  Oscillatory dynamics and quasi-static Campbell regime of the vortex lattice
(VL) in twinned YBa2Cu3O7 single crystals has been explored at low fields near
the peak effect (PE) region by linear and non-linear ac susceptibility
measurements. We show evidence that the PE is a dynamic anomaly observed in the
non-linear response, and is absent in the Labusch constant derived from the
linear Campbell regime. Static properties play a major role however, and we
identify two H(T) lines defining the onset and the end of the effect. At H1(T)
a sudden increase in the curvature of the pinning potential wells with field
coincides with the PE onset. At a higher field, H2(T), a sudden increase in
linear ac losses, where dissipative forces overcome pinning forces, marks the
end of Campbell regime and, simultaneously, the end of the PE anomaly. Vortex
dynamics was probed in frequency dependent measurements, and we find that in
the PE region, vortex dynamics goes beyond the description of a power law with
a finite creep exponent for the constitutive relation.
