Linear low-energy positron diffraction (LLEPD) for the Si(111)-(2 x 1) pi-chain model

We have applied the linear method to low-energy positron diffraction and lo w-energy electron diffraction. The linear method searches for the global mi nimum in a structural analysis by carrying out full dynamical calculations at atomic positions which scale linearly as the number of independent struc tural parameters and variations in each parameter. For the Si(111)-(2 x 1) pi-chain system, linear low-energy electron diffraction (LLEED) is found to work adequately. However, in other systems tested, LLEED breaks down when atoms in the first and second layers are lined up along the incident-beam d irection. On the other hand, the method of linear low-energy positron diffr action (LLEPD) is found to work extremely well in all systems tested. This is due to the absence of forward focusing in low-energy positron diffractio n. Thus, LLEPD should be an excellent tool for locating the global minimum in complex surface-structural searches.