Giant peak effect observed in an ultrapure YBa2Cu3O6.993 crystal

A giant peak in the temperature dependence of the screening current is obse rved in the ac magnetic response of an ultrapure YBa2Cu3O6.993 crystal in a magnetic field. At H = 2.0 T (H//c), the screening current density J(c)(T) exhibits a 35-fold rise with a 0.5 K increase in temperature, indicating a n abrupt 10(3)-fold collapse in the characteristic volume of ordered region s 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 equilib rium magnetization exhibits a small discontinuous jump (for high fields) in side the peak-effect regime, suggesting that the underlying phase transitio n is a weak first-order vortex-lattice melting transition. [S0163-1829(99)5 1042-4].