  We study the variation of electron transmission through Au-S-benzene-S-Au
junctions and related systems as a function of the structure of the Au:S
contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly
coordinated in-hollow and bridge positions are connected with broad
transmission peaks around the Fermi level, due to a broad range of transmission
angles from transverse motion, resulting in high conductivity and weak
dependence on geometrical variations. In contrast, for (unstable) S-adsorption
on top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission
peaks narrow up due to suppression of large transmission angles. Such more
one-dimensional situations may describe more common types of contacts and
junctions, resulting in large variations in conductivity and sensitivity to
bonding sites, tilting and gating. In particular, if S is adsorbed in an Au
vacancy, sharp spectral features appear near the Fermi level due to essential
changes of the level structure and hybridization in the contacts, admitting
order-of-magnitude variations of the conductivity. Possibly such a system, can
it be fabricated, will show extremely strong non-linear effects and might work
as uni- or bi-directional voltage-controlled 2-terminal switches and non-linear
mixing elements. Finally, density-functional-theory (DFT) based transport
calculations seem relevant, being capable of describing a wide range of
transmission peak structures and conductivities. Prediction and interpretation
of experimental results probably require more precise modeling of realistic
experimental situations.
