Oxygen-isotope effect in the optical conductivity of YBa2Cu3OyYBa2Cu3Oy and Bi2Sr2CaCu2O8+δBi2Sr2CaCu2O8+δ

We have investigated the contribution of the electron–phonon coupling in the optical conductivity of YBa2Cu3OyYBa2Cu3Oy and Bi2Sr2CaCu2O8+δBi2Sr2CaCu2O8+δ through the oxygen-isotope effect. We have confirmed that the oxygen-isotope effect in the optical conductivity is larger for the more under-doped samples. The origin of this doping dependence of the isotope effect has been discussed.