Anomalous superconducting surface nucleation fields

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Abstract
Complex a.c. incremental permeability measurements have been made on In-Tl alloys to investigate the dependence of the superconducting surface nucleation field H c3 on surface treatment. Freshly electropolished or etched surfaces of high Tl content alloys had anomalously high H c3/H c2 values which were restored towards the Saint-James-de Gennes value of 1·7 on annealing. For the anomalous surfaces the ratio Hc3 H c2 was found to be temperature dependent. It is suggested that anomalous behaviour is due to increased scattering of normal electrons near the surface caused by preferential solution of In during electropolishing or etching. It is shown that the general features of anomalous behaviour can be accounted for theoretically; this is done by modifying, to first-order perturbation, the surface nucleation condition derived from the Gor'kov linear equation for the energy gap function assuming an extra attractive non-local surface scattering potential.