Delocalization of tightly bound excitons in disordered systems

The localization length of a low-energy tightly bound electron-hole pair (e xcitons) is calculated by exact diagonalization for small interacting disor dered systems. The exciton localization length (which corresponds to the th ermal electronic conductance) is strongly enhanced by electron-electron int eractions, while the localization length (pertaining to the charge conducta nce) is only slightly enhanced. This shows that the two-particle delocaliza tion mechanism widely discussed for the electron pair case is more efficien t close to the Fermi energy for an electron-hole pair. The relevance to exp eriment is also discussed. [S0163-1829(99)08125-4].