We have mapped the electronic bands of the “quasi-(5×5)” monolayer structure of Cu on Si(111) near the Fermi energy at high angular and energy resolution. Although this unusual system does not exhibit a true long-range periodicity, well-defined bands and sharp Fermi surface contours are observed. The fundamental Fermi surface inside the first surface Brillouin zone has the shape of a hexagon and possesses strong nesting features. However, no relation to the unusual translational symmetry could be established. Strong umklapp bands and Fermi surface contours are observed with umklapp vectors that correspond to the reciprocal lattice of the slightly expanded and rotated discommensurate ${\mathrm{Cu}}_2\mathrm{Si}$ layer, and to that of the quasi-$(5\mathrm{}\times 5)$ domain pattern formed by the regular dislocation network. We find thus a one-to-one correspondence between the electronic bands and the complex structure model that has been established by Zegenhagen et al. [Phys. Status Solidi B 204, 587 (1997)]. This suggests that the two-dimensional electron gas formed on this surface does not induce the formation of the discommensurate structure but rather takes a spectator role.

• Received 4 July 2001

DOI:https://doi.org/10.1103/PhysRevB.64.235415