Characteristic transport properties observed in underdoped YBa₂Cu₃O_6+x

By annealing under the equilibrium oxygen pressure, we have succeeded in obtaining YBa₂Cu₃O_6+x with any doping level. Both in bulk and film samples, the doping driven superconductor-to-insulator (S-I) transition has been observed at ρ_ab≃0.8 mΩ cm which corresponds to the universal resistance h/4e² per CuO₂ bi-layer. The observed critical carrier density is n_H^SI≃3 × 10²⁰ cm^-3. Within the critical region of the S-I transition, using a thin film sample, we have succeeded in observing the resistivity scaling as a function of doping. The carrier density obeys the relation n_H ∝x - 0.2 in the underdoped region (0.2≤x< 0.5), where the conductivity is substantially small and transport properties show weaker temperature dependence. The quantum effects are considered to be more important e.g., the quantum phase fluctuation in superconductors with small superfluid density.