Semiconducting phases in the system $\mathrm{Y}{\mathrm{La}}_x{\mathrm{Ba}}_{2-x}{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($x=1.0-0.0\mathrm{}$) exhibit three-dimensional variable-range hopping conduction over a wide temperature ($T$) range (100-900 K). Thermopower ($S$) measurements support this observation. $S$ increases with $T$ for $x=1.0\mathrm{} \mathrm{and} 0.8$ indicative of transport at the chemical potential $E_{\mu }$. The decrease in $S$ with $T$ for $x=0.4\mathrm{} \mathrm{and} 0.3$ is indicative of gap at $E_{\mu }$. A schematic band picture is used to explain the observed behaviors.

• Received 24 July 1995

DOI:https://doi.org/10.1103/PhysRevB.52.16213