High-temperature superconductivity confined to nanometer-size interfaces hasbeen a long standing goal because of potential applications^{1,2} and theopportunity to study quantum phenomena in reduced dimensions^{3,4}. However,this is a challenging target: in conventional metals the high electron densityrestricts interface effects such as carrier depletion/accumulation to a regionmuch narrower than the coherence length, the scale necessary forsuperconductivity to occur. In contrast, in copper oxides the carrier densityis low while the critical temperature (T_c) is high and the coherence lengthvery short; so, this provides a breakthrough opportunity but at a price: theinterface must be atomically perfect. Here we report on superconductivity inbilayers consisting of an insulator (La_2CuO_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 layeringsequence. This highly robust phenomenon is confined within 2-3 nm from theinterface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and thisenhanced superconductivity is also shown to originate from the interface layerabout 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observedpreviously^5 but the essential role of the interface was not recognized at thetime. Our results demonstrate that engineering artificial heterostructuresprovides a novel, unconventional way to fabricate stable, quasi two-dimensionalhigh T_c phases and to significantly enhance superconducting properties inknown or new superconductors.
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