nano-scale real-space observation of surface two-dimensional electron system under ultra-low temperature and magnetic field

我们搭建了可在超低温高磁场下工作的超高真空扫描隧道显微镜（UHV-STM），并确认其在11T磁场下工作在4K。利用该仪器，我们成功地在Au（111）和Cu（111）表面上拍摄了数百埃的驻波模式图像。我们观察了Pd/Au（111）和Pd/Cu（111）表面的驻波图像，试图找到一种控制和降低表面电子态占据的电子密度的方法，发现单层高Pd薄膜上观察到的电子驻波周期比Au（111）或Cu（111）衬底上观察到的电子驻波周期更长。钯层上的周期越长，表明钯层表面态的能级越高，因此这些态的电子密度比衬底低。表面态的改变可以通过考虑表面态的形成机制来解释，并归因于表面态的能级恰好位于Pd电子态L隙的下缘之下。对于我们项目的最终目标：用STM在高空间分辨率下表征表面二维电子系统，上述结果是第一步，因为为了观察磁场对二维电子系统的影响，必须降低表面态的电子密度。我们还在Au（111）表面观察到一些吸附现象，显示出STM随磁场大小的对比变化。它们在没有磁场的情况下表现为亮点，但在高磁场（6T）下，它们与驻波振荡周期相同，呈现出甜甜圈状的对比。我们现在正试图通过更多的控制实验和与理论小组的合作来阐明对比度变化的机制。

We set up ultrahigh vacuum scanning tunneling microscope (UHV-STM) which can be operated under ultra-low temperature and high magnetic field, and confirmed that it works at 4K under a magnetic field of 11T. Using the instrument, we were successfully able to take images of standing wave pattern spreading in several hundred angstrom on Au(111) and Cu(111) surfaces.We observed standing wave images on Pd/Au(111) and Pd/Cu(111) surfaces, trying to find a method to control and reduce electron density occupied in the surface electronic states, and found that a period of electron standing waves observed on monolayer-high Pd thin film is longer than those observed on Au(111) or Cu(111) substrate. The longer period on Pd layers indicates higher energy level of the surface states on the Pd layer, and thus lower electron density in the states than the substrates. The modification of the surface states is explained by considering a formation mechanism of the surface states, and is attributed to the fact that an energy level of the surface states is located just below the lower edge of the L gap of Pd electronic states. For the final goal of our project: characterization of surface two-dimensional electron system with STM in high spatial resolution, the above result is a first step since electron density in the surface states has to be reduced in order to observe effects of magnetic field to the 2D electron system.We also observed some adsorbates on the Au(111) surface which show contrast variation in STM with a amount of magnetic field. They appear as a bright spot under no magnetic field, but at high magnetic field (6T) they show a donut-like contrast with a same period of oscillation with the standing waves. We are now trying to elucidate a mechanism of the contrast variation through more controlled experiments and a collaboration with a theoretical group.