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Surface impurities and the superconducting-to-normal transition - theory

The variation of the superconducting-to-normal transition field as a function of temperature is evaluated for superconductors having impurities which vary in density through a thin surface layer. Numerical results are obtained for tantalum, in which impurities cause the appearance of a tail to the M-H curve above Hc. The precise shape of the tail is also evaluated.