Current Issus

Volume 51, No 1

February 2013

Visitors since 2010/06/28

Accepted paper

• A Study of the Optical Properties of $$\alpha$$-Al$$_2$$O$$_3$$ Using Density Functional Theory

H. Salehi and H. Asareh

The optical properties in $$\alpha$$-Al$$_2$$O$$_3$$ crystal have been calculated by using the full potential linearized augmented plane wave (FP-LAPW) method in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA) by the WIEN2k package. The refraction index of $$\alpha$$-Al$$_2$$O$$_3$$ is found to be 1.64, which, with respect to its experimental value, has a very good agreement.

• Two-Dimensional Self-Similar Rotating Azimuthons in Strongly Nonlocal Nonlinear Media

Xian-Jing Lai, Mei-Zhen Jin, and Jie-Fang Zhang

We derive analytical 2D self-similar rotating azimuthons of a strongly nonlocal nonlinear media with a space-dependent diffractive, a gain (attenuation) coefficient based on the similarity transformation, and a variational approach. Remarkably, these self-similar azimuthons have the azimuthal angle modulated by the distributed diffractive coefficient, apart from the beam width and intensity changing self-similarly.

• The Entanglement Dynamics of Two Atoms in a Double Two-Photon Jaynes--Cummings Model

Qinghong Liao, Wenjie Nie, Nanrun Zhou, Ye Liu, and M. A. Ahmad

By employing the concurrence, we investigate the dynamics of entanglement between two atoms in a double two-photon Jaynes--Cummings model when the cavity field is previously in a maximally entangled state. The influence of the atomic coherence on the time evolution of the atom-atom entanglement is examined. It is shown that the phenomenon of the sudden birth of entanglement occurs and the atomic coherence decreases the degree of the atom-atom entanglement. Moreover, it is found that a maximally entangled two-atom state can be generated.

• Photoabsorption of H$$^-$$ and He via the Modified Atomic Orbital Theory: Application to the $$^1$$P$$^{\circ}$$ - Rydberg States

I. Sakho

We report in this paper the $$E_{Nn}$$ energy resonances of the doubly excited $$^1$$P$$^{\circ}$$ states of H$$^-$$ and He below the $$N=$$ 4$$-$$10 threshold of H and He$$^+$$ respectively, applying the modified orbital atomic theory in the framework of a semi-empirical procedure. The energy positions calculated are specified in terms of the supermultiplet classification scheme based on the $$_N (K,T)_n^A$$, usually new notation and the $$_N (nu)_n^A$$, notation in the framework of the rovibrator model for the autoionizing states considered. Comparison of the $$_N ‹ {r_{12}^{-1} } › _n$$ radial expectation values of H$$^-$$ and He indicates that, the angular quantum number $$K$$ is effectively related to $$-$$, with $$theta _{12}$$ the angle between the position vectors of the two electrons. In addition, comparison of the radial expectation values of H$$^-$$ and He shows that, for both intrashell and intershell $$_N (K,T)_n^A$$, $$^1$$P$$^{\circ}$$ states, it is the Coulomb force which causes the differences observed in the photoabsorption spectra of H$$^-$$ and He. All the results presented in this work compared well with available literature values.

• Effect of Crystal Defects on the Melting Temperature of Ni and Al

Fayyaz Hussain, Sardar Sikandar Hayat, Zulfiqar Ali Shah, and S. A. Ahmad

The molecular dynamics simulation technique has been applied to study the effects of temperature on Ni and Al having point and planar defects. For this purpose a well established program ‘dyn86’ containing the DYNAMO subroutine has been used. Semi-empirical potentials based on the Embedded Atom Method (EAM) have been employed to calculate the lattice parameter and energy per atom in order to determine the melting point. The effects of point defects including self, substitution, and interstitial on the melting point have been investigated. The twin formation energy of some low index (111), (112), (113), and (114) twin-interfaces and their effects on the melting point of Ni and Al are also studied. It is observed that the presence of defects (point or planar) lowers the melting point of metals, as compared to the simulated melting point of a defect-free crystal.

