Phonons in A_3C_(60) Lattice and Structural Dynamics

摘要

The critical temperature ( T_C) of superconductivity in A_3C_(60) compounds is generally lower smaller with alkali atoms (A). Furthermore T_C decreases with applied pressure. In the BCS model, these trends are explained by the lower density of states at the Fermi level for a decreased lattice constant (R). There is more than one counterexample, however, suggesting that BCS does not give the whole truth. The most important one is that the compound with the largest lattice constant, Cs_3C_(60), is not superconducting at all at ambient pressure. In this paper we derive a novel model where a negative lattice contribution to Hubbard U, proportional to 1/R, is taken into account. It is possible to explain why A_3C_(60) compounds with A = Li, and Na have a low T_C or are not superconducting at all, and why Cs_3C_(60) is superconducting only at applied pressure and then with the highest T_C of all C_(60) alkali fullerides. It is concluded that the density of states mechanism derived in the BCS model is in doubt. Nevertheless superconductivity in A_3C_(60) depends on electron-phonon coupling. The dominating phonon is the bond stretching A, phonon, a breathing phonon for the whole fullerene molecular ion.