High-temperature interface superconductivity between metallic and insulating copper oxides

The realization of high- transition- temperature ( high- T-c) superconductivity confined to nanometre- sized interfaces has been a long- standing goal because of potential applications(1,2) and the opportunity to study quantum phenomena in reduced dimensions(3,4). This has been, however, a challenging target: in conventional metals, the high electron density restricts interface effects ( such as carrier depletion or accumulation) to a region much narrower than the coherence length, which is the scale necessary for superconductivity to occur. By contrast, in copper oxides the carrier density is low whereas T-c is high and the coherence length very short, which provides an opportunity - but at a price: the interface must be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator ( La2CuO4) and a metal ( La1.55Sr0.45CuO4), neither of which is superconducting in isolation. In these bilayers, T-c is either similar to 15 K or similar to 30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2 - 3nm of the interface. If such a bilayer is exposed to ozone, T-c exceeds 50 K, and this enhanced superconductivity is also shown to originate from an interface layer about 1 - 2 unit cells thick. Enhancement of T-c in bilayer systems was observed previously(5) but the essential role of the interface was not recognized at the time.