Breakdown of the rigid-band picture in the alkali-metal-doped C/sub 60/; A/sub 2/C/sub 60/ and A/sub 4/C/sub 60/

Summary form only given. Alkali-metal-doped C/sub 60/, A/sub x/C/sub/, is extensively studied in connection with the superconductivity occurring at x=3 with T, - 30K. As a result, it has been shown that the rigid-band picture can´t be applicable to A_-Cro. That is, although A__CrO is expected to be a metal for x=I, 2, 3, 4, and 5 from the results of the theoretical calculations, a number of experimental studies show that A/sub x/C/sub 60/ is a semiconductor for x=2 and 4. The purpose of the present work is to elucidate why A/sub x/C/sub 60/ is a semiconductor for x=2 and 4. We introduce the model which exhaustively takes account of the results of the first-principle calculations; the electron-electron interaction and the electron-phonon interaction are properly included in the present analysis. We find that the ground state of A/sub 2/C/sub 60/ and A/sub 4/C/sub 60/ is a nonmagnetic semiconductor with the energy gap slightly larger than 0.1 eV. This comes from the fact that the electron-phonon interaction is considerably larger than the Hund coupling between the conduction electrons, resulting in the intramolecular negative-U effect. Furthermore, it is pointed out that the space group of the crystal structure is Pa3 if the host C/sub 60/ lattice is fcc; this is certainly the case of Na/sub 2/C/sub 60/