Consideration of the formulas that give the upper critical field ${\mathit{H}}_{\mathit{c}2}$ near the critical temperature ${\mathit{T}}_{\mathit{c}}$ for superconductors in the dirty limit, together with the uncertainty principle, is shown to provide an upper limit to the maximum slope of ${\mathit{H}}_{\mathit{c}2}$. Analysis of reported values shows that amorphous superconductors are in this limit.

• Received 3 January 1990

DOI:https://doi.org/10.1103/PhysRevB.42.934