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Room-temperature single-electron transistors are realized within individual metallic single-wall carbon nanotube molecules. The devices feature a short (down to approximately 20 nanometers) nanotube section that is created by inducing local barriers into the tube with an atomic force microscope. Coulomb charging is observed at room temperature, with an addition energy of 120 millielectron volts, which substantially exceeds the thermal energy. At low temperatures, we resolve the quantum energy levels corresponding to the small island. We observe unconventional power-law dependencies in the measured transport properties for which we suggest a resonant tunneling Luttinger-liquid mechanism.

We investigate quantum Brownian motion in adiabatically rocked ratchet systems. Above a crossover temperature T c tunneling events are rare, yet they already substantially enhance the classical particle current. Below T c , quantum tunneling prevails and the classical predictions grossly underestimate the transport. Upon approaching T 0 the quantum current exhibits a tunneling induced reversal, and tends to a finite limit. [S0031-9007(97)03540-0] PACS numbers: 05.40. + j, 73.23.Ad, 85.25.Dq The quest of extracting usable work from fluctuations has provoked debates ever since the early days of Brownian motion theory [1]. Prima facie, periodic structures with broken spatial symmetry (ratchets) seem able to perform the job. Yet, already Smoluchowski and later Feynman [1] point out that an intriguing probabilistic balance prohibits the emergence of directed motion-in reconciliation with the second law of thermodynamics-if only equilibrium fluctuations are acting. As shown with the seminal studies [2,3], this situation changes drastically in the presence of additional unbiased nonthermal forces. Indeed, such classical nonequilibrium models entail a variety of interesting technological applications [3,4], and may be of relevance for intracellular transport as well [5]. The challenge here consists in the study of a quantum Brownian rectifier operating in a regime where tunneling and other quantum fluctuation effects become important for the transport properties. Our work opens the possibility of exploiting the ratchet mechanism in physical and biological systems in novel temperature regimes, predicting new qualitative effects such as a tunneling-induced current reversal. For example, a new type of superconducting quantum interference device has recently been proposed to investigate the ratchet mechanism [6]. At low temperature, our predictions can be observed in situ in these mesoscopic quantum structures. Moreover, using recent technical developments [7], semiconductor superlattices could be designed which, too, exhibit a quantum ratchet effect.To start out, we consider the quantum Brownian motion of a particle with mass m and viscous damping h, mẍ͑t͒ 2h ᠨ x͑t͒ 2 V 0 ͑ ͑ ͑x͑t͒͒ ͒ ͒ 1 f͑t͒ 1 j͑t͒ , (1) under the simultaneous action of thermal quantum fluctuations j͑t͒, and symmetric, unbiased, external driving forces f͑t͒, in an asymmetric, periodic "ratchet"-potential V ͑x͒ of period L, such as (cf. Fig. 1)(2) Equation (1) follows as the exact Heisenberg equation for the coordinate operator x͑t͒ from a system-plus-reservoir model with HamiltonianHere, H B ͑x, q͒ describes the heat bath interacting with the particle x͑t͒ and we adopt its usual modelization by an ensemble of harmonic oscillators q at thermal equilibrium with a coupling bilinear in the bath and particle coordinates [8]. By a suitable choice of the model parameters in H B ͑x, q͒ one recovers the quantum Langevin equation (1) with the operator valued quantum thermal noise j͑t͒ being self-adjoint, stationary, and Gaussian. With b 1͞k B T, k B Boltzmann's ...

Various theoretical methods address transport effects in quantum dots beyond single-electron tunneling while accounting for the strong interactions in such systems. In this paper we report a detailed comparison between three prominent approaches to quantum transport: the fourth-order Bloch-Redfield quantum master equation ͑BR͒, the real-time diagrammatic technique ͑RT͒, and the scattering rate approach based on the T-matrix ͑TM͒. Central to the BR and RT is the generalized master equation for the reduced density matrix. We demonstrate the exact equivalence of these two techniques. By accounting for coherences ͑nondiagonal elements of the density matrix͒ between nonsecular states, we show how contributions to the transport kernels can be grouped in a physically meaningful way. This not only significantly reduces the numerical cost of evaluating the kernels but also yields expressions similar to those obtained in the TM approach, allowing for a detailed comparison. However, in the TM approach an ad hoc regularization procedure is required to cure spurious divergences in the expressions for the transition rates in the stationary ͑zero-frequency͒ limit. We show that these problems derive from incomplete cancellation of reducible contributions and do not occur in the BR and RT techniques, resulting in well-behaved expressions in the latter two cases. Additionally, we show that a standard regularization procedure of the TM rates employed in the literature does not correctly reproduce the BR and RT expressions. All the results apply to general quantum dot models and we present explicit rules for the simplified calculation of the zero-frequency kernels. Although we focus on fourth-order perturbation theory only, the results and implications generalize to higher orders. We illustrate our findings for the single impurity Anderson model with finite Coulomb interaction in a magnetic field.

We study the dissipative dynamics of a two-level system (TLS) exposed to strong ac driving. By combing Floquet theory with Van Vleck perturbation theory in the TLS tunneling matrix element, we diagonalize the time-dependent Hamiltonian and provide corrections to the renormalized Rabi frequency of the TLS, which are valid for both a biased and unbiased TLS and go beyond the known high-frequency and rotating-wave results. In order to mimic environmental influences on the TLS, we couple the system weakly to a thermal bath and solve analytically the corresponding Floquet-Bloch-Redfield master equation. We give a closed expression for the relaxation and dephasing rates of the TLS and discuss their behavior under variation of the driving amplitude. Further, we examine the robustness of coherent destruction of tunneling (CDT) and driving-induced tunneling oscillations (DITO). We show that also for a moderate driving frequency an almost complete suppression of tunneling can be achieved for short times and demonstrate the sensitiveness of DITO to a change of the external parameters.