The noise properties of pump currents through an open double-quantum-dot setup with nonadiabatic ac driving are investigated. Driving frequencies close to the internal resonances of the double-dot system mark the optimal working points at which the pump current assumes a maximum while its noise power possesses a remarkably low minimum. A rotating-wave approximation provides analytical expressions for the current and its noise power and allows to optimize the noise characteristics. The analytical results are compared to numerical results from a Floquet transport theory.

We investigate a mechanism for extracting heat from metallic conductors based on the energy-selective transmission of electrons through a spatially asymmetric resonant structure subject to ac driving. This quantum refrigerator can operate at zero net electronic current as it replaces hot by cold electrons through two energetically symmetric inelastic channels. We present numerical results for a specific heterostructure and discuss general trends. We also explore the conditions under which the cooling rate may approach the ultimate limit given by the quantum of cooling power.Comment: 4 pages, 3 figures; published version, typos correcte

We study the current and the associated noise for the transport through a two-site molecule driven by an external oscillating field. Within a high-frequency approximation, the time-dependent Hamiltonian is mapped to a static one with effective parameters that depend on the driving amplitude and frequency. This analysis allows an intuitive physical picture explaining the nontrivial structure found in the noise properties as a function of the driving amplitude. The presence of dips in the Fano factor permits a control of the noise level by means of an appropriate external driving.

We derive a master equation for the electron transport through molecular wires in the limit of strong Coulomb repulsion. This approach is applied to two typical situations: First, we study transport through an open conduction channel for which we find that the current exhibits an ohmic-like behaviour. Second, we explore the transport properties of a bridged molecular wire, where the current decays exponentially as a function of the wire length. For both situations, we discuss the differences to the case of non-interacting electrons.

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-