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

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 o r perpendicular to the CuO2 planes. Zero-held resistivity anisotropy measur ements have also been made for the same values of delta. The irreversibilit y 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 cup rates. On the other hand, the anisotropy in Hi,, and the room-temperature r esistivity 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 corres ponds to a free-energy density of k(B)T in a certain field-dependent correl ation volume. Many of the results are consistent with the presence of criti cal thermodynamic fluctuations described by the three-dimensional XY model; these are probably enhanced when the field along the c axis is greater tha n 10 kOe. [S0163-1829(99)02825-8].