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We report magnetotransport measurements of the critical field behavior of thin Al films deposited onto multiply connected substrates. The substrates were fabricated via a standard electrochemical process that produced a triangular array of 66-nm-diameter holes having a lattice constant of 100 nm. The critical field transition of the Al films was measured near Tc as a function of field orientation relative to the substrate normal. With the field oriented along the normal (θ=0), we observe reentrant superconductivity at a characteristic matching field Hm=0.22 T, corresponding to one flux quantum per hole. In tilted fields, the position H∗ of the reentrance feature increases as sec(θ), but the resistivity traces are somewhat more complex than those of a continuous superconducting film. We show that when the tilt angle is tuned such that H∗ is of the order of the upper critical field Hc, the entire critical region is dominated by the enhanced dissipation associated with a submatching perpendicular component of the applied field. At higher tilt angles a local maximum in the critical field is observed when the perpendicular component of the field is equal to the matching field.

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Physical Review B