JOURNAL OF THEORETICAL AND APPLIED PHYSICS (IRANIAN PHYSICAL JOURNAL)
Year:2013 | Volume:7 | Issue:7|Papers Count:17

Oblique propagating electromagnetic whistler mode instability has been studied having a k vector at angle to ambient magnetic field for bi-Maxwellian distribution function with parallel AC electric field in the magnetosphere of Uranus. The dispersion relation and growth rate have been calculated for plasma parameters suited to the magnetosphere of Uranus by using the method of characteristic solutions and kinetic approach. The effect of AC electric field and temperature anisotropy for oblique propagating electromagnetic whistler mode instability has been reported and discussed in the results and discussion section. The comparisons have been made with observations made by Voyager 2. It is found that whistler mode emissions observed at lower radial distances cannot reproduce in our calculations. The present studies should be helpful in making estimates on the scattering properties of whistler mode waves and, thus, contribute to a better understanding of auroral activity in the planetary atmosphere.

Copper thin films of 80-nm thickness were deposited on glass substrate using electron beam deposition at two different deposition angles of 0o and 40o, and they were post-annealed under flow of nitrogen at different temperatures. The structure of the films was analyzed using X-ray diffraction, atomic force microscope, and scanning electron microscope. Investigation on the copper nitride phase formation showed that this phase was not formed in the samples produced at 0o, while those prepared at oblique angle of 40° clearly showed the formation of copper azide phase. This is related to the porosity of the film structure, hence increased surface area for the reaction of nitrogen with copper atoms. Therefore, this is a simple method for preparation of copper nitride films that are not usually formed due to low reactivity of copper (as transition metal) with nitrogen. The results showed that the crystallite size (coherently diffracting domains), grain size, and surface roughness increase with annealing temperature.

The structural, electronic, and magnetic properties of Co2CrGe, a Heusler alloy, have been evaluated by firstprinciples density functional theory and compared with the known experimental and theoretical results. Generalized gradient approximation is used for structural study, whereas local spin density approximation is used for electronic calculation. First-principles structure optimizations were done through total energy calculations at 0 K using the full-potential linearized augmented plane wave method as implemented in the WIEN2K code.

In this paper, we have presented a power law scaling Kaluza-Klein cosmological model dominated by two interacting perfect fluid components during expansion. Barotropic equations of state for pressure and density are considered to get determinate solutions of the field equations. We have shown that the components are not conserved separately because of mutual interaction between the two fluids, and the energy densities are proportional to 1/t2.

Influence of mismatch on quantum dot formation during heteroepitaxial growth of thin films with zinc blende structure on GaAs substrate is investigated. A kinetic Monte Carlo model is used for simulation of thin films with different values of lattice mismatch in the range of 4% to 14%. Simulation is performed at a substrate temperature of 700 K and deposition rate of 0.3 ML/s. Also, ‘reflection high-energy electron diffraction’ (RHEED) intensity of simulated thin films is evaluated during growth. Results of simulation show that at constant temperature and deposition rate, quantum dot size decreases with increasing lattice mismatch, and their distribution function becomes sharp. Simulated RHEED oscillations have a good agreement with morphology of simulated thin films, and results show that growth mode changed from Stranski-Krastanov to Volmer Webber by increasing lattice mismatch.

The influence of a hot filament on the electrical breakdown characteristics is studied for different ratios of argon and nitrogen gases for a wide range of pressure. The vacuum tube consists of two parallel plane stainless steels used as cathode and anode accompanied with a tungsten filament located behind the cathode. Paschen’s curves are obtained for different ratios of argon and nitrogen as a function of pressure for various electric currents of the hot filament. The first and second Townsend coefficients as well as the ionization efficiency and secondary ionization coefficient are obtained for different filament currents. In addition, the influences of the nitrogen partial pressure on the forgoing parameters are obtained. It is shown that, increase of the filament current causes the decrease of the electrical breakdown voltage which is more pronounced in low pressures. Furthermore, introducing the nitrogen gas leads to the increase of the breakdown voltage and decrease of the ionization efficiency as well as the first and second Townsend coefficients. Moreover, it is concluded that, in the middle range of pressure, the presence of the hot filament results to the electrical breakdown which reveals the linear features.

