T_c up to 50 K in superlattices of insulating oxides

Here we report a structural and electronic investigation of the recently discovered CaCuO₂/SrTiO₃ superconducting artificial superlattices based on two insulating oxides. We show that T_c = 50 K is reached in the superlattices grown on NdGaO₃ substrate under optimized growth conditions. If the superlattices are grown on LaAlO₃ substrate the T_c is drastically reduced due to the much higher lattice mismatch between CaCuO₂ and LaAlO₃, which has lattice parameters substantially smaller than NdGaO₃. The consequent in-plane lattice contraction also makes the incorporation of extra oxygen atoms more difficult, leading to a decrease of hole doping and T_c. Finally, x-ray absorption spectroscopy study as a function of the thickness of the CaCuO₂ block in CaCuO₂/SrTiO₃ superconducting superlattices has revealed the important role played by the interfaces as the location of hole doping and superconductivity in these systems. (paper)