Experimental evidence for fast cluster formation of chain oxygen   vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly
  We report on three different and complementary measurements, namely magnetisation measurements, positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin. 
Andreas Erb, Alfred A. Manuel, Marc Dhalle, Frank Marti, Jean-Yves   Genoud, Bernard Revaz, Alain Junod, Dharmavaram Vasumat...