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An experimental study to determine the orientation of surface structures with respect to atomic-bond direction is presented. The As-stabilized (2×4) and C(2×8), and Ga-stabilized (4×2) and C(8×2) surface structures all have their twofold direction parallel to the plane containing the dangling surface bonds and perpendicular to that of the back bonds.
A. Y. Cho (Thu,) studied this question.