Molecular Wires in Electromagnetic Fields

We investigate the role of external electromagnetic fields on the conduction properties of bridged molecular wires. In particular, it is analyzed quantitatively how resonant excitations of electrons enhance the dc current and, simultaneously, lower the noise level of the current. The results from an exact numerical treatment are in good agreement with those obtained within an approximation scheme applicable at resonances. Thirty years ago, Aviram and Ratner proposed in a seminal work [1] to build elements of electronic circuits—in their case a rectifier—with single molecules. In the present days their vision starts to become reality and the experimental and theoretical study of such systems enjoys a vivid activity [2–4]. Recent experimental progress has enabled reproducible measurements [5, 6] of weak tunneling currents through molecules which are coupled by chemisorbed thiol groups to the gold surface of external leads. Typical energy scales in molecules are in the optical and the infrared regime, where basically all of the today’s lasers operate. Hence, lasers represent a natural possibility to control atoms or molecules and also currents through them. It is for example possible to induce by the laser field an oscillating current in the molecule which under certain asymmetry conditions is rectified by the molecule. This results in a directed electron transport even in the absence of any applied voltage [7, 8]. Another theoretically predicted effect is the current suppression by the laser field [9, 10] which offers the possibility to control both the average current and the current noise. Since the considered frequencies lie below typical plasma frequencies of metals, the laser light will be reflected at the metal surface, i.e., it does not penetrate the leads. Consequently, we assume that the leads’ bulk properties are essentially unaffected by the laser field—in particular each lead remains close to equilibrium. Thus, it is sufficient to consider the influence of the driving solely in the molecule Hamiltonian. In addition, the energy of infrared light quanta is by far smaller than the work function of a common metal, which is of the order of 5 eV. This prevents the generation of a photo current, which otherwise would dominate the effects discussed below. For a quantitative description of an experiment, it might be necessary to take into account also the influence of the laser on the leads. Most theoretical descriptions of the molecular conductivity in static situations are based on a scattering approach [11–13], or assume that the unB. Kramer (Ed.): Adv. in Solid State Phys. 44, 157–167 (2004) c © Springer-Verlag Berlin Heidelberg 2004 158 Sigmund Kohler et al.