Effect of doping and impurities on the oxygen isotope effect in high-temperature superconducting cuprates

We report measurements of the oxygen isotope effect in high-temperature superconductors. We extend previous oxygen isotope measurements to high hole concentrations and show that the data can be analyzed in terms of a hole-concentration-independent oxygen isotope effect in the overdoped region. Measurements of the oxygen isotope effect in YBa2(Cut-xNix)(4)O-8 are also presented. We show that the hole-concentration- and impurity-concentration-induced changes in the isotope-effect coefficient can be consistently modeled by including the effect of the normal-state pseudogap.