Electronic transport in low-dimensional nanostructures
The electronic properties of low-dimensional systems are closely related to their geometric structure. Electron confinement is of high importance in one (1D) and two-dimensional (2D) structures leading to strong correlations between electrons and to significant deviations from Fermi liquid behavior, sometimes better described by a Luttinger liquid.
There are actually two directions addressed in the recent past: a) Surface (magneto-) conductance and scattering mechanisms in systems with strong spin-orbit coupling, and b) quasi-1D systems.
a) DC transport is actually sensitive to confinement, to growth modes, and to (electronic) surface roughness. Our experimentally investigated examples show that we can discriminate classical and quantum size effects, pronounced conductance anisotropy or conductance oscillations directly related to a layer-by-layer growth mode. By adding a magnetic field, details of the scattering mechanism at impurities and defects are revealed, since elastic, inelastic, spin–orbit scattering contributions can be separated. Strong changes of scattering properties are evident when going from the multilayers to the monolayer. Contrary to multilayers, spin-orbit scattering dominates in many cases for the monolayer, presumably due to the symmetry reduction at the surface and the appearance of spin-split bands split by the Rashba effect. The intriguing interplay between bulk and surface conductance with large Rashba split states at the surface is exemplified by a study of multilayer growth of Bi on Si(111). By magnetoconductance we were able to separate bulk from surface contributions. Scattering between strongly spin-polarized Rashba-split states spin can be effectively suppressed, so that only the ‘‘classical’’ magneto- conductance effect remains, as observed in this system. Adsorption of small concentrations of impurities with large magnetic moments reveal futher details of the scattering mechanism.
b) The 1D case is an exotic situation, since it inherently unstable. As a consequence, numerous instabilities exist such as spin- and charge density waves or metal-insulator transitions in electronic transport, which reduce both energy and entropy. From a practical point of view, 1D and 2D systems cannot exist in free space. They can be realized either by strongly anisotropic crystals and polymers or on supporting surfaces, which, however, are the source of 3D interaction.
Bundles of chains consisting of metal atoms such as Au, Ag or Pb can easily be formed by various systems on insulating surfaces by either spontaneously breaking the surface symmetry or by ordering on specially prepared, e.g. regularly stepped surfaces like Si(557). These systems are called quasi one-dimensional, but exhibit in certain limits 1D properties. How these properties can be influenced by the substrate, the width of wires formed, adsorbate density etc. is investigated in this project. Thus we get information about how 1D properties are influenced by the interactions of adsorbate and substrate, and directly between the adsorbed wires. Our research focuses on the relation between electronic and geometric properties in these systems.
A particularly fascinating example is the Pb/Si(557) system, which is able to form new facets induced by concentrations of Pb around one monolayer. Depending on Pb concentration we get Fermi nesting. and consequently 1D electronic conductance along the wires. In the perpendicular direction formation of charge and spin density waves was found. As experimental methods we use LEED, tunneling microscopy, angular resolved photoemission and magneto-conductance both on a mesoscopic and a macroscopic scale.