Critical fields of weakly coupled superconductors

The parallel critical field of a stack of superconducting and insulating layers is calculated as a function of the superconducting layer's coherence length ξ and thickness d interlayer coherence length ξ, and insulating thickness s. It is shown that the critical field shows an upturn when the stack crosses over from the strongly coupled or three-dimensional regime (ξ>s+d) to the weakly coupled or two-dimensional regime (ξ<s+d). This upturn is a general property of such stacks, though it is more pronounced when s>d.