Atomic Scale Study of Surface Structures and Phase Transitions with Reflection High-Energy Positron Diffraction

In this work, we studied a few surfaces, of which the structures have not yet been revealed, using reflection high-energy positron diffraction (RHEPD). We studied the Ge(111)/Pb and Ge(111)/Sn surfaces that exhibit the phase transition from 3×3 to √3×√3 periodicities at around 200K. We found that in both phases the equilibrium positions of Pb and Sn adatoms are conserved. That is, in the unit-cell, one of three adatoms is located upper position and two of them are located lower positions (one-up-two-down). The phase transition is interpreted in terms of an order-disorder type. We furthermore studied the quasi-one dimensional Si(111)/In surface which exhibits the metal-insulator transition at around 120K. The high temperature phase is well explained as the zigzag chain structure. We found that a dynamic atomic displacement which leads to the formation of hexagon structure occur below 120 K. We confirmed the appearance of the band gap using the surface structure determined from the RHEPD rocking curves.