First-principles investigation of transient current of molecular devices by using complex absorbing potential

Based on the non-equilibrium Green's function (NEGF) coupled with density function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an efficient formalism to calculate the transient current of molecular devices under a step-like pulse from first principles. By combining NEGF-DFT with the complex absorbing potential (CAP), the computational complexity of our formalism (NEGF-DFT-CAP) is proportional to $\emph{O}(N)$ where $N$ is the number of time steps in the time-dependent transient calculation. Compared with state-of-the-art algorithm of first principles time-dependent calculation that scales with at least $N^2$, this order N technique drastically reduces the computational burden making it possible to tackle realistic molecular devices. To ensure the accuracy of our method, we carry out the benchmark calculation compared with exact NEGF-TDDFT formalism and they agree well with each other. As an illustration, we investigate the transient current of molecular device Al-C$_3$-Al from first principles

Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530

We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (about 2-4 Myr-old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 years, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, a velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is -- at least partially -- due to the way in which star formation has proceeded in these complex star-forming regions (``primordial'' mass segregation). Based on arguments related to the clusters' published initial mass functions, in conjunction with our new measurements of their internal velocity dispersions (35 and 8 km/s for NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of the dissolution of NGC 2244 on very short time-scales, while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation. However, this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic Center.Comment: 36 pages, 10 figures, accepted to A

Hypothesis testing of multiple inequalities: the method of constraint chaining

Econometric inequality hypotheses arise in diverse ways. Examples include concavity restrictions on technological and behavioural functions, monotonicity and dominance relations, one-sided constraints on conditional moments in GMM estimation, bounds on parameters which are only partially identified, and orderings of predictive performance measures for competing models. In this paper we set forth four key properties which tests of multiple inequality constraints should ideally satisfy. These are (1) (asymptotic) exactness, (2) (asymptotic)similarity on the boundary, (3) absence of nuisance parameters from the asymptotic null distribution of the test statistic, (4) low computational complexity and boostrapping cost. We observe that the predominant tests currently used in econometrics do not appear to enjoy all these properties simultaneously. We therefore ask the question : Does there exist any nontrivial test which, as a mathematical fact, satisfies the first three properties and, by any reasonable measure, satisfies the fourth ? Remarkably the answer is affirmative. The paper demonstrates this constructively. We introduce a method of test construction called chaining which begins by writing multiple inequalities as a single equality using zero-one indicator functions. We then smooth the indicator functions. The approximate equality thus obtained is the basis of a well-behaved test. This test may be considered as the baseline of a wider class of tests. A full asymptotic theory is provided for the baseline. Simulation results show that the finite-sample performance of the test matches the theory quite well

Microwave method for high-frequency properties of graphene

Graphene is a remarkable material, which is yet to make the transition from unique laboratory phenomenon to useful industrial material. One missing element in the development process is a quick method of quality control of the electrical properties of graphene which may be applied in, or close to, the graphene growth process on an industrial scale. In this study, the authors describe a non-contact method using microwave resonance which potentially solves this problem. They describe the technique, consider its limitations and accuracy and suggest how the method may have future take up.UK NMS Programme, the EU EMRP project ‘GraphOhm’ and ‘MetNEMS’. The EMRP (European Metrology Research Programme

Structural control by the use of piezoelectric active members

Large Space Structures (LSS) exhibit characteristics which make the LSS control problem different form other control problems. LSS will most likely exhibit low frequency, densely spaced and lightly damped modes. In theory, the number of these modes is infinite. Because these structures are flexible, Vibration Suppression (VS) is an important aspect of LSS operation. In terms of VS, the control actuators should be as low mass as possible, have infinite bandwidth, and be electrically powered. It is proposed that actuators be built into the structure as dual purpose structural elements. A piezoelectric active member is proposed for the control of LSS. Such a device would consist of a piezoelectric actuator and sensor for measuring strain, and screwjack actuator in series for use in quasi-static shape control. An experiment simulates an active member using piezoelectric ceramic thin sheet material on a thin, uniform cantilever beam. The feasibility of using the piezoelectric materials for VS on LSS was demonstrated. Positive positive feedback as a VS control strategy was implemented. Multi-mode VS was achieved with dramatic reduction in dynamic response

Summary of theoretical and experimental investigation of grating type, silicon photovoltaic cells

Theoretical and experimental aspects are summarized for single crystal, silicon photovoltaic devices made by forming a grating pattern of p/n junctions on the light receiving surface of the base crystal. Based on the general semiconductor equations, a mathematical description is presented for the photovoltaic properties of such grating-like structures in a two dimensional form. The resulting second order elliptical equation is solved by computer modeling to give solutions for various, reasonable, initial values of bulk resistivity, excess carrier concentration, and surface recombination velocity. The validity of the computer model is established by comparison with p/n devices produced by alloying an aluminum grating pattern into the surface of n-type silicon wafers. Current voltage characteristics and spectral response curves are presented for cells of this type constructed on wafers of different resistivities and orientations

Control of Spin Dynamics of Excitons in Nanodots for Quantum Operations

This work presents a step furthering a new perspective of proactive control of the spin-exciton dynamics in the quantum limit. Laser manipulation of spin-polarized optical excitations in a semiconductor nanodot is used to control the spin dynamics of two interacting excitons. Shaping of femtosecond laser pulses keeps the quantum operation within the decoherence time. Computation of the fidelity of the operations and application to the complete solution of a basic quantum computing algorithm demonstrate in theory the feasibility of quantum control.Comment: 5 pages, 4 figure