Preferential position for nucleation of order parameter and angular dependence of upper critical fields in Nb/NbZr multilayers

The Nb/NbZr multilayer is a typical one composed of two materials with the same bulk transition temperature and different electron diffusion constants and exhibits an anomalous upturn at T⁺ in the temperature dependence of parallel upper critical field H_c2∥(T). For all samples which show the upturn behavior in H_c2∥(T), we have found the anomalous minimum and maximum at a tilt of angle θ = θ* and a perpendicular direction θ = 0, respectively. Of particular interest is the existence of a different tendency in the variation of θ* and H_c2⊥-peak as a function of T below and above T⁺. From our experimental results, we suggest the dimensional crossover of the localization of Δ(r) for a tilt of angle. The angular dependence of H_c2(T) has been in a good agreement with the numerical calculations.