Scielo RSS <![CDATA[Brazilian Journal of Physics]]> vol. 40 num. 3 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Bianchi type I tilted cosmological model for barotropic perfect fluid distribution with heat conduction in general relativity</b>]]> Bianchi Type I tilted cosmological model for barotropic perfect fluid distribution with heat conduction is investigated.To get the deterministic solution, we have assumed barotropic condition p = γ ∈ ,0 < γ < 1, p being isotropic pressure, ∈ the matter density and a supplementary condition between metric potentials A, B, C as A = (BC)n where n is the constant. To get the model in terms of cosmic time, we have also discussed some special cases. The physical aspects of the model are also discussed. <![CDATA[<b>Hot electron transport properties in characteristics of wurtzite GaN MESFETs using a five-valley model</b>]]> Ensemble Monte Carlo simulations have been carried out to investigate the effects of upper valleys on the characteristics of wurtzite GaN MESFETs. Electronic states within the conduction band valleys at the Γ1, U, M, Γ3 and K are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The following scattering mechanisims, i.e, impurity, polar optical phonon, acoustic phonon, alloy and piezoelectric are inculded in the calculation. Ionized imurity scattering has been treated beyound the Born approximation using the phase-shift analysis. The simulation results show that on the drain side of the gate region, hot electrons attained enough energy to be scattered into the upper satellite conduction valleys. Approximately %20 of the electrons occupy the higher valleys (mainly U and M valley). The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including upper valleys show much closer agreement with the available experimantal data. <![CDATA[<b>Effective cosmology a la Brans-Dicke with a non-minimally coupling massive inflaton field interacting with minimally coupling massless field</b>]]> We discuss an effective cosmology a la Brans-Dicke with two interacting scalar fields: a non-minimally coupling massive inflaton Higgs-like scalar field Φ interacting with a minimally coupling massless scalar field χ. Several features are observed and discussed in some details. <![CDATA[<b>Torsional vibration of carbon nanotubes under initial compression stress</b>]]> This study examines torsional vibration of Single-walled carbon nanotubes (SWCNTs) subjected to initial compression stresses. The nanotube structures are treated within the multilayer thin shell approximation with the elastic properties taken to be those of the graphene sheet. Simplified Flügge shell equations of motion are proposed as the governing equations of vibration for the carbon nanotubes. A new equation of motion and phase velocity of torsional waves propagating in carbon nanotubes (CNTs) subjected to initial compression stresses have been derived. The study reveals that the initial stresses present in the tube has a notable effect on the propagation of torsional waves. The results has been discussed and shown graphically. This investigation is very significant for potential application and design of nanoelectronics and nanodevices. <![CDATA[<b>Polarized Raman spectra of <i>L</i>-arginine hydrochloride monohydrated single crystal</b>]]> Polarized Raman spectra of L-arginine hydrochloride monohydrated single crystal in nine different scattering geometries of the two irreducible representations of factor group C2 were studied at room temperature. The experimental wavenumber values are compared with those obtained from ab-initio calculation and the assignment of the Raman bands to the respective molecular vibrations is also given. Finally, a discussion related to a previously reported phase transition undergone by L-arginine hydrochloride monohydrated single crystal at low temperature is furnished. <![CDATA[<b>Genetic transcriptional regulatory model driven by the time-correlated noises</b>]]> Steady state properties of a kinetic model of Smolen- Baxter- Byrne [P. Smolen, D. A. Baxter, J. H. Byrne, Amer. J. Physiol. Cell. Physiol. 