1.- Ab initio description of the electronic transport in single-molecule junctions

Present trends in the miniaturization of electronic devices suggest that ultimately single molecules may be used as electronically active elements in a variety of applications. Recent advances in the manipulation of single molecules now permit to contact an individual molecule between two electrodes and measure its electronic transport properties. In contrast to single-electron transistors based on metallic islands, molecular devices have a more complicated, but in principle tunable, electronic structure. In addition to generic principles of nanoscale physics, e.g. Coulomb blockade, the chemistry and geometry of the molecular junction emerge as the fundamental tunable characteristics of molecular junctions.

In our group we have developed new theoretical tools to bridge traditional concepts of mesoscopic and molecular physics to describe transport through single molecules. In particular, in order to elucide the relation between the electronic structure of individual molecules and the conductance of the circuits in which they are embedded, we have combined ab initio quantum chemistry calculations, based on the density functional theory, with non-equilibrium Green functions techniques. This approach and constitutes a first step towards a quantitative theory for the a priori design of molecular devices.

We have used the approach mentioned above to describe transport through: single organic molecules [Reichert et al. PRL 88, 176804 (2002); J. Heurich et al., PRL 88, 256803 (2002)], a hydrogen molecule between Pt electrodes [Smit et al., Nature 419, 906 (2002); J.C. Cuevas et al., Nanotechnology 14, 29 (2003)], and atomic contacts [J.C. Cuevas et al., Nanotechnology 14, 29 (2003)].

Our current work is aimed at the understanding of the following issues in the electronic transport of single-molecule:

2.- Electrical conduction in metallic atomic-sized contacts

The appearance of experimental techniques such as the scanning tunneling microscope and breakjunctions has allowed to explore the electronic transport at the atomic scale [see N. Agrait et al., Phys. Rep. 377, 81 (2003)]. With these techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to a single atom upon further stretching. The conductance of these contacts can be described by the Landauer formula: G=G0i Ti, where the sum runs over all the available conduction channels, Ti is the transmission for the i-th channel and G0=2e2/h is the quantum of conductance. It has been established that the channels in atomic contacts are determined by the valence orbitals of the central atoms, and the transmission of each channel is fixed by the atomic environment of the neck region [see E. Scheer et al., Nature 394, 154 (1998)].

In spite of the progress made in the last years in the understanding of the transport properties of these nanowires, there are still several basic open problems. In our group we have focused our theoretical efforts in resolving some of these puzzles. In particular, we have studied the following issues of special interest: