Deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdopedLa2−xSrxCuO4

Abstract

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 ${\text{La}}_{1.8}{\text{Sr}}_{0.2}{\text{Cu}}_{1\ensuremath{-}z}{M}_{z}{\text{O}}_{4}$ ($M=\text{Zn}$ or Mg) single crystals. With moderate impurity substitution concentrations of $z=0.049$ and 0.082 $(M=\text{Zn})$, the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to $z=0.13$ $(M=\text{Zn})$ or 0.15 $(M=\text{Mg})$, the resistivity shows a low-$T$ upturn but its temperature dependence indicates a finite conductivity in the $T\ensuremath{\rightarrow}0$ limit. In this weakly localized metallic state, that is intentionally achieved in the overdoped regime, a negative departure from the WF law is found, which is opposite to the theoretical expectation.