Anomalous upper critical fields of superconducting multilayers: Verification of the Takahashi-Tachiki effect

Abstract

We observe for the first time the anomalous upturn in the parallel upper critical field predicted for metallic multilayers composed of two materials which have different diffusion coefficients but which have the same superconducting transition temperature. We observe this upturn, which reflects the shift of the nucleation field from the clean to the dirty layers, in multilayers of Nb/${\mathrm{Nb}}_{0.6}$${\mathrm{Ti}}_{0.4}$ having composition wavelengths \ensuremath{\Lambda}\ensuremath{\ge}420 A\r{}. The temperature at which the upturn occurs increases with \ensuremath{\Lambda}, as predicted by theory.