High-temperature interface superconductivity between metallic and insulating cuprates

High-temperature superconductivity confined to nanometer-size 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. However, this is a challenging target: in conventional metals the high electron density restricts interface effects such as carrier depletion/accumulation to a region much narrower than the coherence length, the scale necessary for superconductivity to occur. In contrast, in copper oxides the carrier density is low while the critical temperature (T c) is high and the coherence length very short; so, this provides a breakthrough opportunity-but at a price: the interface must be atomically perfect. Here we report on superconductivity in bilayers consisting of an insulator (La 2 CuO 4) and a metal (La 1.55 Sr 0.45 CuO 4), neither of which is superconducting in isolation. However, in bilayers T c is either ~15 K or ~30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm from 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 the interface layer about 1-2 unit cell 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. Our results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi