Correlation of electronic transport and structure in Pb atomic wires on Si(557) surfaces

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Temperature-driven refacetting phase transition in Pb chains on Si(557)

By using quantitative low energy electron diffraction, we have studied the temperature-driven phase transition of Pb chains grown on Si(557) substrates at a surface concentration of 1.3 ML. This concentration, which is still below one physical monolayer, exhibits a unique switching of electrical conductance from one dimensional to two dimensional above 78 K, which is coupled to this phase transition, and was investigated for this reason. Annealing to 640 K causes a concentration-driven refacetting of the whole surface into large (223) facets at low temperatures, while along the chains a so-called (1,5) linear phase is formed, causing a tenfold periodicity. At Tc=78 K, we analyze a temperature-driven order-order transition along the [¯1¯12] direction in detail, which again turns out to be a refacetting transition. The two-dimensional character of this transition was seen by corresponding structural changes along the [1¯10] direction as well. Refacetting causes a change in periodicity and destroys the conditions of Fermi nesting necessary for one-dimensional conductance. © 2008 The American Physical Society.DF

Magnetotransport in anisotropic Pb films and monolayers

The anisotropy induced by atomic steps of a Si(557) substrate in structure and magnetoconductance of ultrathin Pb films adsorbed on this surface is shown to be effectively shielded as a function of layer thickness, as found out by a combined study of low-energy electron diffraction and macroscopic four-point conductivity measurements as a function of Pb coverage, temperature, and magnetic field. In strong contrast to flat Si(111), substrate steps effectively compensate the lateral misfit (10%), leading to crystalline growth starting from the first monolayer. Multilayers already exceeding four physical monolayers (PML) form isotropic and percolated Pb islands even on this uniaxial surface. This structural anisotropy corresponds well to that found in dc conductivity measurements. As a function of temperature, strong localization effects with clear anisotropy become dominant for coverages below 4 PML. Strong anisotropic magnetotransport was found for Pb-wetting layers close to completion of the physical monolayer caused by an enhanced elastic scattering rate in the direction perpendicular to the step direction. While multilayers are characterized by weak localization, antilocalization is found for all monolayer structures due to strong spin-orbit coupling, which is effectively switched off around 1.3 ML (1 PML) below 78 K, where one-dimensional transport was seen along the step direction. © 2010 The American Physical Society.DF

Pb nanowires on vicinal Si(111) surfaces: Effects of refacetting on transport

The conductance of Pb wires grown by self-assembly on Si(557) has been studied in detail as a function of coverage and of the facet structure. Only for 1.31 ML, corresponding to one physical monolayer on the terraces (steps not covered with Pb), and a perfectly ordered wire array along the [¯1¯12] direction quasi-one-dimensional (1D) transport along the [1¯10] direction is found, corroborating the model of one-dimensional band filling in an adsorbate induced (223) facet structure. The transport results recently shown by Morikawa et al. [Phys. Rev. B 82, 045423 (2010)] can also reproduced by our group. In contrast to what was claimed by them, our results clearly show that either a too small coverage or structural imperfections of the surface are responsible for a metal-insulator transition around 140 K irrespective of the crystallographic direction. The variety of different transport scenarios found is caused by strong adsorbate-induced refacetting into an electronically stabilized (223) orientation, which differs from the macrosocopic orientation of the substrate. The crucial interplay between structure and filling factor explains the extremely small parameter window in which the 1D transport channel can be observed. © 2010 The American Physical Society.DF