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 Tc 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 Tc. 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.