THEORETICAL-ANALYSIS OF THE ISOTOPE EFFECT USING THE BIPOLARONIC THEORY

The recent oxygen isotope effect measurement made in the superconducting system YBa2-xLaCu3O7 (0 less-than-or-equal-to x less-than-or-equal-to 0.5) is analyzed using the bipolaronic theory (BT). The hamiltonian model consists of several major contributions: (a) the intra-band interaction among electrons in both the valence and conduction bands, forming Cooper pairs as mediated by phonons, and (b) the ''hop-on'', ''hop-off'' interactions between bipolarons and Cooper pairs in the two bands. A detailed numerical analysis of the consequences of the BT with respect to shift of T(c) due to the oxygen isotope effect on the La-doped Y-Ba-Cu-O system is carried out. Excellent agreement is achieved between the theoretically calculated oxygen isotopic constant and the experimental results of Bornemann and Morris [7], covering a T(c) range of 38.3-92.3 K.