Carrier-concentration-driven superconductor-to-insulator transition in YBa₂Cu₃O_{6 + x}

We have succeeded in controlling carrier concentration near the superconductor-to-insulator transition by sample annealing under the equilibrium oxygen pressure of YBa₂Cu₃O_{6 + x}. Near the critical doping, below 60 K, we observed carrier-concentration-driven resistivity scaling of the pulsed-laser-deposited thin film of YBa₂Cu₃O_{6 + x} indicating the large energy scale of the quantum phase fluctuation. This demonstrates that a considerable area in the phase diagram of the pseudo-gapped underdoped YBa₂Cu₃O_{6 + x} is in the critical region of the quantum phase transition.