Changes in irreversibility line, anisotropy, and condensation energy by oxygen depletion of YBa2Cu3O7-δ

We report results of magnetization measurements on the same YBa2Cu3O7-delta crystal for a range of delta values 0.03 less than or equal to delta less than or equal to 0.35, in magnetic fields up to 120 kOe, applied paralell or perpendicular to the CuO2 planes. Zero-held resistivity anisotropy measurements have also been made for the same values of delta. The irreversibility line Ni,, falls by over a factor of 10 in the range of 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 Hi,, 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) approximate to H-irr (T) is presented. There is also a large held and temperature region where the reversible magnetization varies as H-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 XY model; these are probably enhanced when the field along the c axis is greater than 10 kOe. [S0163-1829(99)02825-8].