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

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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.