Large reductions of the superconducting transition temperature T c and the accompanying loss of the thermal electrical resistivity (electron-phonon interaction) due to radiation damage have been observed for several A15 compounds, Chevrel phase and Ternary superconductors, and NbSe 2 in the high fluence regime. We examine these behaviors based on the recent theory of weak localization effect in superconductors. We find a good fitting to the experimental data. In particular, weak localization correction to the phononmediated interaction is derived from the density correlation function. It is shown that weak localization has a strong influence on both the phononmediated interaction and the electron-phonon interaction, which leads to the universal correlation of T c and resistance ratio.

Weak localization leads to the same correction to both the conductivity and the McMillan's electron-phonon coupling constant λ (and λtr, transport electron-phonon coupling constant). Consequently the temperature dependence of the thermal electrical resistivity is decreasing as the conductivity is decreasing due to weak localization, which results in the decrease of the temperature coefficient of resistivity (TCR) with increasing the residual resistivity. When λ and λtr are approaching zero, only the residual resistivity part remains and it gives rise to the negative TCR. Accordingly, the Mooij rule is a manifestation of weak localization correction to the conductivity and the electron-phonon interaction. This understanding provides a new means of probing the phonon-mechanism in exotic superconductors and an opportunity of fabricating new novel devices

Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e., λ tr ) is decreasing with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of T c (i.e., λ) and the resistance ratio (i.e., λ tr ) follows. This understanding provides a new means to probe the phonon mechanism in superconductors including MgB 2 . The merits of this method are its applicability to any superconductors and its reliability because the McMillan's electron-phonon coupling constant λ and λ tr change in a broad range, from finite values to zero, due to weak localization. Karkin et al's preliminary data of irradiated MgB 2 show the Testardi correlation, indicating that the dominant pairing mechanism in MgB 2 is the phonon-mediated interaction.

It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in a crystalline or an amorphous state. Based on the recent theory of Kim and Overhauser (KO), it is shown that as the system is getting disordered, the initial slope of the T c depression is decreasing by a factor ℓ/ξ 0 , when the mean free path ℓ becomes smaller than the BCS coherence length ξ 0 , which is in agreement with experimental findings. In addition, for a superconductor in a crystalline state in the presence of magnetic impurities the superconducting transition temperature T c drops sharply from about 50% of T c0 (for a pure system) to zero near the critical impurity concentration. This pure limit behavior was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same pure limit behavior in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.2