SCREENED INTERACTION AND COULOMB PSEUDO-POTENTIAL IN C60

Abstract

We study the screening for C 60 in the random phase approximation. For a free molecule we find a substantial reduction of the interaction between close atoms but an “antiscreening” of the interaction between distant atoms. For a C 60 solid there is an efficient screening of the intraband Coulomb interaction (U~0.05 eV ), leading to µ=UN(0)~0.4, where N(0) is the density of states per spin and molecule. We then study the renormalization of the Coulomb pseudo-potential µ* due to higher subbands in a simple two-band model. We find that the renormalization is small, although a traditional approach, summing ladder diagrams in the screened interaction, predicts a large renormalization. This suggests that µ* may not be much smaller than µ~0.4 in C 60. To obtain reasonable transition temperatures, Tc, this value of μ* (~0.4) requires values of the electron-phonon coupling constant λ, which are not very much larger than what has been calculated.