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

The ac screening response of a superconducting epitaxial Tl2Ba2CaCu2O8 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- (TH and TH) and third-harmonic (|TH3|) 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 (h0&lt;~13.2G) and low-frequency (flt;~1073Hz) response is well described by the bean critical-state model applied to thin discs in a transverse ac field. The Jc calculated from the model compares well with the Jc 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 |TH3| 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 (&#956;0H&lt;~0.5T) regime. The irreversibility field derived from the onset temperature of |TH3| 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 Tg with field inferred from the tail of the resistive transition