Local Hall-probe-based susceptometry of Tl 2 Ba 2 CaCu 2 O 8 epitaxial films: critical state and flux dynamics in collinear ac and dc magnetic fields

The ac screening response of a superconducting epitaxial Tl 2 Ba 2 CaCu 2 O 8 film in static and time-varying magnetic fields collinear with the film normal is reported. The complex susceptibility components measured in terms of the fundamental- (T H and T H ) and third-harmonic (|T H3 |) transmittivities of a local hall-probe susceptometer are compared with the models of flux penetration in a disc-shaped thin superconductor. In the absence of a dc field, the low-amplitude (h 0 c calculated from the model compares well with the J c measured in a four-probe transport geometry. When a dc field is superimposed over the ac field, signatures of a creeplike motion of vortices are seen in a temperature regime where |T H3 | is nonzero. Vortex-glass/collective-pinning theories for a three-dimensional vortex system are considered to understand this behavior in a high-temperature (T gt;~60K) low-field (μ 0 H H3 | varies as H(T)~(1-t) n on the H-T plane, with n=1.2. At low fields, this line also represents the variation of the vortex-glass temperature T g with field inferred from the tail of the resistive transition.