On the oxygen isotope effect and apex anharmonicity in high-T c cuprates

Emphasis is put on aprecise knowledge of the oxygen isotope shift exponent _O is the so far synthesized high-T _c superconductors in order to elucidate the basic mechanism. The known data, see _a in Table 6, indicate the presence of a presumably anharmonic coupling of pyramidal apex oxygen motion along thec-axis in these highly in-plane correlated superconductors. _a is computed from _O=r _a , based on the conjecture that the ratior of apex to total oxygen-ion content per unit cell is relevant. The related experimental and theoretical literature is reviewed.