Study of electronic state associated with surface super-structures through electron excitations by totally reflected positrons

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To study the energy loss processes of positrons at solid surfaces under the total reflection condition at small enough glancing angles, we developed an energy-filtered reflection high-energy positron diffraction apparatus. By optimizing the configuration of retarding electrodes, we achieved an energy resolution of 4eV which is adequate to observe surface plasmon excitation. Consequently, we first succeeded in observing the multiple surface plasmon excitation processes for Si(111)-7x7, Al(111) and Bi(001) surfaces. Comparing the case of electron diffraction, it was found that the mean excitation number of surface plasmon by positrons is two times greater than that by electrons. This difference between positrons and electrons is originating from their different diffraction passes. That is, until the eventual escaping to the vacuum, electrons penetrate into the bulk, while positrons channel in the first surface layer. Hence, positrons excite more surface plasmons than electrons. The broadening of reflected beam profile and the degradation of absolute reflectivity of positrons under the total reflection condition could be explained considering the multiple surface plasmon excitations. We also observed the energy loss process of positrons with the Si(111)/In surface exhibiting the metal-insulator transition at low temperatures. Although a small modulation in the energy loss spectra were observed accompanying the phase transition, an improvement of the energy resolution is still needed for the detailed study.