• Frequency Dependent Ferroelectric Properties of BaZrO$$_3$$ Modified Sr$$_{0.8}$$Bi$$_{2.2}$$Ta$$_{2}$$O$$_{9}$$ Thin Films

Mehmet S. Bozgeyik

Frequency dependent ferroelectric properties were investigated in BaZrO$$_3$$ (BZ) modified Sr$$_{0.8}$$Bi$$_{2.2}$$Ta$$_{2}$$O$$_{9}$$ (SBT) thin films deposited by sol-gel deposition. The polarization versus electric field (P-E) hysteresis loops of pure and doped SBT films were comparatively studied for frequencies ranging from 1 to 100 kHz. It was found that coercive field ($$2E_C$$) increases and remanent polarization ($$2P_r$$) decreases with increasing frequency. Such an increasing behavior of $$2E_C$$ is associated with resistance, since domain switching consists of domain wall motion through ferroelectric material. It was found that the frequency dependent coercive field relation obeys the Ishibashi power law. Also, it was noted that the $$2P_r$$ value decreases with a logarithmic function of frequency. These frequency dependent properties are attributed to oxygen vacancies and leakage current which are enhanced after BZ doping.

• Experimental Investigation of a Solar Cell's Characteristics in a Concentrating Photovoltaic System

W. B. Xia, X. D. He, J. T. Liu, Y. Q. Gao, and J. H. Duan

The dependence of the open-circuit voltage and internal resistance on the distance between the lens and solar cell is investigated in a concentrating photovoltaic system. The results show that the open-circuit voltage follows a Gaussian distribution function ($$f(x)$$) with that distance, and the internal resistance can be approximately described by $$1-f(x)$$. Through experimental data fitting, we find that the maximum open-circuit voltage and minimum internal resistance will occur when the light is focused into the depletion layer of the solar cell. The light intensity dependence of the open-circuit voltage and internal resistance provide an indirect proof for proving the above conclusions. This is of great guiding significance for the design and research of current photovoltaic systems.

• First-Principles Study on Structural, Electronic and Magnetic Properties of Cu-Fe, Cu-Co and Cu-Ni Linear and Zigzag Nanowires

Liang-Cai Ma and Hong-Sheng Zhao

By using first-principles calculations, we have systematically investigated the equilibrium structure, electronic and magnetic properties of free-standing Cu-Fe, Cu-Co and Cu-Ni bimetallic linear and zigzag nanowires, and comparison was carried out with those of corresponding monatomic chains. It is found that all the bimetallic linear and zigzag chains have stable ferromagnetic (FM) states and the total energy of all considered zigzag chains is lower than that of corresponding linear chains. The equilibrium bond lengths of the bimetallic Cu-Fe, Cu-Co and Cu-Ni nanowires lie in between the values of the corresponding monatomic systems. The magnetic moments in linear nanowires are generally larger than the ones of corresponding zigzag nanowires and the Fe, Co and Ni atoms in bimetallic nanowires have quite high local magnetic moments. The calculations suggest that there is hybridization between Cu-3d and Fe (Co or Ni) $$3d$$ states, which leads to the lower in cohesive energies of the bimetallic nanowires than those of the corresponding monatomic nanowires. The bimetallic Cu-Fe and Cu-Co nanowires also have high spin polarizations and may be good potential materials for spintronic devices.

• Pressure Dependence of Phonon Dispersion in Bcc Tungsten

Xiao-Jun Zhang and Chang-Le Chen

Combining the modified analytic embedded atom method with the theory of lattice dynamics, we reproduce the experimental results of the phonon dispersion in bcc metal tungsten at zero pressure along major symmetry directions [$$00\zeta$$], [0$$\zeta \zeta$$] and [$$\zeta \zeta \zeta$$] and then predict the phonon dispersion relations of tungsten under high pressures of 40, 50, 80 100 and 120 GPa basing on the EOS by Rose and Vinet. Our simulated results in high pressure are agreement with the finding of other bcc metals when the differences in masses, interatomic spacings, and melting temperatures of these metals are taken into account. It implies that the predicted phonon dispersion relations at different pressures are reasonable, showing that the modified analytic embedded atom method can describe the phonon dispersion relation of solids at zero pressure or in high pressure.

• Chaotic Oscillation on Two-Coupled Chemical Oscillators Under Time Delay with Asymmetric Condition

Minoru Yoshimoto, Namiko Morita, and Hiroyuki Shirahama

The synchronized modes of the two-coupled Oregonators are investigated. The numerical results indicate that the combination of the time delay and asymmetry in coupling between the independent oscillators generates the chaotic oscillations. This study also reveals the two types of the routes to chaos of anti-phase mode $$\to$$ chaos $$\to$$ anti-phase mode and in-phase mode $$\to$$ chaos $$\to$$ anti-phase mode. Furthermore, we propose the equation of one-dimensional map of the chaos observed in the two-coupled Oregonators.

