Superconductivity in fullerides

Abstract

Experimental studies of superconductivity properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why the A 3 C 60 are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential µ * , describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.

Phase diagram of the one-dimensional Hubbard-Holstein model at half and quarter filling

“…One large family of such molecular crystalline materials are the organic conductors and superconductors. 1 While some molecular crystals such as the fullerene superconductors 2 have a threedimensional crystal structure, many other examples are either quasi-one-or quasi-two-dimensional, i.e., charge transport is restricted in certain directions due to anisotropic crystal structure. In addition to strong electron-phonon ͑e-ph͒ coupling to the molecular vibrations, electron-electron ͑e-e͒ interactions are often important in low dimensional materials.…”

Relevance of multiband Jahn-Teller effects on the electron-phonon interaction inA3C60

“…A simple application of the conventional Migdal-Eliashberg theory in these materials is however questioned for different reasons. On one hand, the small value of t gives rise to strong electronic correlation effects since t appears to be of the same order of the intra-molecular Hubbard repulsion U [1]. On the other hand, nonadiabatic effects are also expected to be relevant since the electronic energy scale t is of the same order as well of the phonon frequency scale ω ph [2].…”

“…The electronic band structure along the high-symmetry axes of the fcc Brillouin zone and the total electron DOS has been widely reported in literature [1]. In order to assess to which extent the realistic band structure can be approximated by a degenerate three-band model, it should be noted that (low energy) electronic and transport properties are mainly determined by quantities defined at the Fermi level, as the Fermi surface itself, the electron DOS at the Fermi level, the Fermi velocity, etc.…”

“…Electron and phonon properties in these compounds are described in terms of molecular crystals, where the small hopping integral t between nearest neighbor C 60 molecular orbitals sets the scale of the electronic kinetic energy, while electron-phonon coupling is essentially dominated by the intra-molecular vibrational modes [1]. First-principle calculations suggest that el-ph coupling is mainly dominated by the H g modes, having energies ω Hg ∼ 30 − 200 meV, with some small contribution from the A g modes.…”

Jahn–Teller Instability of Correlated C60 Anions

“…In particular, this interaction is supposed to be responsible for superconductivity of alkali-doped fullerides [1] with fullerenes being in C NÀ 60 multicharged anion forms. The lowest unoccupied t 1u molecular orbital in fullerene anions becomes partially filled by electrons that, according to group-theoretic analysis can interact with some vibrational modes only.…”