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The resistance of several granular NbN films has been measured as a function of temperature as they approach the superconducting state. The lowest-resistance data deviate from earlier reported power-law behavior above a phase-ordering transition Tcj, but are consistent with the exponential expression describing the resistance above a two-dimensional phase-transition temperature, T2D, of the type proposed by Kosterlitz and Thouless. At T2D a finite supercurrent appears.
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Wolf et al. (1981) studied this question.
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