As shown recently by STM and LEED, the adsorption of approximately 1ML Pb at low temperatures on Si(557) followed by annealing to 640K leads to the formation of Pb-chains with an average interachain spacing of d=1.5nm [1]. These wires have unique electrical properties: dc-conductivity measurements below 78K have revealed that electronic transport occurs only along the chain direction, whereas insulating behavior is found in the perpendicular direction. Above 78 K, the system switches into a 2d-regime, i.e. activated transport is found along and across the step direction [2]. The transport properties are coupled with structural changes. Here we report on SPA-LEED experiments, where we have investigated systematically the adsorption of Pb in the monolayer regime close to Tc. The closely packed Pb film on the micro-Si(111) facets forms locally a sqrt3xsqrt3 structure. The 10-fold periodicty, as seen also by STM, is obviously the consequence of sqrt3 domains separated sqrt7xsqrt3 domain walls. This phase is known as the HIC structure form ~1.25ML Pb on Si(111) [3]. In the perpendicular direction the Pb-chains reorganize the structure of the Si(557) substrate into [223] facets with 4 2/3 Si unit cells, as seen by a spot-splitting of 21,3%, i.e. the Pb phase and in particular the interchain distance is an electronically stabilized structure. This model is fully supported by ARPES measurements, where the interchain distance d is seen by Umklapp structures close to the Fermi surface [4]. With respect to the conductivity measurements this finding results in perfect Fermi nesting in the direction normal to the chains below 78 K. In addition, the domain structure along the chains forms split-off valence bands with mesoscopic Fermi--wavelengths, responsible for the 1d conductance along the chains without further instabilities at low temperatures. Above 78K the perfect nesting condition is destroyed in agreement with an incommensurate-incommensurate phase transition of the HIC structure, seen by SPA-LEED.
[1] C. Tegenkamp, H. Pfnür, Surf.Sci. (2007), doi:10.1016/j.susc.2006.11.068
[2] C. Tegenkamp, et.al., PRL 95 (2005) 176804.
[3] S. Stepanovsky et.al., Surf. Sci. 600 (2006) 1417.
[4] C. Tegenkamp, H. Pfnür, T. Ohta, J.L. McChesney, E. Rotenberg, K. Horn, submitted.
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