Deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdoped La_{2-x}Sr_{x}CuO_{4}

TL;DR

This study examines how nonmagnetic impurities affect the validity of the Wiedemann-Franz law in overdoped La_{2-x}Sr_{x}CuO_{4}, revealing deviations in a weakly-localized metallic state contrary to theoretical predictions.

Contribution

It provides experimental evidence of deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdoped cuprates, challenging existing theoretical expectations.

Findings

WF law holds at moderate impurity levels with low resistivity.

Negative deviation from WF law observed at higher impurity concentrations.

Deviations occur in a weakly-localized metallic state, contrary to theoretical predictions.

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 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.