Oxygen isotope study of YBa2Cu3O7

The magnitude of the oxygen isotope effect in the high-T_c superconductor YBa₂Cu₃O₇ has been determined by substitution of ¹⁸O for ¹⁶O. A series of cross exchanges was performed on high-quality polycrystalline specimens to eliminate uncertainties due to sample heat treatments and sample inhomogeneities. Magnetic measurements suggest a relative isotope shift for the bulk material of -0.17+/-0.03 K at 80 at. % ¹⁸O substitution, yielding α_{bulk=0.019+/-0.004} where T_c~M^-α and M is the oxygen mass. Resistivity measurements in freshly prepared specimens reveal filamentary superconductivity 1 or 2 K above the bulk superconducting transition temperature. The isotope shift associated with the filamentary superconductivity is similar to but slightly larger than the bulk shift: α_{fil=0.028+/-0.003}. The filamentary superconductivity is time dependent and disappears several months after sample preparation. We show that the above values of α are inconsistent with the standard three-dimensional phonon-mediated Bardeen-Cooper-Schrieffer theory, and discuss implications for alternative possibilities.