PHONONS, ELECTRON-PHONON, AND ELECTRON-PLASMON COUPLING IN C-60 COMPOUNDS

We have performed ab initio calculations for the A(g) and H(g) intramo lecular phonon frequencies, using a full-potential linear muffin-tin-o rbital (LMTO) method. The deviation from experiment is typically 5%. W e have further calculated the electron-phonon coupling for these modes , as well as for some alkali-metal and libration modes, using both tig ht-binding and ab initio, full-potential LMTO methods. The tight-bindi ng calculations illustrate that for the intramolecular modes the tedio us Brillouin-zone sums can be replaced by calculations for a free mole cule, while for the libration modes a simplification of this type is n ot possible. The ab initio calculations show a strong coupling to the two highest and, to a lesser extent, to the second lowest H(g) intramo lecular modes, while the coupling is weak to the librations and the st udied alkali-metal modes. The total coupling to the H(g) intramolecula r modes is lambda = 0.068N(0), where N(0) is the density of states (st ates/eV spin C60), which corresponds to lambda approximately 0.6-0.7 f or the alkali-metal-doped C60 compounds. The difference in coupling be tween superconductivity and photoemission, where the molecule is charg ed during the emission process, is studied for the A(g) modes. Further more, the electron-plasmon coupling is calculated in a tight-binding f ormalism. This coupling (g/omega(p)2 approximately 1 is found to be su bstantial. The calculated electron-phonon couplings are finally tested by calculating the superconductivity transition temperature using the Eliashberg equation and the temperature-dependent resistivity. These calculations show that the calculated couplings are of the right order of magnitude, but probably somewhat small, and that the coupling to l ibrations and intermolecular modes should be weak.