Current Noise in ac-Driven Nanoscale Conductors

Abstract

The theory for current fluctuations in ac-driven transport through nanoscale systems is put forward. By use of a generalized, non-Hermitian Floquet theory we derive novel explicit expressions for the time-averaged current and the zero-frequency component of the power spectrum of current fluctuations. A distinct suppression of both the zero-frequency noise and the dc-current occurs for suitably tailored ac-fields. The relative level of transport noise, being characterized by a Fano factor, can selectively be manipulated by ac-sources; in particular, it exhibits both characteristic maxima and minima near current suppression.PACS numbers: 05.60. Gg, 85.65.+h, 72.40.+w Recent experimental successes in the coherent coupling of quantum dots [1] and in the reproducible measurement of electronic currents through molecules [2,3] have given rise to renewed theoretical interest in the transport properties of nanoscale systems [4,5]. Thereby, new ideas in order to exploit the quantum coherence of such systems for the construction of novel electronic devices [5] have emerged. One possible construction element is based on the manipulation of quantum dots or single molecules by use of an oscillating gate voltage or an infrared laser, respectively. A prominent effect of such ac-fields consists in the adiabatic [6,7,8,9] and nonadiabatic [10, 11] pumping of electrons. Moreover, laser irradiated molecular wires provide novel devices such as coherent quantum rectifiers [12] and optically controlled transistors [13]. However, such time-dependent control schemes can be valuable in practice only if they operate at tolerable noise levels. Thus, the question whether noise properties of nanoscale systems can be selectively manipulated becomes of foremost interest.Electron transport through time-independent, mesoscopic systems is commonly described within the framework of a scattering formalism. Both the average current [14] and the transport noise characteristics [15,16] can be expressed in terms of the quantum transmission coefficients for the corresponding transport channels. By contrast, the theory for driven quantum transport is much less developed. Expressions for the spectral density of the current fluctuations have been derived for the low-frequency ac-conductance [17] and the scattering by a slowly time-dependent potential [18]. However, the situation becomes more opaque in the presence of rapidly varying time-dependent fields. Within a Green function approach, a formal expression for the current through a time-dependent conductor has been presented in Refs. [19,20]. Here, we derive explicit expressions for both the current and the noise properties of electron transport through a nanoscale conductor under the influence of time-dependent forces at arbitrary frequency and strength. The dynamics of the electrons is solved by integrating the Heisenberg equations of motion for the electron creation/annihilation operators within a generalized Floquet approach. We then use the resulting expressions to explore the possibility of an ...

Controlling currents through molecular wires

“…we find after some algebra that it assumes the commonly expected "scattering form" [18] but with periodically time-dependent transmission probabilities and, as well, an additional contribution that accounts for a T -periodic charging/discharging of the wire [19]. Only the former contributes to the time-averaged current…”

Molecular Wires in Electromagnetic Fields

“…For the retarded Green function of the wire electrons, one finds after eliminating the leads the equation of motion [10] …”

“…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.…”

“…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.…”

“…This suggests utilizing the tools that have been acquired in that area, thus, developing a transport formalism that combines Floquet theory for a driven molecule with the many-particle description of transport through a system that is coupled to ideal leads [8,10,16].…”