Surface structure analysis of the domain-wall phase of S/Ru(0001) using an efficient parameter optimization method

Abstract

A structural analysis of the striped domain wall phase of S/Ru(0001) is performed using LEED-IV techniques. For handling the large number of geometrical parameters, a new technique for multi-dimensional data optimization designed for the analysis of LEED-IV data was developed. It is based on the Newton-Raphson method, which includes all parameter couplings to first order in the fit. However, instead of using first- and second-order derivatives to evaluate the required quadratic expansion of the R-factor hypersurface, we use a weighted least-squares fit to adapt a paraboloid to the R-factor data (PLEED). The structural analysis, which requires the determination of 27 geometrical parameters, reveals that the local structure of the domain walls resembles that of the c(2 × 4) structure, but additional relaxations have been found which can be identified as the origin of the effective lateral repulsion between the domain walls. The main mechanisms of local structure formation in this system are discussed. The formation of a strong local bond is modified by lateral relaxations due to effective repulsive interactions. They involve tilting of the tetrahedron formed by the S atom and its three nearest neighbors and lateral shifts.