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Magnetic impurity effect on superconductivity in systems with comparable Fermi and Debye energies
M. E. Palistrant Institute of Applied Physics Academy of Sciences of Moldova
Abstract:
We obtain the basic system of equations for a superconductivity theory of a system with a chaotically distributed paramagnetic substitution impurity in which the Migdal theorem is violated (the condition
$\omega_{\operatorname D}\ll E_{\operatorname F}$ cannot be assumed). The electron-phonon and impurity diagrams and also the additional diagrams corresponding to intersections of the electron-phonon and electron-impurity lines are taken into account. In the weak electron-phonon coupling limit, we obtain an equation for the superconducting transition temperature
$T_C$ that differs from the corresponding equation for the usual superconductors by renormalizations of
$T_{C0}$ and of the impurity scattering parameter
$\rho$. These quantities depend essentially on the Migdal parameter
$m=\omega_{\operatorname D}/E_{\operatorname F}$ and on the transferred momentum
$q_c$. We show that the decrease of
$T_C$ with the increase of the impurity concentration is slowed, as compared with usual superconductors, to an extent determined by
$m$ and
$q_c$. We also evaluate the isotopic coefficient
$\alpha$, whose behavior as a function of the impurity concentration depends on
$m$ and
$q_c$.
Received: 03.11.1998
DOI:
10.4213/tmf752
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