Electron Correlation Effects in Theoretical Model of Doped Fullerides

摘要

6.1 Introduction The diversity of physical properties of doped fullerides has not been explained so far at a microscopic level despite the intensive experimental and theoretical studies conducted in recent decades. A variety of fullerene-based organic compounds have been synthesized with metals [1], hydrogen [2, 3], halogens [4-6], and benzene [7] that form a new class of organic conductors and semiconductors with tunable parameters. In polycrystalline C_(60) doped with alkali metals, at temperatures under 33 K superconductivity has been observed [8-13] with critical temperatures varying from 2.5 K for Na_2KC_(60) to 33 K for RbCs_2C_(60). Along with the phonon mechanism of Cooper pairing [14, 15], purely electronic pairing mechanism has been proposed [16]. To date, superconductivity of molecular conductors remains an open problem. According to the theoretical band structure calculations (see [17] for a review), fullerides with integer band-filling parameter n should be Mott-Hubbard insulators because all of them possess large enough intra-atomic Coulomb correlation. At the same time, the doped systems A_3C_(60) (where A= K, Rb, Cs) turn out to be metallic at low temperatures [1]. It has been noted in papers [18, 19] that for a proper description of the metallic behavior of A_3C_(60) (with x= 3 corresponding to the half filling of conduction band), the orbital degeneracy of the energy band is to be taken into account.