274, 531 (1998)] are investigated in presence of two time- correlated noises. The steady state probability distribution can be obtained by solving the Fokker- Planck equation. It is found that both the correlated- time between the white noise and the colored noise and that between the colored noises can convert the bistability to monostability while the former activates the transcription and the latter suppresses it. <![CDATA[<b>Tuning luminescence of 3d transition- metal doped quantum particles: Ni<sup>+2</sup>: CdS and Fe<sup>+3</sup>: CdS</b>]]> The room- temperature photoluminescence of Cd1- xMxS (M=Ni, Fe) nanoparticles were investigated. Compared with the photoluminescence of CdS which peaks at 475 nm, the photoemission of CdS:Fe nanoparticles was peaking at 537 nm because of Fe acting as luminescent centers. On the other hand, the green emission (503 nm) of CdS:Ni attributed to the ¹T2g(D)→ ³A2g(F) raditive transition. With the increase of the Ni+2 concentration, photoluminescence intensity is increased while by Fe replacement with Cd ions, PL intensity is decreased. Relative to bulk crystals, due to the quantum confinement effect the band gap of CdS clusters is significantly blue- shifted with decreasing cluster size. CdS nanoclusters present a mixed hexagonal/cubic structure and with increasing doping concentration the peaks position of doped CdS shifts to higher angle. <![CDATA[<b>Identification of free radicals induced by gamma irradiation in amino acid derivatives</b>]]> Powders of N- acetyl- L- leucine and N- methyl- L- glutamic acid were gamma irradiated and the free radicals formed were investigated at room temperature by electron paramagnetic resonance technique. The free radicals formed in compounds were attributed to the (CH3)2CCH2CH(NHCOCH3)COOH and HOOCCH2CH2C (NHCH3)COOH radicals respectively. Both radicals are very stable and we could observe them for five months without almost intensity diminution at room temperature. The g values of the radicals and the hyperfine structure constants of the free electron with nearby protons and 14N nucleus were determined. The results were found to be in good agreement with the existing literature data. <![CDATA[<b>Final state interactions effects on kinetic energy sum spectra in nonmesonic weak decay</b>]]> We analyze the effect of final state interactions (FSI) on coincidence spectra in nonmesonic hypernuclear weak decay, ΛN → nN, as a function of the kinetic energy sum, EnN = En+ E N, both for np and nn events. Adopting a formalism recently developed, the effects of FSI originated from the interaction between the outgoing nucleons and those in the residual core are included analytically in a very simple way within the eikonal approximation through the modification of the emerging particles momenta. Numerical results are shown for 5ΛHe and 12ΛC hypernucleus. We found that coincidence spectra are only slightly modified and the disagreement between theory and experimental data still persists, mainly for nn events in 5ΛHe where enough statistics exists. We conclude that admixtures of excitations in the final state produced by FSI need to be added to our approach in order to improve the agreement with data. <![CDATA[<b>Energy momentum complex</b>]]> We show that the definition of the energy- momentum complex given by Møller using Weitzenböck space- time in the calculations of gravitational energy gives results which are different from those obtained from other definitions given in the framework of general relativity. <![CDATA[<b>Is it possible to accommodate massive photons in the framework of a gauge-invariant electrodynamics?</b>]]> The construction of an alternative electromagnetic theory that preserves Lorentz and gauge symmetries, is considered. We start off by building up Maxwell electrodynamics in (3+1)D from the assumption that the associated Lagrangian is a gauge- invariant functional that depends on the electron and photon fields and their first derivatives only. In this scenario, as well- known, it is not possible to set up a Lorentz invariant gauge theory containing a massive photon. We show nevertheless that there exist two radically different electrodynamics, namely, the Chern- Simons and the Podolsky formulations, in which this problem can be overcome. The former is only valid in odd space- time dimensions, while the latter requires the presence of higher- order derivatives of the gauge field in the Lagrangian. This theory, usually known as Podolsky electrodynamics, is simultaneously gauge and Lorentz invariant; in addition, it contains a massive photon. Therefore, a massive photon, unlike the popular belief, can be adequately accommodated within the