Letter

1. Brookhaven National Laboratory, Upton, New York 11973-5000, USA
   
2. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
   
3. FEI Company, Hillsboro, Oregon 97124, USA

Correspondence to: I. Bozovic¹ Correspondence and requests for materials should be addressed to I.B. (Email: bozovic@bnl.gov).

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 (La_1.55Sr_0.45CuO₄), neither of which is superconducting in isolation. In these 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 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 previously⁵ but the essential role of the interface was not recognized at the time.