Oxygen isotope study of YBa2Cu3O7.

Abstract

The magnitude of the oxygen isotope effect in the high-${T}_{c}$ superconductor Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ has been determined by substitution of $^{18}\mathrm{O}$ for $^{16}\mathrm{O}$. A series of cross exchanges was performed on highquality 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\ifmmode\pm\else\textpm\fi{}0.03 K at 80% $^{18}\mathrm{O}$ substitution, yielding ${\ensuremath{\alpha}}_{\mathrm{bulk}}=0.019\ifmmode\pm\else\textpm\fi{}0.004$ where ${T}_{c}\ensuremath{\sim}{M}^{\ensuremath{-}\ensuremath{\alpha}}$ 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: ${\ensuremath{\alpha}}_{\mathrm{fil}}=0.028\ifmmode\pm\else\textpm\fi{}0.003$. The filamentary superconductivity is time dependent and disappears several months after sample preparation. We show that the above values of $\ensuremath{\alpha}$ are inconsistent with the standard three-dimensional phonon-mediated Bardeen-Cooper-Schrieffer theory, and discuss implications for alternative possibilities.