Anisotropic conductance of Pb-induced chain structures on Si(557) in the monolayer regime

We present data of a study of four-point conductance of adsorbed Pb films on Si(557) in the thickness range between 0.6 up to several monolayers (ML) at various annealing stages. These measurements are combined with tunneling microscopy (STM). Onset of conductance is found close to the percolation limit. Pb layers annealed to room temperature are characterized by activated contributions to conductance up to 3 ML, a purely metallic temperature dependence at thicker layers, and an anisotropy of at most a factor of 2. On the contrary, annealing to 640 K, leaving only the first monolayer on the Si(557) surface results in extremely high surface state conductance which is quasi one-dimensional below a critical temperature of Tc=78 K, associated with an order-disorder phase transition. Induced by a 10-fold superperiodicity along the Pb chains and their lateral ordering, the system switches from low to high conductance anisotropy, with a metal-insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a 1/T + const. temperature dependence was found.