Explanation of the dissipation observed in several high-temperature superconductors using a modified Ambegaokar-Halperin model

We have measured the current-voltage (I-V) characteristics of several high-temperature-superconducting materials with widely different morphologies (bulk Ag/Pb-Bi-Sr-Ca-Cu-O tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-O samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current Ic, the normal-state resistance Rₙ, and {γ}, which is proportional to the pinning potential U(H,T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines Ic, we conclude that its use puts this parameter on a solid physical basis.