Non–Linear Transport through a Molecular Nanojunction

– We present a simple model of electrical transport through a metal–molecule– metal nanojunction that includes charging effects as well as aspects of the electronic structure of the molecule. The interplay of a large charging energy and an asymmetry of the metal– molecule coupling can lead to various effects in non–linear electrical transport. In particular, strong negative differential conductance is observed under certain conditions. Introduction. Single molecule electron transistors offer exciting perspectives for further minituarisation of electronic devices with a potentially large impact in applications. To date several experiments have shown the possibility to attach individual molecules to leads and to measure the electrical transport. Two terminal transport through a single molecule [1–4] or other nanoscale objects [5–7] has been achieved by deposition of the object between two fixed electrodes or a conducting-tip STM above an object attached to a conducting substrate [8–11]. Interesting and novel effects, such as negative differential conductance (NDC), were observed in one experiment [12], which still needs satisfactory theoretical explanation. Several factors are important for single-molecule transport: For nanoscale objects the capacitance C is very small. Consequently, the energy to charge (or uncharge) the molecule EC = e /2C can be very large, of the order of electron volts. This leads to the phenomenon of Coulomb blockade and makes room temperature single-electron transistors (SET) based on such molecules a distinct possibility. In contrast to SETs based on metallic islands [13, 14], molecular devices have a more complicated electronic structure that, in principle, can be chemically ’designed’. Gaps in the I–V curve are not only determined by EC but predominantly by the structure of the molecular bands [2, 3]. Therefore, it is important to consider the interplay of charging effects with the specific structure of the molecular orbitals. For electronic transport we will see that, in particular, the specific spatial structure of the molecular orbitals is crucial. In this letter we study the impact of such a low-energy electronic structure on electronic transport and demonstrate that it can result in non–trivial conductance under reasonable generic assumptions. A full quantitative treatment of a molecule in contact with metal electrodes including many–body interactions on the molecule and strong molecule-electrode coupling is still out of reach. In this paper, we therefore study a simple model for a ’generic’