MANY-BODY EFFECTS IN THE HIGHLY EXCITED-STATE OF FULLERENES - COMPARISON TO INDIRECT BAND-GAP SEMICONDUCTORS

The highly excited state of fullerenes is characterised by a luminesce nce output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation condi tions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited stat e is observed to be largely independent of temperature, contrasting sh arply with the thermally activated behaviour at low excitation densiti es. The degree of nonlinearity of the observed phenomena exclude an in terpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-li ke transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phen omena in the highly excited state luminescence and photoconductivity a re explained in terms of electron-hole droplet formation. The similari ties of the behaviours leads to a consideration of exchange and correl ation energies in fullerenes, which are calculated according to a phen omenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exc hange/correlation energy in the highly excited state of approximately 150 meV.