Deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdoped La_(2-x)Sr_(x)CuO₄

To investigate the validity of the Wiedemann-Franz (WF) law in disordered but metallic cuprates, the low-temperature charge and heat transport properties are carefully studied for a series of impurity-substituted and carrier-overdoped La_{1.8}Sr_{0.2}Cu_{1-z}M_zO_4 (M = Zn or Mg) single crystals. With moderate impurity substitution concentrations of z = 0.049 and 0.082 (M = Zn), the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to z = 0.13 (M = Zn) or 0.15 (M = Mg), the resistivity shows a low-T upturn but its temperature dependence indicates a finite conductivity in the T \to 0 limit. In this weakly-localized metallic state that is intentionally achieved in the overdoped regime, a {\it negative} departure from the WF law is found, which is opposite to the theoretical expectation.