Molecular Wires in Electromagnetic Fields

Abstract

Abstract. 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][3][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][12][13], or assume that the un-

Formulation of charge transport in molecular junctions with time‐dependent molecule‐leads coupling operators

“…Significant effort was devoted in recent years to the development of theories that account for nano‐scale transport in the presence of time‐dependent perturbations (for extensive reviews see, e.g., ). Considering periodic perturbations , the Floquet theory in combination with quantum scattering theory enabled to account for the effect of oscillating potentials on the transport characteristics . The Floquet theory was integrated also into density matrix or Green's functions based treatments of monochromatically driven transport.…”

“…Considering periodic perturbations , the Floquet theory in combination with quantum scattering theory enabled to account for the effect of oscillating potentials on the transport characteristics . The Floquet theory was integrated also into density matrix or Green's functions based treatments of monochromatically driven transport. Other theoretical treatments considered pulsed excitations, using a reduced density matrix approach or non‐equilibrium Green's functions techniques .…”

Photon-assistant Fano resonance in serially coupled triple quantum dots

“…In this paper we study the photon-assistant transport in MCQDs (see Fig.1) paying special attention to the consequences of those interesting interference phenomena. In previous theoretical work on MCQDs, the quantum levels, which depend on the structures of the system, are often assumed as parameters 30,31 , and the quantum properties of MCQDs can not be presented quantitatively. Therefore, it is necessary to reveal the quantum behavior of MCQDs based on more realistic model.…”

AC-Driven Transport Through Molecular Wires

“…At the resonance peaks, the current no longer decays exponentially fast with increasing wire length, but scales asĪ ∝ A 2 /(N −1)Γ [28]. This photon-assisted tunnelling effect is shown in Fig.…”

“…The resulting changes of the current through a contacted molecule due to the influence of a laser field are studied. In particular, we focus on the modification of the length dependence of the conductivity [26][27][28]. At present, the corresponding experiments are attempted, but still no clearcut effect has been reported.…”