• Order of Magnitude Enhancement in Axial Run-Down Velocity of Current Sheath and Focus Duration of a PF Device with Preionization

H. U .Khan

The effects of the Neon gas pressure and preionization on the current sheath dynamics in a low energy (3.3 kJ) Mather type plasma focus (PF) has investigated. The formation and dynamics of the current sheath is monitored by using Rogowski coil and high voltage probe. The experiment is performed to explore the evolution of the plasma sheath and to estimate the range of its velocity during the run-down phases. The Rogowski coil and high voltage probe signals reveals an axisymmetric parabolic current sheath which propagates down the co-axial tube and longer confinement time of it during collapse phase with preionization source.

• Fabrication and Characteristics of Ion-implanted 4H Silicon Carbide Metal-Semiconductor Field-Effect Transistors on P-Type Epilayer

Shou Guo Wang

The methods for design and fabrication of ion-implanted 4H Silicon Carbide (SiC) Metal-Semiconductor Field-Effect Transistors (MESFETs) have been studied. The N-well regions are formed by three-fold (Sample A) and four-fold (Sample B) nitrogen ion-implantation on Si-face P-type epilayer with net accept doping concentration of 6.5 $$\times$$ 10$$^{15}$$ cm$$^{-3}$$ grown at Cree. The layout with testing structures has been designed. The locations of peak concentration and the longitudinal straggles of implanted ions are simulated by Monte Carle simulator TRIM. The box-like profile for the implantation layer is calculated by Gauss expression. According to the structure of the device and the energy band for the implantation layer, theoretical equation of channel depth for ion-implantation technique is given and calculated. The processes to determine implantation energies and doses are discussed. The experiment for ion-implanted 4H-SiC MESFETs is performed. The Ohmic and Schottky contacts metals are annealed Ni/Cr alloy and Ti/Pt double-metal layers respectively, and the pad metallization is Au. The performance of current-voltage characteristics of two samples ion-implanted 4H-SiC MESFETs has been given lastly.

• Drag Force Experienced by a Body Moving through a Rarefied Gas

Bertúlio de Lima Bernardo, Fernando Moraes and Alexandre Rosas

Understanding how the drag force acts on a body is important in many areas of science and technology, and substantial efforts have been made to evaluate this force with high precision, particularly in the realm of fluid mechanics. In this paper we introduce a simple model based on kinetic theory that allows us to estimate the drag force on a body moving through a rarefied gas, where the assumptions of fluid mechanics no longer apply. Despite the simplicity of this model, the results agree quite well with the exact solutions. However, the simplicity of the model allows it to be used with undergraduate students in an introductory course.

• Magnetization Reversal through a Soliton in a Site-Dependent Weak Ferromagnet

L. Kavitha, M. Saravanan, P. Sathishkumar, and D. Gopi

Magnetization reversal or switching is the process by which the magnetization of a specimen is changed from one stable direction into another. Switching the magnetization of a magnetic bit through the flipping of a soliton offers the possibility of developing a new innovative approach for data storage technologies. The spin dynamics of a site-dependent ferromagnet with antisymmetric Dzyaloshinskii-Moriya interaction is governed by a generalized inhomogeneous higher order nonlinear Schrödinger equation. We demonstrate the magnetization reversal through the flipping of a soliton in the ferromagnetic medium by solving numerically the two coupled evolution equations for the velocity and amplitude of the soliton using the fourth order Runge-Kutta method. We propose a new approach to inducing the flipping behaviour of a soliton in the presence of an inhomogeneity by tuning the parameter associated with the Dzyaloshinskii-Moriya interaction, which causes the soliton to move with constant velocity and amplitude along the spin lattice.

• Simulation Studies on the Time Delay Effects in an Intracellular Calcium Oscillation System with Correlated Noises

Wei-Long Duan and Dong-Cheng Mei

The effects of multiple time delays on an intracellular calcium oscillation (ICO) system with correlated noises were investigated by means of stochastic simulation. Suppose the pumping of Ca$$^{2+}$$ into the calcium store takes time $$\tau_{1}$$, and the release of Ca$$^{2+}$$ from that store into the cytosol takes time $$\tau_{2}$$. The simulation results indicate that: (i) There is a critical value $$\tau_{c} \simeq$$ 0.1 s of $$\tau_{1}$$, i.e., as $$\tau_{1}<\tau_{c}$$, increasing $$\tau_{1}$$ can restrain ICO and convert the structure of the stationary probability distribution (SPD) of the intracellular Ca$$^{2+}$$ concentrations from two peaks to one peak, however, as $$\tau_{1}>\tau_{c}$$, increasing $$\tau_{1}$$ can convert the SPD to the contrary and transit Ca$$^{2+}$$ concentrations from desynchronous oscillation to periodically synchronous oscillation; (ii) Increasing $$\tau_{2}$$ converts the SPD from two peaks to one peak and restrains ICO; (iii) As $$\tau_{2}$$ increases the coherence resonance with respect to $$\tau_{1}$$ occurs in the system, but it is inhibited by increasing external noise.

