Angle-resolved photoemission has been utilized to study the surface electronic structure of $\frac{1}{3}$ monolayer of Sn on Ge(111) in both the room-temperature $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ phase and the low-temperature ( $3\times 3$) charge-density-wave phase. The results reveal a gap opening around the ( $3\times 3$) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting. A highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the ( $3\times 3$) symmetry.

• Received 4 May 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.3684