Temperature dependence of the upper critical magnetic field of amorphous bismuth films

The effect of enhancement of disorder on temperature dependences of the upper critical magnetic field Hc2 (T) in two-dimensional amorphous bismuth films is investigated. It is shown that as the “sheet” resistance R□ (which is a measure of disorder) increases, the deviations of the Hc2 (T) curves from those predicted by the Werthamer–Helfand–Hohenberg (WHH) theory for dirty superconductors become larger. The values of Hc2 (T) and of the derivative (dHc2 (T)/dT)T = Tc (Tc is the superconducting transition temperature) are found to be higher than those in the WHH theory. The essential distinction of this work from previous investigations of amorphous bismuth consists in that the Hc2 (T) dependences for films with sufficiently large values of R□ (≳ 0.5 kΩ) have positive curvature which, according to modern ideas, is an indication of competition between localization and superconductivity of electrons. The obtained results are analyzed on the basis of available theories of the effect of disorder on Tc and Hc2 (T) and of the mutual effect of localization and superconductivity. For extremely thin films with R□ ≫ 1 kΩ, a strong increase (to − ∞ in the limit) in (dHc2 (T)/dT)T = Tc is detected with a large negative curvature in the Hc2 (T) dependences in the absence of a linear segment for T ≤ Tc, which is typical of the WHH theory. Such a behavior is satisfactorily described by the influence of structural heterogeneities of extremely thin films (of thickness 15−20 Å) and percolation effects associated with it.

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