Changes in irreversibility line, anisotropy, and condensation energy by oxygen depletion ofYBa2Cu3O7−δ

Abstract

We report results of magnetization measurements on the same ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystal for a range of $\ensuremath{\delta}$ values $0.03<~\ensuremath{\delta}<~0.35,$ in magnetic fields up to 120 kOe, applied parallel or perpendicular to the ${\mathrm{CuO}}_{2}$ planes. Zero-field resistivity anisotropy measurements have also been made for the same values of $\ensuremath{\delta}.$ The irreversibility line ${H}_{\mathrm{irr}}$ falls by over a factor of 10 in the range of $\ensuremath{\delta}$ studied, and there is a similar strong decrease in superconducting condensation energy associated with the growth of the normal-state pseudogap in under-doped cuprates. On the other hand, the anisotropy in ${H}_{\mathrm{irr}}$ and the room-temperature resistivity anisotropy only increase by a factor of 2. Evidence that for our crystals the so-called ``vortex lattice melting line'' ${H}_{m}(T)\ensuremath{\approx}{H}_{\mathrm{irr}}(T)$ is presented. There is also a large field and temperature region where the reversible magnetization varies as ${H}^{\ensuremath{-}1/2}$ and its magnitude corresponds to a free-energy density of ${k}_{B}T$ in a certain field-dependent correlation volume. Many of the results are consistent with the presence of critical thermodynamic fluctuations described by the three-dimensional $\mathrm{XY}$ model; these are probably enhanced when the field along the c axis is greater than 10 kOe.