Magnetic irreversibility and zero resistance in melt-textured YBaCuO

Abstract

We have investigated a possible correlation between the magnetic irreversibility limit and the zero resistance as a function of applied magnetic field up to 50 kOe in a melt-textured YBa 2Cu 3O 7− δ sample doped with Y211 phase. The main goal is to disclose the connection between the magnetic irreversibility and zero resistance and to appoint the physical grounds of the discrepancies. The electric resistivity falls steeply as a function of temperature at the pairing transition. However, just before falling to zero value it abruptly changes drastically the slope and achieves zero resistance, T c0( H), only after a long and flat tail. Along this tail grain coupling and phase coherence of the grain aggregate takes place. The magnetic irreversibility data as a function of applied field, T irr( H), follow the trends of granular superconductors. Above 2 kOe the data follow the usual giant-flux-creep regime for H parallel to the c-axis as well as for H parallel to the ab-plane. However, in low fields the T irr( H) data exhibit the signature of a frustrated superconducting grain aggregate. Besides this, the irreversibility data T irr( H) exhibit a much lowered anisotropy, which we impute to imperfect c-axis alignment of the crystallites. Within the high-field region the T c0( H) and the irreversibility T irr( H) data agree within the experimental precision just for H parallel to the ab-plane. However, in a low-field region, where the T irr( H) data display the signature of frustration, the T c0( H) data fall visibly below the irreversibility line, which we impute to persistence of resistivity within the grain junctions.