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±0.03 K at 80% ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution, yielding ${\alpha }_{\mathrm{bulk}}=0.019\mathrm{}\pm 0.004$ where $T_c\sim M^{-\alpha }$ 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: ${\alpha }_{\mathrm{fil}}=0.028\mathrm{}\pm 0.003$. The filamentary superconductivity is time dependent and disappears several months after sample preparation. We show that the above values of $\alpha$ are inconsistent with the standard three-dimensional phonon-mediated Bardeen-Cooper-Schrieffer theory, and discuss implications for alternative possibilities.