Surface study using positron diffraction

Kawasuso, Atsuo; Fukaya, Yuki   ; Hashimoto, Mie; Ichimiya, Ayahiko

Well-focused and highly parallel positron beam is totally reflected at the first surface layer at a small enough glancing angles. The reflection high-energy positron diffraction is suited to determine surface structures without any disturbance from the bulk. Using electro-magnetic lenses we developed a coherent positron beam. We found that the RHEPD pattern from the Si-77 surface is different from that of electron diffraction. The difference is reasonably explained as a strong thermal diffuse scattering of positrons. We studied the phase transition of Si-() ()-Ag surface and found that the surface-sensitive diffraction spots exhibit significant temperature dependences, which is explained considering the order-disorder phase transition. We also succeeded in determining the structure of Si-() ()-Ag surface. Furthermore we studied the metal/insulator phase transition of Si-In surface. We found a large displacement of In atomic chains at low temperatures.