Mechanisms determining the broadening of resistive transitions of thin superconducting films

Resistive transitions in thin superconducting films of simple (In, Sn) and transition (V, Ta) metals in a perpendicular magnetic field are studied. In transition-metal films the relative broadening of the transitions ΔH⊥/Hc⊥ (T) increases with the temperature and varies inversely proportionally to the electron mean-free path length l, while the ratio of the broadening of the transitions in the field H⊥ and at H = 0 as a function of l remains unchanged in accordance with the theory of critical fluctuations in two-dimensional superconductors. The observed smearing of the transitions is quantitatively in agreement with the values of the Ginzburg magnetic number obtained on the basis of the theory Feat and Rickayzen. The resistive behavior of In and Sn differs substantially from that observed in transition-metal films. The relative broadening of the transition is independent of the temperature, and its dependence on l is nonmonotonic. In implanted helium films with a variable electron mean-free path length there is no correlation between the broadening of the transitions in the perpendicular field and at H = 0. The experimental values of the broadening of transitions exceed by several orders of magnitude the computed width of the critical region. In this case the smearing of the phase transitions is caused by nonuniformities of the parameters of the superconductor.

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