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
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.
PACS: 74.70.Wz – Fullerenes and related materials / 74.20.-z – Theories and models of superconducting state / 63.20.Kr – Phonon-electron and phonon-phonon interaction
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2001
