Fractional surface doping by topological neutral wall intersections on Ge(111)

We show that a fluctuating incommensurate domain wall structure such as that hypothesized for clean Ge(111) above 540 K and the observed weakly metallic behavior can be mutually related in an unexpected manner. The wall structure implies a liquid of defects-adatom trimers at the intersection of three concurrent topological walls-that carry fractional charge, one half extra electron each. These electrons are delocalized among defects, giving rise to a narrow band 2D metal, whose Fermi level density of states grows with the density of walls and thus with temperature. This model agrees strikingly with new photoemission measurements carded out on Ge(111) across the 540 K transition and beyond.