Conduction mechanism in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$ in the normal state

The temperature coefficient of the conductivity of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{z}}$, (∂ lnσ/∂T${)}_{\mathit{z}}$, has been measured by a new method as a function of composition z at 873 K. It shows a λ peak at the orthorhombic (O)–to–tetragonal (T) phase transition. In the O phase, it is negative, whereas in the T phase, it increases as z decreases, being at first negative and then positive. Its variations are qualitatively interpreted by a two-mechanism conduction in the range 6.3≤z≤6.8: a metallic one due to the holes in the ${\mathrm{CuO}}_2$ plane and a hopping one due to the holes in the chains.