Characteristic transport properties observed in underdoped YBa2Cu3O6+x

Abstract

By annealing under the equilibrium oxygen pressure, we have succeeded in obtaining YBa2Cu3O6+x with any doping level. Both in bulk and film samples, the doping driven superconductor-to-insulator (S–I) transition has been observed at ρab≃0.8mΩcm which corresponds to the universal resistance h/4e2 per CuO2 bi-layer. The observed critical carrier density is nHSI≃3×1020cm−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 nH∝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.