Gozar A, Logvenov G, Kourkoutis L Fitting, Bollinger A T, Giannuzzi L A, Muller D A, Bozovic I
Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
Nature. 2008 Oct 9;455(7214):782-5. doi: 10.1038/nature07293.
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(2)CuO(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 approximately 15 K or approximately 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.
由于潜在的应用价值以及在低维空间研究量子现象的机会，实现局限于纳米尺度界面的高转变温度（高Tc）超导一直是一个长期目标。然而，这是一个具有挑战性的目标：在传统金属中，高电子密度将界面效应（如载流子耗尽或积累）限制在比超导发生所需的相干长度窄得多的区域。相比之下，在铜氧化物中，载流子密度低而Tc高，且相干长度非常短，这提供了一个机会——但有代价：界面必须是原子级完美的。在此，我们报道了由绝缘体（La₂CuO₄）和金属（La₁.₅₅Sr₀.₄₅CuO₄）组成的双层结构中的超导现象，单独的这两种材料都不具有超导性。在这些双层结构中，Tc约为15K或约为30K，具体取决于层序。这种高度稳健的现象局限于界面的2 - 3纳米范围内。如果将这样的双层结构暴露于臭氧中，Tc超过50K，并且这种增强的超导性也被证明源自约1 - 2个晶胞厚的界面层。此前在双层系统中观察到了Tc的增强，但当时并未认识到界面的关键作用。
