Reflection high-energy positron diffraction study of surface super-structures

Kawasuso, Atsuo; Fukaya, Yuki*; Hayashi, Kazuhiko; Hashimoto, Mie; Ichimiya, Ayahiko

Well-focused and highly parallel positron beam is totally reflected at the first surface layer when grazing incidence at a small enough angles. Therefore, the reflection high-energy positron diffraction (RHEPD) is suited to determine the surface structures without any disturbance from the bulk. Using the electro-magnetic lenses we developed a positron beam which has more coherence than before. We found that the RHEPD pattern from the Si-77 surface is considerably different from that of electron diffraction. The difference is reasonably explained as a strong thermal diffuse scattering of positrons. As a further application, we studied the phase transition of Si-Ag ultra-thin film. We found that the surface-sensitive diffraction spots exhibit significant temperature dependences. Based on the dynamical diffraction theory, the temperature dependences are reproduced considering the order-disorder phase transition. This will terminate the long debates in the surface science community. Furthermore, we recently found that the positron-surface interaction is different from the electron-surface interaction, especially in the elastic processes. We present the above recent advances of surface studies using RHEPD.