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The electrostatic potential around a single charge in vacuum is described with the Coulomb potential. If it is situated in a metal, the potential is modified by electrons in the metals. The modification of the potential, called screening, is one of the fundamental phenomena in the condensed matter physics. Using low-temperature scanning tunneling microscopy / spectroscopy (STM/S) we have developed a method for measuring electrostatic potential in high spatial and energy resolutions and performed a real-space observation of the potential around external charges screened by two-dimensional surface electron system. In the real-space potential mappings, characteristic decay and oscillation in the potential, so-called the Friedel oscillation, were clearly visible around the charges [1].
As a sample having a two-dimensional electron system, we used the Si(111)-√3×√3-Ag surface. The electron standing wave patterns are observed in tunneling conductance (dI/dV) images and an obtained energy dispersion curve indicates free-electron like behaviors of the electrons. It has been known that when the surface potential is changed by, for instance, additional Ag adsorption on the surface, an energy level of the surface states shifts accordingly. Therefore, by measuring the energy level of the surface states by using STS, a potential mapping is obtained. The obtained potential profiles around step edges, where Ag adsorbates are accumulated, were fitted well with theoretically predicted screened potential profiles. If the oscillation in the potential is assumed to be the Friedel oscillation, it should have a period of the half Fermi wavelength of the electron system. The assumption was confirmed by an observation that the potential oscillation has the same period as the electron standing wave at the Fermi level. Despite the same oscillation wavelength, however, their shapes and phases are different.
This work has been done in collaboration with Masanori Ono, Takahiro Nishio, and Toyoaki Eguchi
References [1] M. Ono et al., Phys. Rev. Lett., 96, 016801 (2006)
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