A giant peak in the temperature dependence of the screening current is observed in the ac magnetic response of an ultra-pure YBa2Cu3O6.993 crystal in a magnetic field. At H = 2.0 T ({\bf H}∣∣{\bf c}), the screening current density Jc(T) exhibits a 35-fold rise with 0.5 K increase in temperature, indicating an abrupt 103-fold collapse in the characteristic volume of ordered regions in the vortex array. The peak-effect anomaly is most pronounced for H< 4.0 T, but detectable up to 7.0 T. The temperature dependence of the equilibrium magnetization exhibits a small discontinuous jump (for high fields) inside the peak-effect regime, suggesting that the underlying phase transition is a weak first-order vortex-lattice melting transition.
@article{arxiv.cond-mat/9910523,
title = {Giant Peak Effect Observed in Ultrapure YBCO Crystal},
author = {Jing Shi and X. S. Ling and Ruixing Liang and D. A. Bonn and W. N. Hardy},
journal= {arXiv preprint arXiv:cond-mat/9910523},
year = {2009}
}