Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
Nature. 2011 Apr 28;472(7344):458-60. doi: 10.1038/nature09998.
High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor-insulator transition is a key open question. The cleanest approach is to tune the carrier density using the electric field effect; for example, it was learned in this way that weak electron localization transforms superconducting SrTiO(3) into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge, because perfect ultrathin films and huge local fields (>10(9) V m(-1)) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration. Here we report synthesis of epitaxial films of La(2- x)Sr(x)CuO(4) that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor-insulator transition. The observed critical resistance is precisely the quantum resistance for pairs, R(Q) = h/(2e) = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.
铜氧化物中的高温超导性出现在母体绝缘化合物被掺杂超过某个临界浓度时；在这个超导-绝缘转变中究竟发生了什么，是一个关键的开放性问题。最干净的方法是使用电场效应来调谐载流子密度；例如，通过这种方法得知，弱电子局域化将超导 SrTiO(3)转变为费米玻璃绝缘体。但在铜氧化物中，这一直是一个长期存在的技术挑战，因为需要完美的超薄薄膜和巨大的局部场（>10(9)V m(-1)）。最近，在双电层晶体管构型中使用电解质或离子液体已经获得了这种场。在这里，我们报告了外延 La(2- x)Sr(x)CuO(4) 薄膜的合成，其厚度为一个单胞，以及双层晶体管的制造。非常大的场和诱导的表面载流子密度变化使临界温度的变化高达 30 K。记录了数百个电阻对温度和载流子密度的曲线，并将其折叠到单个函数上，正如二维超导-绝缘转变所预测的那样。观察到的临界电阻恰好是量子电阻对，R(Q) = h/(2e) = 6.45 kΩ，表明是由量子相位涨落和库珀对（去）局域化驱动的相转变。
