Theoretical study of isotope effect in La-doped Y-123

The recent oxygen isotope effect measurement made in the system YBa_2-xLa_xCu₃O₇ (0x0.5) is analyzed using the two-carrier model which describes a superconducting system as a mass anisotropic system consisting of charge carriers with two different masses as a result of lattice defects or oxygen deficiency. The extent to which Ba, Cu, and O atoms participate in the phonon-mediated Cooper pairing interaction is elucidated. The relationship between the theoretically deduced mass anisotropic factor andT_c agrees closely with the experimental results of Uemuraet al. [28]. Furthermore, the superconducting energy gap toT_c ratio is found to be close to 3.5. Excellent agreement is also achieved between the theoretically calculated oxygen isotopic constant and the experimental results of Bornemann and Morris [12].