Oxygen-isotope effects in Bi₂Sr₂CaCu₂O_8+y from deeply underdoped to heavily overdoped regimes

Oxygen-isotope effects have been studied in double-layer superconducting Bi₂Sr₂CaCu₂O_8+y in a wide doping range, which crosses a region from the deeply underdoped to the heavily overdoped regimes. The doping levels of the samples are controlled by varying the oxygen content, which can be achieved through annealing them in different oxygen partial pressures up to 600 bars. The oxygen-isotope effect on the superconducting transition temperature T_c is the smallest for optimal doping and increases systematically as T_c drops in both the underdoped and overdoped regions. The present result provides an essential constraint on the microscopic pairing mechanism of high-temperature superconductivity.