مجله پژوهش فیزیک ایران
سال سیزدهم شماره 1 (بهار 1392)

We have considered the self-focusing of a Gaussian laser pulse in unmagnetized plasma. High-intensity electromagnetic fields cause the variation of electron density in plasma. These changes in the special conditions cause the acceleration of electrons to the higher energy and velocities. Thus the equation of plasma density evolution was obtained considering the electrons ponderomotive force. Then، an equation for the width of laser pulse with a relativistic mass correction term and the effect of ion channel were derived and the propagation of high-intensity laser pulse in an underdense plasma with weak relativistic approximation was investigated. It is shown that the ratio of ion channel radius to spot size could result in different forms of self focusing for the laser pulse in plasma.

In recent years، semiconductor nanostructures have become the model systems of choice for investigation of electrical conduction on short length scales. Quantum transport is studied in a two dimensional electron gas because of the combination of a large Fermi wavelength and large mean free path. In the present work، a numerical method is implemented in order to contribute to the understanding of quantum transport in narrow channels in different conditions of disorder and magnetic fields. We have used an approach that has proved to be very useful in describing mesoscopic transport. We have assumed zero temperature and phase coherent transport. By using the trick that a conductor connected to infinite leads can be replaced by a finite conductor with the effect of the leads incorporated through a ''self-energy'' function، a convenient method was provided for evaluating the Green''s function of the whole device numerically. Then، Fisher-Lee relations was used for calculating the transmission coefficients through coherent mesoscopic conductors. Our calculations were done in a model system with Hard-wall boundary conditions in the transverse direction، and the Anderson model of disorder was used in disordered samples. We have presented the results of quantum transport for different strengths of disorder and introduced magnetic fields. Our results confirmed the Landauer formalism for calculation of electronic transport. We observed that weak localization effect can be removed by application of a weak perpendicular magnetic field. Finally، we numerically showed the transition to the integral quantum Hall effect regime through the suppression of backscattering on a disordered model system by calculating the two­ terminal conductance of a quasi-one-dimensional quantum conductor as a strong magnetic field is applied. Our results showed that this regime is entered when there is a negligible overlap between electron edge states localized at opposite sides of the sample.

This paper considers the Landau problem in an elected static space time and the are erased levels shifts which are erased as a metric deviation from the Minkowski space time. This research is based on the Weber’s method. We try to rewrite the equation of motion of particles in the presence of the gravitational effects and consider the regions limited with the tangent spaces conditions. I t would be reasonable to assume the nonrelativistic particles with low speed. We show that due to the Weber’s method، the tangent space is always available. Another assumption of this article is time independent tangent space of Schwarzschild universe and use of Riemann’s normal coordinates.

This paper reports on physical principles and the relations between extinction cross section and geometrical properties of silver and gold nanostructures. We introduce some simple relations for determining geometrical properties of silver and gold nanospheres based on the situation of their plasmonic peak. We also applied، investigated and compared the accuracy of these relations using other published works in order to make clear the effects of shape، size distribution and refractive index of particles’ embedding medium. Finally، we extended the equations to non-spherical particles and investigated their accuracy. We found that modified forms of the equations may lead to more exact results for non-spherical metal particles، but for better results، modified equations should depend on shape and size distribution of particles. It seems that these equations are not applicable to particles with corners sharper than cubes'' corners i. e. nanostructures with spatial angles less than π/2 sr.

The structure of longitudinal modes of a passively Q-switched، non-planar unidirectional ring-resonator،with Nd: YAG active medium is described in this article. Two different techniques are used to study the longitudinal mode structure of the laser resonator. At first، the fast-fourier transform technique is applied for analyzing the mode beating of the optical fields by intensity frequency structure of the laser pulses to determine the number of longitudinal modes. Then، an analyzer etalon is used to observe Fabry-Perot fringes to compute the numbers of the resonator longitudinal modes. The results of two techniques are in good agreement with each other. Under the proper conditions، a reliable single longitudinal mode of the non-planar ring-resonator can be achieved with a good spatial mode profile that originates from the unidirectional travelling optical field propagation in the resonator having a very low sensitivity of the non-planar ring resonator to the optical elements misalignment.

The aim of this investigation was to calculate the thermal properties of silver crystal in the presence of linear imperfection. The simulations were performed by molecular dynamics simulation technique in NPT as well as NVT ensemble based on quantum Sutton-Chen many body potential. The thermal properties including cohesive energy، melting temperature، isobaric heat capacity and thermal expansion of imperfect silver crystal were calculated and compared to those of the perfect crystal. Moreover، the quantities such as radial distribution function، order parameter and lindemann index were calculated in order to obtain information on crystal structure and disorder in atoms. All calculations were done both with liner imperfection in [001] direction and without imperfection at different temperature. The simulation results show that cohesive energy، linear thermal expansion coefficient increase and melting temperature، latent heat of fusion decrease with increasing linear imperfection. Also، the results show that linear imperfection has no effect on the heat capacity.

