  At interfaces between complex oxides it is possible to generate electronic
systems with unusual electronic properties, which are not present in the
isolated oxides. One important example is the appearance of superconductivity
at the interface between insulating oxides, although, until now, with very low
Tc. We report the occurrence of high Tc superconductivity in the bilayer
CaCuO2/SrTiO3, where both the constituent oxides are insulating. In order to
obtain a superconducting state, the CaCuO2/SrTiO3 interface must be realized
between the Ca plane of CaCuO2 and the TiO2 plane of SrTiO3. Only in this case
extra oxygen ions can be incorporated in the interface Ca plane, acting as
apical oxygen for Cu and providing holes to the CuO2 planes. A detailed hole
doping spatial profile has been obtained by STEM/EELS at the O K-edge, clearly
showing that the (super)conductivity is confined to about 1-2 CaCuO2 unit cells
close to the interface with SrTiO3. The results obtained for the CaCuO2/SrTiO3
interface can be extended to multilayered high Tc cuprates, contributing to
explain the dependence of Tc on the number of CuO2 planes in these systems.
