Electronic transport through nanostructures: From thin wires to single atoms and molecules

H. v. Löhneysen
Universität Karlsruhe und Forschungszentrum Karlsruhe, DFG Center for Functional Nanostructures

Electronic transport via restricted geometries reveals new phenomena that are not found in macroscopic samples. Phenomena like weak localization, Aharonov-Bohm effects, universal conductance fluctuations, and conductance quantization arise from quantum interference of electron waves, while Coulomb blockade is due to charge quantization. In addition, electron-electron interactions in one-dimensional conductors may lead to break down of the Fermi-liquid quasiparticle concept. All these features tend to be more pronounced as the system size and/or the dimensionality is reduced. We will discuss experiments on Aharonov-Bohm rings and small metal bridges out of equilibrium to study interference and electron-electron interaction effects. Interference effects (universal conductance fluctuations) will also be employed to investigate the phase coherence time in metallic spin glasses. Of course, the ultimate material unit to carry an electric current is a single atom or a single molecule. In microfabricated break junctions where the transition from a conducting metallic state to a state of tunnelling through a vacuum barrier can tuned reproducibly, the observed conductance plateaus in this ``wave guide for electrons'' can be analysed in terms of the contribution of different modes with the help of superconductivity. In addition, distinct differences between opening or closing of a contact are found in conductance histograms, indicating ``deformations'' on an atomic level.