Optical properties of 4 A single-walled carbon nanotubes inside the zeolite channels studied from first principles calculations

The structural, electronic, and optical properties of 4 A single-walled carbon nanotubes (SWNTs) contained inside the zeolite channels have been studied based upon the density-functional theory in the local-density approximation (LDA). Our calculated results indicate that the relaxed geometrical structures for the smallest SWNTs in the zeolite channels are much different from those of the ideal isolated SWNTs, producing a great effect on their physical properties. It is found that all three kinds of 4 A SWNTs can possibly exist inside the Zeolite channels. Especially, as an example, we have also studied the coupling effect between the ALPO_4-5 zeolite and the tube (5,0) inside it, and found that the zeolite has real effects on the electronic structure and optical properties of the inside (5,0) tube.Comment: 9 pages, 6figure

Effect of flow forecasting quality on benefits of reservoir operation - a case study for the Geheyan reservoir (China)

This paper presents a methodology to determine the effect of flow forecasting quality on the benefits of reservoir operation. The benefits are calculated in terms of the electricity generated, and the quality of the flow forecasting is defined in terms of lead time and accuracy of the forecasts. In order to determine such an effect, an optimization model for reservoir operation was developed which consists of two sub-models: a long-term (monthly) and a short-term (daily) optimization sub-model. A methodology was developed to couple these two sub-models, so that both short-term benefits (time span in the order of the flow forecasting lead time) and long-term benefits (one year) were considered and balanced. Both sub-models use Discretized Dynamic Programming (DDP) as their optimization algorithms. The Geheyan reservoir on the Qingjiang River in China was taken as case study. Observed (from the 1997 hydrological year) and forecasted flow series were used to calculate the benefits. Forecasted flow series were created by adding noises to the observed series. Different magnitudes of noise reflected different levels of forecasting accuracies. The results reveal, first of all, a threshold lead time of 33 days, beyond which further extension of the forecasting lead time will not lead to a significant increase in benefits. Secondly, for lead times shorter than 33 days, a longer lead time will generally lead to a higher benefit. Thirdly, a perfect inflow forecasting with a lead time of 4 days will realize 87% of the theoretical maximum electricity generated in one year. Fourthly, for a certain lead time, more accurate forecasting leads to higher benefits. For inflow forecasting with a fixed lead time of 4 days and different forecasting accuracies, the benefits can increase by 5 to 9% compared to the actual operation results. It is concluded that the definition of the appropriate lead time will depend mainly on the physical conditions of the basin and on the characteristics of the reservoir. The derived threshold lead time (33 days) gives a theoretical upper limit for the extension of forecasting lead time. Criteria for the appropriate forecasting accuracy for a specific feasible lead-time should be defined from the benefit-accuracy relationship, starting from setting a preferred benefit level, in terms of percentage of the theoretical maximum. Inflow forecasting with a higher accuracy does not always increase the benefits, because these also depend on the operation strategies of the reservoir.\u

Elimination of negative differential conductance in an asymmetric molecular transistor by an ac-voltage

We analyze resonant tunneling subject to a non-adiabatic time-dependent bias-voltage through an asymmetric single molecular quantum dot with coupling between the electronic and vibrational degrees of freedom using a {\em Tien-Gordon-type} rate equation. Our results clearly exhibit the appearance of photon-assisted satellites in the current-voltage characteristics and the elimination of hot-phonon-induced negative differential conductance with increasing ac driving amplitude for an asymmetric system. This can be ascribed to an {\em ac-induced suppression} of unequilibrated (hot) phonons in an asymmetric system.Comment: Accepted by Appl. Phys. Let

Finite-frequency current (shot) noise in coherent resonant tunneling through a coupled-quantum-dot interferometer

We examine the shot noise spectrum properties of coherent resonant tunneling in coupled quantum dots in both series and parallel arrangements by means of quantum rate equations and MacDonald's formula. Our results show that, for a series-CQD with a relatively high dot-dot hopping $\Omega$, $\Omega/\Gamma\gtrsim 1$ ($\Gamma$ denotes the dot-lead tunnel-coupling strength), the noise spectrum exhibits a dip at the Rabi frequency, $2\Omega$, in the case of noninteracting electrons, but the dip is supplanted by a peak in the case of strong Coulomb repulsion; furthermore, it becomes a dip again for a completely symmetric parallel-CQD by tuning enclosed magnetic-flux.Comment: 8 pages, 5 figure