Metallic to variable-range-hopping transition controlled by oxygen content in La-Sr-Cu-O

We have performed electrical resistivity measurements on the superconductor ${\mathrm{La}}_{1.8}{\mathrm{Sr}}_{0.2}\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ over a wide range of temperatures and oxygen concentration. The results provide experimental evidence for a transition from a metallic behavior at high temperatures to a variable-range hopping regime at low temperatures. The transition is controlled by the oxygen content. Correlation between both regimes is also shown.