Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-δ as the origin of the fishtail anomaly

Abstract We report on three different and complementary measurements, namely magnetization measurements, positron annihilation and NMR spectroscopy, which give evidence that the formation of oxygen vacancy clusters is the origin of the fishtail anomaly in YBa 2 Cu 3 O 7− δ . In contrast to YBa 2 Cu 3 O 7.0 , where the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. Thus, clusters of oxygen vacancies rather than single vacancies or point defects are responsible for this anomaly.