Electronic properties for bcc Fe and hcp Co in bulk state and also Fe-Co alloy were calculated by quantum calculation based on density functional theory and pseudopotential method. Combination of Wannier function and Berry phase theory was used for calculation of anomalous Hall conductivity in above structures. It was seen that split of band by the spin orbit interaction، lying on the Fermi level، has a major role in Berry curvature. Acquired results agree with experimental ones and thus it seems that conductivity in bulk state is intrinsic.

In this paper، Ba0. 5Sr0. 5Co0. 8Fe0. 2O3-δ (BSCF) thin films were deposited on single crystal SrTiO3 (STO) (100) by pulsed laser deposition (PLD) technique at different pressures of oxygen. Crystal structure of bulk and thin film samples was studied by x-ray diffraction (XRD). The XRD results indicate that both bulk and thin film samples have cubic structures. AFM micrographs showed an increase in RMS roughness by oxygen pressure. The electrical resistance was measured at room temperature up to 600 and 800 °C in air using four probe method for bulk and thin films، respectively. A sharp drop in resistance was observed by increasing temperature up to 400 °C، that was explained with the small polaron hopping model. Polaron activation energy was calculated by Arrhenius relation. It was decreased over increasing oxygen pressure. The surface exchange coefficient (Kchem) of the 300 mTorr sample was measured by electrical conductivity relaxation (ECR) technique. The results suggested a linear relationship between Kchem and reciprocal of absolute temperature.

In this paper، by using the nonlinear coherent states on a sphere، we introduce superposition of the aforementioned coherent states. Then، we consider quantum optical properties of these new superposed states and compare these properties with the corresponding properties of the nonlinear coherent states on the sphere. Specifically، we investigate their characteristics function، photon-number distribution، Mandel parameter، quadrature squeezing، anti-bunching effect and Wigner function، and obtain the curvature effect on the properties of the superposed states. Finally، by using the trapped atom system، we introduce a theoretical scheme to generate superposition of the coherent states on the sphere.

In this work، total excitation cross section of atomic hydrogen in the collision of bare ion was calculated employing a three body Faddeev formalism. In the present calculation، initially the first order electronic amplitude was calculated using the interaction potential which led to inelastic form factor. Secondly، the first order nuclear amplitude was calculated and added to the first order electronic amplitude. This second term was calculated employing the near-the-shell two body transition operator. The interaction energy was assumed to be in the intermediate and high energy limits. Finally، the results were compared with the relevant cross sections calculated under monocentric close-coupling data in the literature.

In this paper، current-voltage characteristic of a resonant tunneling diode under electromagnetic radiation has been calculated and compared with the results when there is no electromagnetic radiation. For calculating current -voltage characteristic، it is required to calculate the transmission coefficient of electrons from the well and barrier structures of this device. For calculating the transmission coefficient of electrons at the presence of electromagnetic radiation، Finite Difference Time Domain (FDTD) method has been used and when there is no electromagnetic radiation Transfer Matrix Method (TMM) and finite diffirence time domain method have been used. The results show that the presence of electromagnetic radiation causes resonant states other than principal resonant state (without presence of electromagnetic radiation) to appear on the transmition coefficient curve where they are in distances from the principal peak and from each other. Also، the presence of electromagnetic radiation causes peaks other than principal peak to appear on the current-voltage characteristics of the device. Under electromagnetic radiation، the number of peaks on the current-voltage curve is smaller than the number of peaks on the current-voltage transmission coefficient. This is due to the fact that current-voltage curve is the result of integration on the energy of electrons، Thus، the sharper and low height peaks on the transmission coefficient do not appear on the current-voltage characteristic curve.

Synchrony is significant in brain neural network. In this study we investigate the collective firing in an excitable media and modeling the brain network by an small-world one. The Gaussian white noise is taken to the system of phase oscillators، and then to the frequency distribution. An order parameter in non stationary situation and other usefull statistical parameters such as firing are computed. Three regimes are identified in such a network: no firing regime، where all elements are confined near the fixed point; coherent pulsation، where a macroscopic fraction fire simultaneously; and incoherent pulsation، where units fire in a disordered fashion.

Scintillation detectors are widely used in the experimental setup for the detection of charged particles. These detectors are able to measure the energy and time-of-flight of the charged particles. Also، they can be used to identify the detected particle. The probability of hadronic interaction between the detected particle and the nuclei of the scintillator atoms is one important issue that must be considered in the analysis of the detectors output. The hadronic interaction causes particles the deposit only a part of their energy inside the detector. In this case، particle will appear in the tail of energy spectrum and is mixed with the background events. Therefore، the measured cross section، which is calculated using the number of particles that deposit their full energy in the detector will be underestimated and one should correct the cross section for the lost events. The percentage of incident particles for which the hadronic interaction occurs is determined by different methods. In this paper، using two different methods، Monte-Carlo simulation and experimental data for several different channels in the proton-deuteron and deuteron-deuteron scattering at intermediate energy are introduced. The obtained results from the two methods are consistent with each other.