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

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Abstract
A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa2Cu3O6+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 ρab 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 σab=1/ ρ ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σc=1/ρc followed a power law, and neither σc(T), nor ρc(T) could be fitted by log T. It follows from the ρc 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 Δx≈0.025.