context of a gauge- invariant electrodynamics. <![CDATA[<b>Measurement of the plasma boundary shift and approximation of the magnetic surfaces on the IR-T1 Tokamak</b>]]> In this research we measured the plasma boundary shift using array of magnetic pick- up coils on the IR- T1 tokamak. Also we approximated the magnetic surfaces by an equilibrium calculation. Firstly, four magnetic probes were designed, constructed, and installed on outer surface of the IR- T1 tokamak chamber and then plasma boundary displacement measured from them. On the other hand, magnetic surfaces approximated by equilibrium calculation of the Grad- Shafranov equation based on expansion of free functions as quadratic in flux function. <![CDATA[<b>Study of the parameters affecting ion beam emerging from cold conical cathode ion source</b>]]> The aim of this work is study of the parameters affecting ion beam emerging from cold conical cathode ion source. The input discharge and output ion beam characteristics have been measured at different pressures using nitrogen and argon gases. The optimum distance between the ion exit aperture of the cathode and the movable copper ion collector plate has been determined using nitrogen and argon gases. The ion collector plate has been placed at different distances from the ion exit aperture of the cathode equal to 2, 3, 4, 4.5, 5, 5.5 and 6 cm respectively. It is found that the optimum distance between the ion exit aperture of the cathode and the ion collector plate equals 5 cm for high output ion beam current. At this optimum distance, the efficiency of the ion source reaches 28.3% and 21.3% using nitrogen and argon gases respectively. The divergence angle of the ion beam exit from the cathode aperture has been determined for each distance by measuring the ion beam diameter which obtained on the ion collector plate. It is found that at the optimum distance between the ion exit aperture of the cathode and the ion collector plate, a minimum divergence angle of the ion beam emerging from the ion source equal to 1.14º and 2.29º using nitrogen and argon gases respectively. Also the aspect ratio of the ion source, the ratio between the radius of the ion exit aperture of the cathode to the distance between the ion exit aperture of the cathode and ion collector plate, has been determined. <![CDATA[<b>Effect of electron inertial delay on Debye sheath formation</b>]]> Present contribution deals with the role of weak but finite electron inertia on the sheath formation condition. As reported earlier this becomes effective when the ions' drift velocity exceeds the phase velocity of the acoustic wave fluctuations. Such situation has natural existence near the sheath edge. Keeping this in mind we have revisited the problem of usual Bohm sheath condition. Analytical and numerical analysis have been performed to re- derive the local condition for plasma sheath formation. It is found that the weak but finite electron inertia reduces the threshold value of ion Mach number that may be, at least in principle, of qualitative value to define the sheath edge boundary. Consideration of finite but weak equilibrium electron flow at the defined sheath edge shrinks the width of non- neutral space charge layer over which major potential drop and charge imbalance occurs. Detailed numerical analysis and results of quantitative and qualitative importance are included in the text. <![CDATA[<b>Influence of process parameters on the growth of pure-phase anatase and rutile TiO<sub>2</sub> thin films deposited by low temperature reactive magnetron sputtering</b>]]> In this work is investigated the optimal conditions for deposition of pure- phase anatase and rutile thin films prepared at low temperatures (less than 150ºC) by reactive dc magnetron sputtering onto well- cleaned p- type Si substrates. For this, the variation of deposition plasma parameters as substrate- to- target distance, total gas pressure, oxygen concentration, and substrate bias were studied and correlated with the characteristics of the deposited films. The XRD analysis indicates the formation of pure rutile phase when the substrate is biased at voltages between - 200 and - 300 V. Pure anatase phase is only attained when the total pressure is higher than 0.7 Pa. Moreover, it's noticeable a strong dependence of surface roughness with parameters studied. <![CDATA[<b>Theoretical studies of the