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45 SUPERCONDUCTIVITY IN NON-ADIABATIC SYSTEMS WITH MAGNETIC IMPURITY

M Palistrant, F Kochorbe

2002
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Moldavian Journal of the Physical Sciences
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unpublished

An important factor in fullerenes and oxide materials is the presence of strong electron correlations as well as low Fermi energy (E F ∼ ω 0). To our mind in papers [1], [2] very interesting results were obtained for the case of pure nonadiabatic materials. The main effects are due to the vertex corrections and the cross diagrams that show a complex behavior with respect to the exchanged momentum (q) and frequency (ω). Positive corrections and a corresponding enhancement of T C arise naturally
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... f the electron-phonon scattering is characterized by small q values. The cutoff q c for the electron-phonon interaction is assumed to be small because of the effect of strong electron correlations on electron-phonon interaction [3]. The account of the effects of no-adiabaticity has been shown to produce an essential renormalization of the electron-phonon coupling λ in the Eliashberg equations [4] as well as an increase of T C at low values of the cutoff momentum q c up to values of high-T C materials at λ ∼ 1. Since materials mentioned above usually contain impurity (nonmagnetic or magnetic) there is a necessity to study influence of non-adiabaticity on impurty's dependences of thermodynamic quantities. The main purpose of this paper is to study the influence of a magnetic impurity on the properties of non-adiabatic superconductors (ω D ≤ ε F) and to answer the question in what degree the Abrikosov-Gor'kov theory [5] is valid to describe suppressing superconductivity in high-T C materials and other systems with strong electron correlations in which the Debye energy and Fermi energy are comparable. The considered system is described by the Hamiltonian of Frolich kind adds by the interaction of electrons with chaotically distributed paramagnetic impurity. In this Hamiltonian the presence of strong electron correlations are taken into account. It is performed by cutting off the electron-phonon and impurity interaction in the momentum space under the low values of the transferred momentum q c and q c1 , respectively. This approach was used in [1], [2] and was based on the studies about influence of strong electron correlations on the electron-phonon interaction [3]. Low values of q c promote both positive values of the vertex functions and dramatic rise of the temperature of superconducting transition. In the diagrams below expressions for the mass operators can be given in the form: Here () M p N 0 r ,Ω and () Ξ Ω S p 0 r , contain diagrams corresponding to the electron-phonon interaction (including all possible "intersecting" lines) [2]. The solid lines in M N 0 , Ξ S 0 and also in (1), (2) represent the complete Green functions averaging over the positions of the chaotically distributed impurity and the orientations of their spins. The wavy line refers to the electron-phonon interaction and the dashed line to the electron impurity one. So, mass operators contain the diagrams with the intersection of the line of

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