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

通过全反射正电子激发电子来研究与表面超结构相关的电子态

基本信息

批准号：
19540349
负责人：
KAWASUSO Atsuo
金额：
$ 2.08万
依托单位：
Japan Atomic Energy Agency
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2007
资助国家：
日本
起止时间：
2007 至 2009
项目状态：
已结题

项目摘要

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.

为了研究在足够小的掠射角下全反射条件下正电子在固体表面的能量损失过程，我们开发了能量过滤反射高能正电子衍射装置。通过优化延迟电极的配置，我们实现了 4eV 的能量分辨率，足以观察表面等离子体激发。因此，我们首先成功地观察到了Si(111)-7x7、Al(111)和Bi(001)表面的多重表面等离子体激发过程。比较电子衍射的情况，发现正电子对表面等离子体的平均激发数是电子的两倍。正电子和电子之间的这种差异源于它们不同的衍射路径。也就是说，直到最终逃逸到真空中，电子穿透到体中，而正电子在第一表面层中通道。因此，正电子比电子激发更多的表面等离子体。考虑到多重表面等离子体激发，可以解释全反射条件下反射光束轮廓的展宽和正电子绝对反射率的降低。我们还观察了 Si(111)/In 表面在低温下表现出金属-绝缘体转变的正电子的能量损失过程。尽管在相变过程中观察到能量损失谱的微小调制，但详细研究仍需要提高能量分辨率。