High-temperature interface superconductivity between metallic and insulating copper oxides

摘要

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 and the opportunity to study quantum phenomena in reduced dimensions. 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 (La_2CuO_4) and a metal (La_(1.55)Sr_(0.45)CuO_4), 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 previously but the essential role of the interface was not recognized at the time.