New sets of molecules containing tri-phenyl-amine (TPA) core and thiophene unit with amide and imine functional groups are designed, synthesized, characterized, and compared. These are solution processable small molecules with high mobility. The newly designed molecules have better solubility due to the C=N (imine) and CONH2 (amide) moiety as compared to the established molecules with CH=CH (methine) for optoelectronic applications. They have an optimal energy band gap, which indicates their potential utility in a variety of optoelectronic applications. These molecules also show efficient intermolecular charge transfer mechanisms similar to conventional organic semiconducting molecules as evidenced by optical measurements. Density functional theory simulation results show that the localization of the frontier highest occupied molecular orbital is around the TPA core for molecules coupled with imine and amide, and is reasonably stable.

In the present paper, we have calculated diffusion coefficient, viscosity coefficient, and surface tension of liquid metals near melting point (Li, Na, K, Rb, Cs, Mg, Al, Pb, and Bi). We have applied our newly constructed model potential to describe electron ion interaction with one component plasma reference system. We have also investigated the effect of different correction functions such as those of Hartree, Hubbard and Sham, Vashista and Singwi, Taylor, Ichimaru and Utsumi, Farid et al., and Sarkar et al. on the above-said properties. It is observed that the present results are found to be in good agreement with those of experimental data as well as with other theoretical results.

The conformally invariant linearized de Sitter gravitational wave equation has recently received a lot of attention. In this article, using the ambient space notations, we solve this field equation in five various cases. In each case, it has been shown that the solution can be written as the product of a generalized symmetric polarization tensor of rank 2 and a massless minimally or conformally coupled scalar field in de Sitter space-time. The conformally covariant graviton two-point functions have been calculated in terms of the massless minimally or conformally coupled scalar two-point functions, using ambient space notations. In the case of massless minimally coupled scalar field, the Krien space quantization has been used to avoid the violation of de Sitter invariance. The two-point functions are expressed in terms of de Sitter intrinsic coordinates from their ambient space counterparts, which are free of any theoretical problem.

Several semiempirical methods were utilized to analyze the structures and stabilities of X-(H2O)n (X=F, Cl; n=1-4) clusters with respect to the number of water molecules through their comparison with ab initio molecular orbital calculations. Our results show that the recently developed PM6-DH+semiempirical method can provide reasonable binding energies of hydrated fluoride and chloride ion clusters, which are consistent with the corresponding experimental results. For the optimized geometries of X=F, however, the semiempirical methods show that the global minima are close to HF (OH)-(H2O)n-1 structures, which are different from the ab initio calculations.Meanwhile, the topological characteristics for the global minima of X=Cl obtained by semiempirical methods have the same symmetries with ab initio calculations. All calculation levels agree on the trend of decreasing ion-water interaction with the increasing number of water molecules. We also found a new structure of Cl-(H2O)4 with a second hydration shell as a complement of previous studies. Those are very important data for our near-future study of on-the-fly semiempirical molecular dynamics (MD) or path integral MD simulation.

The present paper is concerned with the free vibration analysis of double-walled carbon nanotubes embedded in an elastic medium and based on Eringen’s nonlocal elasticity theory. The effects of the transverse shear deformation and rotary inertia are included according to the Timoshenko beam theory. The governing equations of motion which are coupled with each other via the van der Waals interlayer forces have been derived using Hamilton’s principle. The thermal effect is also incorporated into the formulation. Using the statically exact beam element with displacement fields based on the first order shear deformation theory, the finite element method is employed to discretize the coupled governing equations which are then solved to find the natural frequencies. The effects of the small scale parameter, boundary conditions, thermal effect, changes in material constant of the surrounding elastic medium, and geometric parameters on the vibration characteristics are investigated. Furthermore, our analysis includes nonlocal double-walled carbon nanotubes with different boundary conditions between inner and outer tubes which seem to be scarcely considered in the literature, and the corresponding given results for this case can be considered as a benchmark for further studies. Comparison of the present numerical results with those from the open literature shows an excellent agreement.

