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

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，我们研制了一台能量过滤反射高能正电子衍射仪。通过优化缓变电极的结构，我们获得了4 eV的能量分辨率，这足以观察到表面等离子体激发。因此，我们首次成功地观测到了Si(111)-7x7，Al(111)和Bi(001)表面的多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激元的平均激发数比电子的平均激发数大一倍。正电子和电子之间的这种差异源于它们不同的衍射程。也就是说，直到最终逃逸到真空中，电子穿透到块体中，而正电子则在第一表面层中穿行。因此，正电子比电子激发更多的表面等离子体。考虑表面等离子激元的多次激发，可以解释全反射条件下反射光束轮廓展宽和正电子绝对反射率下降的原因。我们还观察到了正电子的能量损失过程，在低温下，Si(111)/In表面发生了金属-绝缘体的转变。虽然在相变过程中观察到了能量损失谱的微小调制，但仍需要提高能量分辨率以进行详细的研究。

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.