Evidence for 2 × 1 building block in the atomic structure of Si{111} 7 × 7

The well known experimental Si {111} 7 × 7 LEED pattern is shown to exhibit systematic enhancement of diffracted intensity at the $\text{1}\text{2}$-order positions, thus revealing the existence of 2 × 1 or 2 × 2 structural components in the 7 × 7 structure with a single domain Si {111} 2 × 1 cleaved structure reveals remarkable similarities. The conclusion is that the fundamental building block of the 7 × 7 structure is a 2 × 1 subunit which closely resembles the unit cell of the cleaved 2 × 1 structure.