Nonadiabatic Channels in the Superconducting Pairing of Fullerides

Abstract

We show the intrinsic inconsistency of the conventional phonon mediated theory of superconductivity in relation to the observed properties of Rb3C60. The recent, highly accurate measurement of the carbon isotope coefficient alpha(C) = 0.21, together with the high value of T(c) (30 K) and the very small Fermi energy E(F) (0.25 eV), unavoidably implies the opening of nonadiabatic channels in the superconducting pairing. We estimate these effects and show that they are actually the key elements for the high value of T(c) in these materials compared to the very low values of graphite intercalation compounds.

High-Temperature Superconductivity in Lattice-Expanded C 60

“…It is generally thought that superconductivity in C 60 is mediated by electron-phonon interaction. In light of T c exceeding 100 K, it will be particularly revealing to evaluate the effective Coulomb interaction and the role of nonadiabatic pairing channels, because the Fermi energy is not much greater than the phonon energy (12,13,17,18 A mechanism to enhance hardness in multilayer coatings is proposed. Using the technologically important hard transition metal carbides as prototypes, although the principle is transferable also to other systems, we demonstrate, from first-principles calculations, that by suitable alloying the energy difference between several competing structures in the transition metal carbides is small or tunable.…”

HighTcSuperconductivity inMgB2by Nonadiabatic Pairing

“…Even there, LDA estimates of the el-ph coupling λ were insufficient to account for the high T c and for the small isotope effect. Such a discrepancy has been explained in terms of opening of nonadiabatic channels which, under favourable conditions fulfilled in fullerenes, can effectively enhance the superconducting pairing [14]. A key role is played by the small Fermi energy E F that in fullerenes is of the same order of the phonon frequency, violating the adiabatic assumption (ω ph ≪ E F ).…”

Nonadiabatic Pauli susceptibility in fullerene compounds

“…The main difference with respect to the vertex function lies in the momentum dependence, which we handle in the same way as reported in Ref. 8. Therefore, in Eq.…”

“…6 However, the description of superconductivity of Rb 3 C 60 within the traditional Migdal-Eliashberg (ME) theory is found to be inconsistent with respect to the adiabatic hypothesis λω ph /E F ≪ 1 which is at the basis of the ME theory itself. 8 Instead, by relaxing the adiabatic hypothesis, a generalized formulation which includes nonadiabatic channels in the electron-phonon interaction provides a more self-consistent decription of superconductivity, and suggests that the key ingredient for the high values of T c in C 60 -based materials is a constructive nonadiabatic interference rather than strong electron-phonon couplings.…”