Shape invariance is an important factor of many exactly solvable quantum mechanics. Several examples of shape-invariant’discrete quantum mechanical systems’ are introduced and discussed in some detail. We present the spectral properties of supersymmetric shape-invariant potentials (SIP). Here we are interested in some time-independent integrable systems which are exactly solvable owing to the existence of supersymmetric shape-invariant symmetry. In 1981 Witten proposed (0+1) -dimensional limit of supersymmetry (SUSY) quantum field theory, where the supercharges of SUSY quantum mechanics generate transformation between two orthogonal eigenstates of a given Hamiltonian wit degenerate eigenvaluesfor the non-SIP as very few lower eigenvalues can be known analytically, which are small to calculate spectral fluctuation.

In this paper, we consider the nonlinear boundary value problem for the electrohydrodynamic (EHD) flow of a fluid in an ion-drag configuration in a circular cylindrical conduit. This phenomenon is governed by a nonlinear second-order differential equation. The degree of nonlinearity is determined by a nondimensional parameter a. We present two semi-analytic algorithms to solve the EHD flow equation for various values of relevant parameters based on optimal homotopy asymptotic method (OHAM) and optimal homotopy analysis method. In 1999, Paullet has shown that for large a, the solutions are qualitatively different from those calculated by Mckee in 1997. Both of our solutions obtained by OHAM and optimal homotopy analysis method are qualitatively similar with Paullet’s solutions.

Radiofrequency (RF) plasma polymerization is a convenient thin film deposition process as it facilitates the synthesis of polymer films with stable physico-chemical properties suitable for various applications in microelectronic, optical, and biomedical fields. The unique properties of these plasma polymerized films as compared to the conventional ones are strongly related to the proper adjustment of the external plasma discharge parameters and selection of suitable monomer. It is also important to study the fundamental chemistry of RF plasma polymerization process, so that one can successfully correlate the internal features of the discharge with the film properties and explore their possible technological applications. The possibility of using styrene-based plasma polymer (SPP) films on bell metal as protective coatings is explored in this work. Depositions of the films are carried out in RF Ar/styrene discharge at working pressure of 1.2´10-1 mbar and at the RF power range of 20 to 110 W. Optical emission spectroscopy (OES) is used to study the active species generated during plasma polymerization, while Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) are used to analyze the internal chemical structures of the films. The protective performances of the SPP films are attempted to correlate with the results obtained from OES, FT-IR, and XPS analyses.

This paper deals with the determination of maximum beam deflection using homotopy perturbation method (HPM) and finite difference method (FDM). By providing some examples, we compare the results with exact solutions and conclude that HPM is more accurate, more stable and effective and can therefore be found widely applicable in structure engineering.

In this work, we have investigated one-sided and two-sided fully saturated hydrocarbons with C: H formula derived from a single sheet of graphene, which are called table-like and chair-like graphanes. The effects of full hydrogenation of graphene sheet, graphane, were studied using generalized gradient approximation of the density functional theory and the pseudopotential method. Total energy calculations show that chair-like conformer of graphane is more favorable than table-like one. A change of hybridization from sp2 to sp3 for chair-like conformer of graphane has been achieved. After full geometric relaxation of graphene and two phases of graphane, we obtain the adsorption geometry, adsorption energies, charge transfer, energy gap, density of states, quantum conductivity, and current for graphene and graphane. We show that the band gap induced by hydrogenation of graphene sheet in table-like conformer can greatly improve the electrical characteristics of graphene-based field effect transistors and its on/off ratio. We identify that graphene is acting as an acceptor after saturation with hydrogen.

The effect of MgO doping on the structural, magnetic, and magnetotransport properties of La0.8Sr0.2MnO3 (LSMO) / xMgO has been investigated. All samples were prepared by a solid-state reaction method. Alternating current susceptibility measurements for LSMO/xMgO samples show that the Curie transition temperature (Tc) and magnetization decrease with the increase of MgO concentration. The rate of the decrease of Tc at higher doping level is very fast. Also, samples with low doping level (x£2) show insulator-metal transition, but the transition temperature decreases and resistivity increases with the amount of MgO. It is observed that there is no insulator-metal transition at higher MgO doping level (x³3). Also, the results show that the value of low-field magnetoresistance decreases with the increase of MgO doping level. It seems that due to the higher sintering temperature and almost the same ionic radii of Mg2+and Mn3+, Mg2+mostly replaced Mn3+and weakened the double-exchange interaction, and consequently, Tc and magnetoresistance decrease and resistivity increases with MgO doping.