• Characterization and Aggregation of Silver Nanoparticles Dispersed in Aqueous Solution

F. Y. Alzoubi and S. A. Abu Bidier

Colloidal solutions of silver nanoparticles were prepared by the chemical reduction method in which silver nitrate was used as a metal precursor and sodium citrate was used as a reducing as well as a stabilizing agent. UV-vis absorption spectra of the prepared colloids showed a surface plasmon resonance peak around 420 nm, indicating the formation of the nanoparticles. Size distribution analysis using a particle size analyzer showed that the average diameter of the synthesized nanoparticles is about 68 nm. This result was supported by the transmission electron microscope (TEM) images of the colloidal solutions. An X-ray diffraction (XRD) pattern was obtained to figure out the crystalline structure of the silver nanoparticles. The aggregation of silver nanoparticles was investigated using different chemicals mainly chlorides and hydroxides. These two categories of chemicals revealed different activity in aggregating silver nanoparticles.

• Optical Properties of Nd$$^{3+}$$/Ho$$^{3+}$$ Codoped Oxyfluoride Glass Ceramics

Atabaki Zohreh, Vaez-zadeh Mehdi, and Poorhashemi Aboozar

Oxy-fluoride glass-ceramics containing rare earths ions Nd$$^{3+}$$/Ho$$^{3+}$$ are fabricated. Heat treatment for two different times caused the precipitation of calcium-fluoride nanocrystals in which doped Nd$$^{3+}$$/Ho$$^{3+}$$ ions are concentrated. The scanning electron microscopy (SEM) images were obtained, and a uniform nanocrystal distribution was observed. The transmission electron microscopy (TEM) images of the best sample proved the existence of nanocrystals. Optical properties and the up and down-conversion processes among Nd$$^{3+}$$ ions in oxy-fluoride glass-ceramics have been investigated. The absorption intensities of the sample which was heat treated for 2 more hours were stronger. The up- and down-conversion emission spectra of the best sample under different excitation wavelengths have been explained.

• Phase Stability, Site Preference and Mechanical Properties of Th$$_2$$Co$$_6$$Al$$_{19}$$ and U$$_2$$Co$$_6$$Al$$_{19}$$

Ping Qian, Jiang Shen, and Nan-Xian Chen

An atomistic simulation of the phase stability and site preference of the Th$$_{2}$$Co$$_{6+x}$$Al$$_{19-x}$$ and U$$_{2}$$Co$$_{6+x}$$Al$$_{19-x}$$ compounds has been carried out using a series of interatomic pair potentials based on the lattice inversion method. Calculated results show that the lattice parameters are found in good agreement with the experimental values. The Co atoms substitute for Al without changing the crystal symmetry and preferentially occupy the 4$$g$$Al(1) site. Moreover, some simple mechanical properties, such as the elastic constants and bulk modulus, are investigated for the actinide compounds with complex structures. This may be regarded as an attempt to predict the structural and mechanical properties for the actinide materials with complex structure.

• First Principle Study of Electronic and Optical Properties of Magnesium based Chalcogenides

A. Sajid, A. Afaq, and G. Murtaza

The electronic and optical properties of MgS$$_{x}$$X$$_{1-x}$$ ($$X$$= Se, Te), and MgSe$$_{x}$$Te$$_{1-x}$$ (0 $$\le x \le$$ 1) ternary alloys are investigated using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT). The direct band gap energies vary in the ranges 2.854-3.864 eV, 2.470-3.864 eV, and 2.470-2.854 eV for MgS$$_{x}$$Se$$_{1-x}$$, MgS$$_{x}$$Te$$_{1-x}$$, and MgSe$$_{x}$$Te$$_{1-x}$$, respectively. These band gaps show a closer agreement with the experimental observations than the calculations by Hassan and Amrani [$${\it J. Phys. Condens. Matter}$$ $${\bf 19}$$, 386234 (2007)] using the same potential. Dielectric constants of the alloys are found to vary inversely with dopant concentrations as well as band gaps. Concentration dependent optical conductivity and reflectivity of the same alloys are also discussed.