Indirect Interaction Involving Impurity States in Superconductors

A new theoretical mechanism of the observed increase in transition temperature Tc in some superconductors containing magnetic-element impurities in low concentration (e.g. Fe in Ti etc.) has been suggested. In the concentration region of interest, these systems do not show the existence of any net localized magnetic moment. In the proposed mechanism, we have invoked indirect Coulomb and exchange-type interactions involving BCS pairs and impurity electron pairs in the singlet states interacting through empty impurity states. The resulting interaction between BCS pairs is attractive. The theoretical expression for Tc, inclusive of the present (attractive) and earlier (repulsive) interactions, gives the right dependence as observed experimentally. The estimated strength of the interaction parameter J turns out to be of the order of 0.5 eV. Such mechanisms seem to be important for systems containing impurities which provide empty orbital states as visualized above.