First-principles study of structural, electronic and thermodynamic properties of (ZnO)n_n(n=2-16) clusters

The structural, electronic, and vibrational thermodynamic properties of the (ZnO)$_n$ (n=2-16) clusters are studied using density functional - full potential computations. The results show, small clusters up to $n=9$ stabilize in the 2D ring shape geometries while the larger clusters prefer the 3D cage like structures. The ring to cage structural cross over in ZnO clusters is studied by investigating the behavior of the Zn-O-Zn bond angle, the Zn-O bond strength, and the number of bonds in the systems. It is argued that 12 is the lowest magic number of ZnO clusters at ground state, while finite temperature vibrational excitations enhance the relative stability of the (ZnO)$_9$ cluster and make it a magic system at temperatures above about 170 K. The obtained electronic structure of ZnO clusters before and after applying the many-body GW corrections evidence a size induced red shift originated from the ring to cage structural cross over in these systems. The behavior of the extremal points of electron density of the clusters along with the extrapolated cluster binding energies at very large sizes may be evidences for existence of a metastable structure for large ZnO nanostructures, different with the bulk ZnO structure.Comment: 8 pages, 8 figures and 1 tabl

A computerized system for beam measurements and control at the KFUPM 350 kV ion accelerator

In this paper the computerized measurement and control system which is recently developed for the 350 kV ion accelerator at KFUPM is described. Waveforms from a beam profile monitor are captured by the facility's VAX 11/785 data acquisition and analysis computer. Derived beam parameters allow accurate measurement of beam stability and more precise beam focusing and steering compared to schemes relying on dc current measurement at a Faraday cup. Measurement and control processes run simultaneously with experimental data acquisition. Hardware and software aspects are described and effects of simultaneous acquisition of experimental and waveform data are discussed. Results of beam stability runs are presented together with measured transfer function characteristics of focusing and steering elements

Numerical investigations of optically induced Gunn oscillations in MESFET-like structures

The availability of an optically controlled integrated microwave source that is compatible with MMIC technology is highly desirable. In this paper, a time-domain energy-based transport model is used for the investigation of the generation of sustained Gunn domains inside a MESFET structure in response to incident light. The time-domain investigations include the effects of optical flux density and biasing conditions on the creation and properties of the induced oscillations

A computerized system for beam measurements and control at the KFUPM 350 kV ion accelerator

In this paper the computerized measurement and control system which is recently developed for the 350 kV ion accelerator at KFUPM is described. Waveforms from a beam profile monitor are captured by the facility's VAX 11/785 data acquisition and analysis computer. Derived beam parameters allow accurate measurement of beam stability and more precise beam focusing and steering compared to schemes relying on dc current measurement at a Faraday cup. Measurement and control processes run simultaneously with experimental data acquisition. Hardware and software aspects are described and effects of simultaneous acquisition of experimental and waveform data are discussed. Results of beam stability runs are presented together with measured transfer function characteristics of focusing and steering elements

Numerical investigations of optically induced Gunn oscillations in MESFET-like structures

The availability of an optically controlled integrated microwave source that is compatible with MMIC technology is highly desirable. In this paper, a time-domain energy-based transport model is used for the investigation of the generation of sustained Gunn domains inside a MESFET structure in response to incident light. The time-domain investigations include the effects of optical flux density and biasing conditions on the creation and properties of the induced oscillations

A time-domain algorithm for the analysis of second-harmonicgeneration in nonlinear optical structures

A time-domain simulator of integrated optical structures containing second-order nonlinearities is presented. The simulation algorithm is based on nonlinear wave equations representing the propagating fields and is solved using the finite-difference time-domain method. The simulation results for a continuous-wave operation are compared with beam propagation method simulations showing excellent agreement for the particular examples considered. Because the proposed algorithm does not suffer from the inaccuracies associated with the paraxial approximation, it should find application in a wide range of device structures and in the analysis of short-pulse propagation in second-order nonlinear device

Electromagnetic wave effects on microwave transistors using afull-wave time-domain model

A detailed full-wave time-domain simulation model for the analysis of electromagnetic effects on the behavior of the submicrometer-gate field-effect transistor (FET's) is presented. The full wave simulation model couples a three-dimensional (3-D) time-domain solution of Maxwell's equations to the active device model. The active device model is based on the moments of the Boltzmann's transport equation obtained by integration over the momentum space. The coupling between the two models is established by using fields obtained from the solution of Maxwell's equations in the active device model to calculate the current densities inside the device. These current densities are used to update the electric and magnetic fields. Numerical results are generated using the coupled model to investigate the effects of electron-wave interaction on the behavior of microwave FET's. The results show that the voltage gain increases along the device width. While the amplitude of the input-voltage wave decays along the device width, due to the electromagnetic energy loss to the conducting electrons, the amplitude of the output-voltage wave increases as more and more energy is transferred from the electrons to the propagating wave along the device width. The simulation confirms that there is an optimum device width for highest voltage gain for a given device structure. Fourier analysis is performed on the device output characteristics to obtain the gain-frequency and phase-frequency dependencies. The analysis shows a nonlinear energy build-up and wave dispersion at higher frequencie

An explicit finite-difference scheme for wave propagation in nonlinear optical structures

In this paper, we present an algorithm that solves a time-domain nonlinear coupled system arising in nonlinear optics. The algorithm is an explicit nonlinear finite-difference method (NFDM) based on the exact solution of the nonlinear discrete equations. It enables simulations that preserve the characteristics of nonlinearity as well as coupling, and can be extended to arbitrary input waveform conditions