Peak effect in YBCO crystals: statics and dynamics of the vortex lattice

Abstract

The oscillatory dynamics and quasi-static Campbell regime of the vortex lattice (VL) in twinnedYBa2Cu3O7 single crystals have been explored at low fields near the peak effect (PE) region by linear andnon-linear ac susceptibility measurements. We show evidence that the PE is a dynamic anomalyobserved in the non-linear response, and is absent in the Labusch constant derived from thelinear Campbell regime. Static properties play a major role however, and we identify twoH(T) lines defining the onset and the end of the effect. AtH1(T) a suddenincrease in the curvature of the pinning potential wells with field coincides with the PE onset. At a higherfield, H2(T), a sudden increase in linear ac losses, where dissipative forces overcome pinning forces,marks the end of the Campbell regime and, simultaneously, the end of the PE anomaly.Vortex dynamics was probed in frequency dependent measurements, and we find that inthe PE region, vortex dynamics goes beyond the description of a power law with a finitecreep exponent for the constitutive relation.