local structure and the EPR parameters for substitutional Mo<sup>5+</sup> in TiO<sub>2</sub></b>]]> The electron paramagnetic resonance(EPR) parameters g-factors g i(i = x,y,z) and the hyperfine structure constants Ai for the substitutional Mo5+ in rutile are theoretically studied from the perturbation formulas of these parameters for a d¹ ion in a rhombically compressed octahedron. On the basis of the studies, The oxygen octahedron around Mo5+ is found to transform from the original elongation on the host Ti4+ site to compression in the impurity center due to the Jahn-Teller effect. The calculated EPR parameters based on the above local structure in this work are in good agreement with the experimental data. <![CDATA[<b>Blue thermoluminescence emission of annealed lithium rich aluminosilicates</b>]]> The blue thermoluminescence (TL) emission of different thermally annealed (i) β-eucryptite (LiAlSiO4), (ii) virgilite-petalite (LiAlSi5O12) and (iii) virgilite-petalite-bikitaite (LiAlSi10O22) mixed crystals have been studied. The observed changes in the TL glow curves could be linked to simultaneous processes taking place in the lithium aluminosilicate lattice structure (phase transitions, consecutive breaking linking of bonds, alkali self-diffusion, redox reactions, etc). The stability of the TL signal after four months of storage performed at RT under red light, shows big differences between annealed (12 hours at 1200ºC) and non-annealed samples. The fading process in non-annealed samples can be fitted to a first-order decay mathematical expression; however preheated samples could not be reasonably fitted due to the highly dispersion detected. The changes observed in the X-ray diffractograms are in the intensity of the peaks that denote modifications in the degree of crystallinity and, in addition, there are some differences in the appeareance of new peaks that could suppose new phases (e.g. β-spodumene). <![CDATA[<b>Dielectric and piezoelectric properties of <i>Ba</i>(<i>Zr<sub>x</sub>Ti</i><sub>1-<i>x</i></sub>)<i>O</i><sub>3</sub> lead-free ceramics</b>]]> Lead-free ceramics Ba(Zr xTi1-x)O3 (x = 0.02-0.2) were prepared using a solid-state reaction technique. The structural and electrical properties were systemically investigated. Crystalline structures and microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. All the samples possess pure perovskite structure. A small amount of Zr content has great effect on the microstructure of Ba(Zr xTi1-x)O3 ceramics. The homogeneous microstructure with grain size about 30µm is obtained for the sample at x = 0.05. The phase transitions merge together in one peak for the samples at x = 0.10 and the highest dielectric constant 15900 is obtained for the sample at x = 0.15. The Ba(Zr xTi1-x)O3 ceramics at x=0.05 exhibit excellent piezoelectric properties of high d33 = 208 pC/N, k p = 31.5% and Qm = 500. <![CDATA[<b>Pentacene based thin film transistors with high-k dielectric <i>Nd</i><sub>2</sub><i>O</i><sub>3</sub> as a gate insulator</b>]]> We have investigated the pentacene based Organic Thin Film Transistors (OTFTs) with High-k Dielectric Nd2O3. Use of high dielectric constant (high-k) gate insulator Nd2O3 reduces the threshold voltage and sub threshold swing of the OTFTs. The calculated threshold voltage -2.2V and sub-threshold swing 1V/decade, current ON-OFF ratio is 1.7 × 10(4) and mobility is 0.13cm²/V.s. Pentacene film is deposited on Nd2O3 surface using two step deposition method. Deposited pentacene film is found poly crystalline in nature. <![CDATA[<b>Theoretical studies of the EPR parameters for <i>Ni</i><sup>2+</sup> and <i>Co</i><sup>+</sup> in <i>MgO</i></b>]]> The electron paramagnetic resonance (EPR) parameters (g factors and the hyperfine structure constants) for Ni2+ and Co+ in MgO are theoretically studied from the perturbation formulas of these parameters for a 3d8 ion in octahedral crystal-fields. In the computations, the ligand orbital and spin-orbit coupling contributions are taken into account using the cluster approach. The calculated EPR parameters are in good agreement with the experimental data. The larger g factor and the smaller magnitude of the hyperfine structure constant for Ni2+ as compared with those for Co+ can be attributed to the higher spin-orbit coupling coefficient and the lower dipolar hyperfine structure parameter of the former, respectively.