Influence of disorder on the vortex-lattice melting in twinned oxygen-deficient ortho-II YBa2Cu3Oy (y≈6.6)

We investigated with ac susceptibility the mixed state of twinned, deoxygenated single crystals and highly c-axis-oriented melt-textured samples of YBa2Cu3O6-x with oxygen stoichometry corresponding to that of the orthorhombic II phase. In the single crystal we found, below H* approximate to 700 Oe and over almost four orders of magnitude in frequency, a frequency-independent "irreversibility line" (IL). Complementry de magnetization measurements seem to disregard an IL arising from geometric effects when H\\c, and they suggest a possible meltinglike transition of the flux-line lattice below H*. The reduced value of H* is associated with the increased anisotropy induced by the oxygen depletion, and an upper limit of the anisotropy parameter is estimated as gamma(max) approximate to 147. However, in the twinned melt-textured sample having a major grade of disorder due to the presence of micrometric nonsuperconducting precipitates of Y2BaCuO5, the IL is frequency dependent for all applied magnetic fields, suggesting a cancellation of H* in the sufficiently disordered specimen.