Abstract
A metal-insulator transition was observed near oxygen content parameter δ∼ 0.018(5) for the Ce-optimal-doped (x=0.15) Pr 1.85 Ce 0.15 CuO 4+δ (-0.003≤δ≤0.03) electron superconductors. The synthesis of Pr 1.85 Ce 0.15 CuO 4+δ samples by the standard solid-state reaction was also discussed. It was found that the superfluid density n s ∼ λ ab -2 increased from the underdoped to the optimal-doped region.