Influence of disorder on the vortex-lattice melting in twinned oxygen-deficient ortho-IIYBa2Cu3O

Abstract

We investigated with ac susceptibility the mixed state of twinned, deoxygenated single crystals and highly c-axis-oriented melt-textured samples of YBa 2 Cu 3 O 6ϩx with oxygen stoichometry corresponding to that of the orthorhombic II phase. In the single crystal we found, below H*Ϸ700 Oe and over almost four orders of magnitude in frequency, a frequency-independent ''irreversibility line'' ͑IL͒. Complementry dc magnetization measurements seem to disregard an IL arising from geometric effects when Hʈc, 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 ␥ max Ϸ147. However, in the twinned melt-textured sample having a major grade of disorder due to the presence of micrometric nonsuperconducting precipitates of Y 2 BaCuO 5 , the IL is frequency dependent for all applied magnetic fields, suggesting a cancellation of H* in the sufficiently disordered specimen.