Abstract
Starting from high quality Nd
1.85Ce
0.15CuO
4 polycrystalline samples with a T
c of 24K, we have exchanged ∼85% of
16O by
18O. The magnetically measured T
c was found to change by less than 0.1K, equivalent to an oxygen-isotope exponent
0α⩽0.05. Thus, the isotope effect is small for all classes of cuprate superconductors when T
c is optimized, irrespective of CuO coordination and conduction type.