SYSTEMATIC STUDY OF PINNING ENERGY FOR OXYGEN-DEFICIENT YBCO

A systematic study of the pinning energy for three oxygen-deficient YB a2Cu3O7-delta ceramic samples, with delta in the range 0.05-0.35, has been undertaken using the techniques of AC susceptibility, magnetic re laxation, electrical resistivity and thermoelectric power. The values of the activation energies obtained from the different techniques usin g the flux-creep model of Anderson are comparable and consistent with those previously reported for different, individual, oxygen-deficient samples and a similar experimental technique. However, only for the la tter three techniques were values of the activation energy internally consistent and therefore considered intrinsic to the different materia ls. It has also been found that (1) the activation energy decreases as the oxygen deficiency increases, (2) the activation energy decreases as the magnetic field increases and follows the relationship U proport ional to H--1/2 for all techniques except that of TEP which follows a H--1/4 field dependence, and (3) the field dependence of the activatio n energy decreases as the oxygen deficiency increases. These results a re interpreted in terms of oxygen depletion from the chains, which in turn decreases the electronic coupling between the CuO2 double layers, and consequently, gives rise to the oxygen-poor samples being more an isotropic, thus reducing the correlation length along the flux line an d hence lowers the activation energy.