Geri Gopir

Compact multiband VHF antenna for transient radio telescope

Development of antenna for a radio telescope system has become a fascinating research area in the field of radio astronomy. In this paper, a design of compact multiband VHF antenna for a transient radio telescope system is proposed. The multiband has been achieved by combining V-shape half-wavelength dipole, traps and parasitic elements. Return losses of about −12.45 dB, −25.15 dB and −29.42 dB are obtained at three different frequencies, where the maximum gains are achieved at zenith direction for all operating frequencies. The proposed antenna is suitable to be used in a transient radio telescope which is operated in urban centers.

Development of a Multiband VHF Antenna for Low-Frequency Transient Radio Telescope

Low-frequency radio astronomy has become a fascinating research area in the field of astronomy, including on its instrumentation development. In this paper, a multiband VHF antenna for low-frequency transient radio telescope is developed. The multiband has been achieved by combining V-shape half-wavelength dipole, traps, and parasitic elements. Measurement result shows that the antenna can work properly in three different frequencies designated by International Telecommunication Union (ITU) for radio astronomical purposes.

The Hurst exponents of the geomagnetic horizontal component during quiet and active periods

The focus in this paper is mainly to characterize the fractal properties denoted by the Hurst exponent of geomagnetic field during quiet and active periods of geomagnetism activity. Analysis is made on the time series of the geomagnetic horizontal component data, H, which were acquired by the Magnetic Data Acquisition System (MAGDAS) developed by the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet and active periods in August 2005 at the equatorial stations of Cebu and Davao in the Philippines. This month had very intense geomagnetic storm with the Dstmin, Kp and Ap indices for the quietest and active days of the month are −20 nT, 2, 6 and −216 nT, 9, 110 respectively. For both stations, the data sampling interval is 1 second for each time periods or one day giving a sample size of 86,400. Using the fast Fourier transform, the power law of the data time series was obtained in the shorter time periods of 10 minutes to 6 hours. This observed power spectrum law exhibit scaling with corresponding Hurst exponents of 0.3–0.5 for quiet periods and 0.5–0.7 for active periods for both stations. For comparison, other fractal techniques were performed which are rescaled range analysis (RS) and detrended fluctuation analysis (DFA) on the data set, and the same ranges of the Hurst exponents were obtained. The work also includes simulation of the similar sized data sets by generating the fractional Brownian motions (FBM) with the Hurst exponents of 0.3–0.7. Our results show that the H component at these equatorial stations is a fractal in nature and become more persistent during active periods of geomagnetism activity. Thus, the Hurst exponent could be used to characterize the geomagnetic time series during quiet and active periods.

Effect of Parasitic Element on 408 MHz Antenna for Radio Astronomy Application

Antenna is one of the important subsystem components in a radio telescope system. In this paper, analysis on the effect of parasitic element on 408 MHz antenna in a radio telescope system is presented. Higher gain up to 10.24 dBi with reduction on beamwidth size has been achieved by optimizing the position of parasitic element relative to the driven element. The proposed antenna is suitable to be utilized in a transient radio telescope array.

Electronic and optical properties of small hydrogenated silicon quantum dots using time-dependent density functional theory

This paper presents a systematic study of the absorption spectrum of various sizes of small hydrogenated silicon quantum dots of quasi-spherical symmetry using the time-dependent density functional theory (TDDFT). In this study, real-time and real-space implementation of TDDFT involving full propagation of the time-dependent Kohn-Sham equations were used. The experimental results for SiH4 and Si5H15 showed good agreement with other earlier calculations and experimental data. Then these calculations were extended to study larger hydrogenated silicon quantum dots with diameter up to 1.6 nm. It was found that, for small Quantum dots, the absorption spectrums atomic-like while, for relatively larger (1.6 nm) structure, it shows bulk-like behavior with Continuous plateau with noticeable peak. This paper also studied the absorption coefficient of silicon quantum dots as a function of their size. Precisely, the dependence of dot size on the absorption threshold is elucidated. It was found that the silicon quantum dots exhibit direct transition of electron from HOMO to LUMO states; hence this theoretical contribution can be very valuable in discerning the microscopic processes for the future realization of optoelectronic devices.

Surface Passivation Effect of Hydrogen and Methyl on the Structural and Electronic Properties of Silicon Quantum Dots: Density Functional Calculation

We have carried out a series of DFT calculations to investigate changes on the structural and electronic properties of Silicon (Si) quantum dots as a function of surface passivation. In particular, we have study non-polar passivation effect of hydrogen (H) and methyl (CH3) at the surface of quantum dots. From geometry optimization result, we find that clusters with reconstructed surfaces a complete methyl passivation is possible and steric repulsion prevents full passivation of Si dots with unreconstructed surfaces. On the electronic properties point of view, it is noticed for small nanocrystals, the presence of mini-gaps are more pronounced which can limit the non-radiative relaxation of excitons. Obviously, methyl passivation weakly affects the band gap values of silicon quantum dots, while it substantially decreases the band gap and reduce mini-gap appearance compared to hydrogen passivation Si QDs. On the basis of our results we propose that methyl terminated quantum dots may be size selected taking advantage of the reduction on mini-gap and the localization of electron as a function of the cluster size.

