Oxygen isotope effect in disordered underdoped and overdoped La2?xSrxCu1?yZnyO4 superconductors

The effect of oxygen isotopic substitution on the superconducting transition temperature has been studied for heavily underdoped and overdoped La_2?xSr_xCu_1?yZn_yO₄ compounds with different Zn contents in the CuO₂ plane. The effect of Zn on the isotope coefficient, α, was significantly more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with disorder content can be described quite well within a model based solely on Cooper pair-breaking in the case of the underdoped compounds. This model fails to describe the behavior of α(y) for the overdoped samples, even though Zn still suppresses T_c very effectively at this hole (Sr) content, indicating that the Zn induced pair-breaking is still very much at play. We discuss the implications of these findings in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as hole content in the CuO₂ plane, p (≡x), is varied.