Zero point motion and the isotope effect in oxide superconductors.

We investigate the possibility of an isotope effect in the high-T sub c oxide superconductors due to mechanisms other than the conventional dependence of phonon frequency on isotopic mass. In particular, we show that the effect of the oxygen zero point motion on normal state electronic properties coupled with the extreme sensitivity of T sub c to pressure could lead to an isotope effect in La sub 1.85 Sr sub .15 CuO sub 4 of the observed magnitude. The same mechanism yields a considerably smaller isotope effect in Ba sub 2 YCu sub 3 O sub 7; thus the isotope effect experiments may be consistent with a purely electronic mechanism for superconductivity in the new oxide superconductors. Experimental tests of this possibility are proposed.