Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa 2 Cu 3 O 6+ x ( x ≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρ ab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σ ab =1/ ρ ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σ c =1/ρ c followed a power law, and neither σ c ( T ), nor ρ c ( T ) could be fitted by log T . It follows from the ρ 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 separated in oxygen content by Δ x ≈0.025.