Impurity Doping Effect in High $T_{c}$ Superconductors

It has been observed that impurity doping and/or ion-beam-induced damage in high $T_{c}$ superconductors cause a metal-insulator transition and thereby suppress the critical temperature. Based on our recent theory of the weak localization effect on superconductivity, we examine the variation of $T_{c}$ with increasing of impurity concentration $\rm (x)$ in $\rm{La_{1.85}Sr_{0.15}Cu_{1-x}A_{x}O_{4}}$ systems, where A $=$ Fe, Co, Ni, Zn, or Ga. We find that the doping impurity decreases the scattering matrix elements for electron-electron attractions, such as $V_{nn'}=-V[1-{2\over πk_{F}\ell} ln(L/\ell)]$, where $L$ and $\ell$ are the inelastic and elastic mean free paths, respectively. Using the mean free path $\ell$ determined from resistivity data, we find good agreements between our calculated values for $T_{c}$ and experimental data except for Ni-doped samples, where Ni impurities may enhance the pairing interaction.

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