Strain modification of band edge energies in a pyramidal InAs-GaAs quantum dot system

We present the calculated band edge energies altered by strain in a nanostructure system of a pyramidal InAs quantum dot buried in a GaAs substrate. Our zinc-blende supercell system of dimension 11.9 nm × 11.9 nm × 8.5 nm and 55119 atoms contains a pyramidal In770As886 quantum dot of 1656 atoms with height of 3.03 nm and square base of length 6.06 nm. The strain energy of this nanostructure system is minimized by employing the Keating formulation of interatomic potential and Monte Carlo relaxation method via the Metropolis algorithm. This relaxation is run for 20 million Monte Carlo steps at simulation temperature of 4.2 K. The calculated strain is then used to determine the conduction and valence band edge energies of the nanostructure. We find that along the [001] growth direction in the InAs quantum dot region, strain increases the conduction band edge energy by 0.6 eV and in the valence band strain results in relatively sharp wells at the dot base for heavy holes and at the dot tip for light holes. Thus, our calculation predicts that strain leads to increased band gap and spatial splitting of holes in this nanostructure system.

Scaling and Fractal Properties of the Horizontal Geomagnetic Field at the Tropical Stations of Langkawi and Davao in February 2007

We investigate the scaling and fractal properties of the horizontal component of the geomagnetic field time series acquired by the Magnetic Data Acquisition System (MAGDAS) of the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet period of geomagnetic activity in February 2007 at the near equatorial stations of Langkawi in Malaysia and Davao in the Philippines. These data were sampled every minute for 28 days giving a sample size of 40,320. The power spectra of the time series are obtained, showing dominant periodicities at 24, 12, 8 and 6 hours due to the effect of the sun on the geomagnetic field. The power spectra also indicate scaling with Hurst exponents of 0.50–0.77 in the period of 10 minutes to 6 hours. Then, rescaled range analysis and detrended fluctuation analysis are performed, producing similar ranges of Hurst exponents. Finally, these fractal methods are used to determine the Hurst exponents of similar sized data sets artificially generated...

Discontinuity mass of finite difference calculation in InAs-GaAs quantum dots

Recently, theoretical analysis of the electronic properties of quantum dot has attracted a great attention when modern nanotechnology has made it possible to fabricate a realistic quantum dots in laboratory [. Quantum dot structures which provide electron confinement in three dimensions can be grown by the so called self-assembly effect or Stranski-Krastanov growth mode. Particular interest attracts ordering effects in StranskiKrastanow growth which proceeds on a lattice-mismatched substrate via formation of essentially three-dimensional islands. This is especially true for the InAs-GaAs system where the lattice mismatch is high and the nucleation process is rapid. Although, quantum dots have being studied experimentally but large amount of numerical studies of electron confined states also have been developed to simulate electronic and optical properties in quantum dots. The single band effective mass is one of the formalism of envelope function which has been widely used to solve quantum dot systems. However, the effective mass m* is usually position dependent in semiconductor heterostrutures. Consequently, the concerning about the form of the boundary conditions to impose on different material interface arisen [3]. According to the present works [2, , the position dependent Hamiltonian is given by: . where m = m (r) is the position dependent effective mass of an electron in conduction band. The constant α, β, and γ is arbitrary set to satisfy α + β + γ = -1. Various approximations regarding the actual constant of α, β, and γ in position dependent effective mass have been observed, example Gora & William (by putting α = -1 and β = γ = 0), Zhu & Kroemer (α = γ = -1/2 and β = 0), and BenDaniel-Duke (α = γ = 0 and β = -1). Among them, β = 1 (known as the Ben DanielDuke Hamiltonian [) is most popular method for solving mass continuity problem on the classic Hamiltonian [. Extensively, these interface condition was been used to solved most of the heterostructure problem such as quantum dots [. However, there is a qualitative argument based upon the Ben DanielDuke choice violates the Heisenberg uncertainty principle and the issue of the correct effective-mass equation was further questioned by Pistol, M. E. which he claims that all the possible equations lead to the same interfacial conditions on the envelope function [. In this paper, we will investigate the effect of discontinuity mass within interface of two semiconductor materials inside InAs-GaAs quantum dot by using the classic constant mass Hamiltonian (CH), position dependent effective mass Hamiltonian (PDH) and Ben Daniel and Duke Hamiltonian (BDH). The most common analytic methods are solving the transcendental equation obtained by matching the interface boundary condition on the envelope function. But this kind of method will suffer from complexity of model quantum dots that contain multiple layer or geometry that unable to derive into analytic formulation. Thus, this study will focus on comparison between difference finite difference formalism to illustrate the mass discontinuity effect on the numerical solution.

Calculation of Confined Electron and Hole States in a Strained InAs-GaAs Pyramidal Quantum Dot System Based on Effective Mass Approximation

Computational studies on zero dimensional semiconductor structure have been centred on typically produced quantum dot of various geometries namely pyramidal and lens with lateral sizes ranging from 10 nm to 24 nm. In the case of an epitaxially grown quantum dot, strain plays another essential role apart from its size and shape in determining its electronic properties [. Among the most studied strained structures is the self-assembled InAs quantum dot capped by a GaAs matrix. A study by [ on InAs pyramidal quantum dot predicted no confined electron states for quantum dot with base lengths 6 nm and below. Nevertheless, a calculation by [3] based on atomistic psedudopotential predicted at most two confined states for both electron and hole in a self-assembled InAs-GaAs quantum pyramid system of base length 6.06 nm.