexes leading to condensation of localized excitons. These complexes can act as nucleation centers of loc al moments.