Superconducting anisotropy in (CaCuO2)n/(SrTiO3)m superlattices

The superconducting properties of (CaCuO2)(n)/SrTiO3/(m) artificial superlattices have been investigated via transport measurements under the application of external magnetic fields. The coherence lengths in the plane of the substrate and in the direction perpendicular to it (xi(ab) and xi(c), respectively) have been measured while varying m, the thickness of the SrTiO3 block. The results show that with increasing m, i.e. with increasing structural anisotropy, the superconducting anisotropy gamma = xi(ab)/xi(c) decreases. This apparent anomalous relation between the structural and the superconducting anisotropies suggests that gamma is more affected by local doping at the interface rather than by the separation between the superconducting blocks. This interpretation of the experimental results has been confirmed by both the irreversibility lines and the magnetic field dependence of the activation energy for fluxon motion.