We explore the prospects to control by use of time-dependent fields quantum transport phenomena in nanoscale systems. In particular, we study for driven conductors the electron current and its noise properties. We review recent corresponding theoretical descriptions which are based on Floquet theory. Alternative approaches, as well as various limiting approximation schemes are investigated and compared. The general theory is subsequently applied to different representative nanoscale devices, like the non-adiabatic pumps, molecular gates, molecular quantum ratchets, and molecular transistors. Potential applications range from molecular wires under the influence of strong laser fields to microwave-irradiated quantum dots.Comment: 82 pages, 19 figures, elsart.cls, solved LaTeX/hyperref problem

Spin qubits offer one of the most promising routes to the implementation of quantum computers. Very recent results in semiconductor quantum dots show that electrically-controlled gating schemes are particularly well-suited for the realization of a universal set of quantum logical gates. Scalability to a larger number of qubits, however, remains an issue for such semiconductor quantum dots. In contrast, a chemical bottom-up approach allows one to produce identical units in which localized spins represent the qubits. Molecular magnetism has produced a wide range of systems with properties that can be tailored, but so far, there have been no molecules in which the spin state can be controlled by an electrical gate. Here we propose to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core. Through electrical manipulation of the molecular redox potential, the charge of the core can be changed. With this setup, two-qubit gates and qubit readout can be implemented.

The effect of laser fields on electron transport through a molecular wire weakly coupled to two leads is investigated. The molecular wire acts as a coherent quantum ratchet if the molecule is composed of periodically arranged, asymmetric chemical groups. This setup presents a quantum rectifier with a finite dc response in the absence of a static bias. The nonlinear current is evaluated in closed form within the Floquet basis of the isolated, driven wire. The current response reveals multiple current reversals together with a nonlinear dependence on the amplitude and the frequency of the laser field. The current saturates for long wires at a nonzero value, while it may change sign upon decreasing its length.

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 ...

Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.

No abstract

Thermally activated escape over a potential barrier in the presence of periodic driving is considered. By means of novel time-dependent path-integral methods we derive asymptotically exact weak-noise expressions for both the instantaneous and the time-averaged escape rate. The agreement with accurate numerical results is excellent over a wide range of driving strengths and driving frequencies.PACS numbers: 82.20.Mj, 82.20.Pm The problem of noise driven escape over a potential barrier is ubiquitous in natural sciences [1]. Typically, the noise is weak and the escape time is governed by an exponentially leading Arrhenius factor. This scheme, however, meets formidable difficulties in far from equilibrium systems due to the extremely complicated interplay between global properties of the metastable potential and the noise [1,2]. Prominent examples are systems driven by time-periodic forces [3,4], exemplified by strong laser driven semiconductor heterostructures, stochastic resonance [5], directed transport in rocked Brownian motors [6], or periodically driven "resonant activation" processes [7] like AC driven biochemical reactions in protein membranes. Despite its experimental importance, the theory of oscillating barrier crossing is still in its infancy. Previous attempts have been restricted to weak (linear response), slow (adiabatic regime), or fast (sudden regime) driving [3][4][5]. In this Letter we address the most challenging regime of strong and moderately fast driving by means of path-integral methods. In fact, our approach becomes asymptotically exact for any finite amplitude and period of the driving as the noise strength tends to zero, and comprises a conceptionally new, systematic treatment of the rate prefactor multiplying the exponentially leading Arrhenius factor. Closest in spirit is the recent work [4], which is restricted, however, to the linear response regime for the exponentially leading part and treats the prefactor with a matching procedure, involving the barrier region only. Our analytical theory is tested for a sinusoidally rocked metastable potential against very precise numerical results. Conceptionally, our approach should be of considerable interest for many related problems: generalizations for higher dimensional systems and for non-periodic driving forces.Model -We consider the overdamped escape dynamics of a Brownian particle x(t) in properly scaled unitṡwith unbiased δ-correlated Gaussian noise ξ(t) (thermal fluctuations) of strength D. The force-field F (x, t) is assumed to derive from a metastable potential with a well atx s and a barrier atx u >x s , subject to periodic modulations with period T . For D = 0, the deterministic dynamics (1) is furthermore assumed to exhibit a stable periodic orbit (attractor) x s (t) and an unstable periodic orbit (basin boundary) x u (t) > x s (t). For weak noise D, there is a small probability that a particle obeying (1) escapes from the basin of attraction A(t) := (−∞, x u (t)] of x s (t) and disappears towards infinity. For an ensemble of part...

We study the influence of laser radiation on the electron transport through a molecular wire weakly coupled to two leads. In the absence of a generalized parity symmetry, the molecule rectifies the laser induced current, resulting in directed electron transport without any applied voltage. We consider two generic ways of dynamical symmetry breaking: mixing of different harmonics of the laser field and molecules consisting of asymmetric groups. For the evaluation of the nonlinear current, a numerically efficient formalism is derived which is based upon the Floquet solutions of the driven molecule. This permits a treatment in the non-adiabatic regime and beyond linear response.Comment: 12 pages, 10 figures, REVTeX