Electronic stabilization of a 5×4 dopant superlattice on Si(111)5×2-Au

The Si(111)5×2-Au surface exhibits a chain structure with additional Si atoms on top of the chains. They dope the chains to achieve the optimum band filling, according to recent local density calculations. Surprisingly, the Si atoms form a half-filled, disordered 5×4 lattice fluid, not an ordered 5×8 lattice. From their autocorrelation function an interatomic potential with 5×4 periodicity was deduced. An explanation for the 5×4 periodicity is provided by establishing a connection to the electronic structure near the Fermi level E_F, which is mapped by angle-resolved photoemission. The constant energy surfaces near E_F consist of one-dimensional lines at the boundaries of a 5×4 Brillouin zone. Such nested features of high density of states are capable of triggering a 5×4 superlattice interaction. The measurements establish a two-way connection between electronic structure and interatomic potentials: A one-dimensional Fermi surface instability triggers a superlattice of extra atoms, and the atoms provide the correct number of electrons for such an instability to occur. The band structure is discussed in view of the recently observed phase-separation into nanometer-sized segments of metallic and semiconducting character.