Anisotropic critical fields in superconducting superlattices

The temperature and angular dependence of critical fields ($H_c$) have been studied as a function of layer thickness for superconducting Nb/Cu superlattices. For layer thicknesses between 100 and 300 Å, dimensional crossover has been observed in the temperature dependence of $H_c$. Associated with the crossover we find a change in the angular dependence of $H_c$ to that given by the effective-mass theory. This is the first time that a relationship has been found between dimensional crossover observed in the temperature dependence and that in the angular dependence of critical fields.