1] Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany [2] Experimental Physics VI, Center for Electronic Correlations and Magnetism, Augsburg University, 86135 Augsburg, Germany.
Nature. 2013 Oct 24;502(7472):528-31. doi: 10.1038/nature12494. Epub 2013 Oct 6.
The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.
二维（2D）电子系统中的超导态物理学与理解高温铜氧化物超导体有关，也与基于界面电子系统的未来超导体的发展有关。但是，当这些超导电子系统耗尽载流子时，基本超导参数（如态密度谱）如何变化，这一点尚未得到理解。在这里，我们使用平面结隧道光谱学来测量电子态密度谱作为载流子密度的函数的行为，从而从实验上澄清了这个问题。我们选择导电 LaAlO3-SrTiO3 界面作为 2D 超导体，因为这个电子系统可以通过栅极电场连续调节。我们在态密度中观察到约 40 微电子伏特的能隙，其形状由 Bardeen-Cooper-Schrieffer 超导能隙函数很好地描述。与临界温度的穹顶形依赖性相反，在欠掺杂区域和过掺杂区域中，随着载流子耗尽，能隙都会增加。这些结果类似于高温铜氧化物超导体的赝能隙行为，这意味着超导能隙平滑地延续到赝能隙行为可能是 2D 超导的普遍特性。
