Normal-State Resistivity Anisotropy in UnderdopedRBa2Cu3O6+xCrystals

We have revealed new features in the out-of-plane resistivity ${\rho }_c$ of heavily underdoped $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ ( $R=\mathrm{Tm},\mathrm{Lu}$) single crystals, which give evidence for two distinct mechanisms contributing to the $c$-axis transport. We have observed a crossover towards “metal-like” ( $\partial {\rho }_c/\partial T>\mathrm{}0\mathrm{}$) behavior at the temperature $T_m$ which quickly increases with decreasing doping. The metal-like conductivity contribution dominates at $T<\mathrm{}{T}_m$ and provides a saturation of the resistivity anisotropy, ${\rho }_c/{\rho }_{\mathrm{ab}}$. The antiferromagnetic ordering is found to block this metal-like part of the $c$-axis conductivity and complete decoupling of ${\mathrm{CuO}}_2$ planes, which may be the reason of superconductivity disappearance.