Low-temperature resistivity of single crystals YBa_2Cu_3O_{6+x} in the normal state

Abstract

A scan of the superconductor -- nonsuperconductor transformation in single crystals YBa_2Cu_3O_{6+x} (x about 0.37) was done in two alternative ways, namely, by applying the magnetic field and by reducing the hole concentration through the oxygen rearrangement. The in-plane normal-state resistivity \rho_{ab} obtained in both cases was quite similar; its temperature dependence can be fitted by logarithmic law in the temperature range of almost two decades. However, a different representation of the \sigma_{ab}=1/\rho_{ab} by a power law typical for a 3D-material near a metal -- insulator transition is also plausible. The vertical conductivity \sigma_c=1/\rho_c followed the power law and neither \sigma_c(T), nor \rho_c(T) could be fitted by log(T). It follows from the \rho_c measurements that the transformation at T=0 is split into two transitions: superconductor -- normal-metal and normal-metal -- insulator. In our samples, they are distanced in the oxygen content by \Delta x\approx0.025.