Eur. Phys. J. B 21, 383-391 (2001)
https://doi.org/10.1007/PL00011123

Superconductivity of Rb₃C₆₀: breakdown of the Migdal-Eliashberg theory

¹ Dipartimento di Fisica, Universitá "La Sapienza", P.le A. Moro 2, 00185 Roma, and INFM Roma1, Italy
² École Polytechnique Fédérale de Lausanne, Département de Microtechnique IPM, 1015 Lausanne, Switzerland
³ INFM - Dipartimento di Fisica, Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Torino, Italy

Corresponding author: ^a emmcapp@pil.phys.uniroma1.it

Received: 30 March 2001
Published online: 15 June 2001

Abstract

In this paper, through an exhaustive analysis within the Migdal-Eliashberg theory, we show the incompatibility of experimental data of Rb₃C₆₀ with the basic assumptions of the standard theory of superconductivity. For different models of the electron-phonon spectral function we solve numerically the Eliashberg equations to find which values of the electron-phonon coupling λ, of the logarithmic phonon frequency and of the Coulomb pseudopotential reproduce the experimental data of Rb₃C₆₀. We find that the solutions are essentially independent of the particular shape of and that, to explain the experimental data of Rb₃C₆₀, one has to resort to extremely large couplings: . This results differs from the usual partial analyses reported up to now and we claim that this value exceeds the maximum allowed λ compatible with the crystal lattice stability. Moreover, we show quantitatively that the obtained values of λ and strongly violate Migdal's theorem and consequently are incompatible with the Migdal-Eliashberg theory. One has therefore to consider the generalization of the theory of superconductivity in the nonadiabatic regime to account for the experimental properties of fullerides.