The origin of the peak effect on Jc is studied for Nd1+xBa2-xCu3O7-δ single crystals grown by the slow cooling method under various partial O2 pressures, PO2 = 0.04 ∼ 1% in Ar atmosphere. We find that the peak effect disappears for the crystals grown under PO2 ∼ 0.04%, where the Nd/Ba substitution can be negligible. This result demonstrates the important role of Nd/Ba substitution for the peak effect. We also investigate the oxygen vacancy effect on the peak effect by using oxygen deficient samples. The large peak effect at relatively high fields seems to be brought by controlling the amount of Nd/Ba substitution rather than oxygen vacancies in almost fully oxygenated crystals.
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