Conductance transition and interwire ordering of Pb nanowires on Si(557)

A Pb-nanowire array on a stepped Si surface, Si (557) α×2-Pb, is investigated by surface-state conductivity measurements, scanning-tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES). No clear indication was found for the abrupt phase transition at 78 K, which was reported to exhibit a switching from a two-dimensional semiconducting conductivity to an one-dimensional metallic behavior and to accompany an intrawire ordering together with a change in the interwire periodicity (so-called "refacetting"). In contrast, a significant reduction in the surface conductivity is found at around 140 K. This transition is accompanied by a subtle out-of-phase interwire ordering of the protrusions along wires in STM images. However, ARPES finds no sign of a significant electronic structure change. While we cannot rule out the possibility of a phase transition at 78 K within a very limited coverage range, this metal-insulator-type transition is found to prevail for most of Pb coverages with a good α×2 ordering. The conductivity below 80 K can be described by the weak localization in two dimensions probably due to the interwire ordering but the abrupt transition at 140 K itself cannot easily be explained at present.