Europhys. Lett.
Volume 47, Number 5, September 1999
Page(s) 588 - 594
Section Condensed matter: electronic structure, electrical, magnetic and optical properties
Published online 01 September 2002
DOI: 10.1209/epl/i1999-00429-5

Europhys. Lett, 47 (5), pp. 588-594 (1999)

Anomalous impurity effects in nonadiabatic superconductors

M. Scattoni 1, C. Grimaldi 1,2 and L. Pietronero 1,2

1 Dipartimento di Fisica, Università di Roma I "La Sapienza"
Piazzale A. Moro, 2, 00185 Roma, Italy
2 Istituto Nazionale Fisica della Materia, Unità di Roma 1 - Roma, Italy

(received 26 March 1999; accepted 5 July 1999)

PACS. 71.38$\rm +i$ - Polarons and electron-phonon interactions.
PACS. 74.20Mn - Nonconventional mechanisms (spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.).
PACS. 74.62Dh - Effects of crystal defects, doping and substitution.


We show that, in contrast with the usual electron-phonon Migdal-Eliashberg theory, the critical temperature $T_{\rm c}$ of an isotropic s-wave nonadiabatic superconductor is strongly reduced by the presence of diluted nonmagnetic impurities. Our results suggest that the recently observed $T_{\rm c}$-suppression driven by disorder in K3C60 (Phys. Rev. B, 55 (1997) 3866) and in Nd2-xCexCuO$_{4-\delta}$ (Phys. Rev. B, 58 (1998) 8800) could be explained in terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the isotope effect on $T_{\rm c}$ has an impurity dependence qualitatively different from the one expected for anisotropic superconductors.


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