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Variation of the vortex activation energy, U(T, H), with hole content in YBa2Cu3O7-delta thin films

The nature of in-plane resistive transition of high-quality c-axis oriented crystalline thin films of YBa2Cu3O7-delta have been studied under magnetic fields (H) applied along the c-direction over a wide range of doped holes, p, in the CuO2 planes. The field and temperature dependent in-plane resistivity, rho_ab(T, H), below the mean field superconducting transition temperature, Tp, has been analyzed within the thermally assisted flux-flow (TAFF) scenario. We have extracted the temperature and field dependent flux activation energy, U(T, H) from this analysis. The low-T part of the rho_ab(T, H) data can be described quite well by a dimensionless activation energy having the functional form of U(T, H) = (1-t)^m(H0/H)^-β, where t = T/Tp, is the reduced temperature and H0, is a characteristic field scale that primarily determines the magnitude of the activation energy for a given sample composition. The temperature exponent, m, shows a systematic variation with the hole content, whereas the field exponent, β, is insensitive to the p-values and is close to unity irrespective of the film composition. H0, on the other hand, changes rapidly as hole concentration is varied. Possible implications of these results are discussed in this paper.