Iranian Journal of Physics Research
Year:2001 | Volume:2 | Issue:5|Papers Count:9

In this paper Fresnel diffraction of light from I-dimensional and circular steps in reflection and transmission modes is theoretically and experimentally studied. The study shows that the diffraction pattern from each side of a step resembles the diffraction pattern from a semi infinite obstacle, but the visibility of fringes is a sensitive periodic function of the step height and the incident angle. This sensitive dependence can be exploited as a useful means for measuring the film thickness, the refraction index and small displacements of objects in a manner easier and more precise than one can manage by the conventional interferometric methods.

We inspected the exact solution of double barrier quantum well. The choice of proper boundary conditions has been taken into account. We evaluated the mechanism of resonant in this device. The density correlation matrix was calculated by using the exact solution of the time-dependent generalized nonlinear Schrodinger equation in the presence of electron- electron interaction. The result shows that there is no correlation dependence among the electrons at the equilibrium between contact regions. After biasing, we have calculated the density correlation matrix in the transient and steady state. The results of our calculations show the oscillatory plasmon current in the state of transient, while in the steady state the correlation among the phase of electrons observed to be oscillatory in the whole region of the device.

Time-dependent wormhole solution of the BD theory in an anisotropic radiation background is presented. It is also found that the BD scalar field depends only on time. This time dependency is in power-law form. It is shown that the wormhole geometry is valid for w≥-3/2, and for any arbitrary positive values of w. The GR limit of our solution is obtained for w=0, not for w→∞. Though the BD field can be non- exotic, the background material is entirely exotic.

We present numerical solutions of the equations of motion of the driven spring-block model with stick-slip dynamics. The system consists of an array of N masses which describes the statistical and dynamic properties of dissipative systems. In this study, we are interested in very low and very high velocity regimes. We show that there is a clear shift in the distribution of slip-force when we move from a low velocity regime to an intermediate one and then to high velocity regime. The observed transitional behaviour corresponds to the Gaussian distribution in low velocity, to δ-shape behaviour in the intermediate velocity and to an inverse-Gaussian distribution for the high values of velocity.

The study of a two-dimensional (2-D) system started nearly half a century ago when Peierls and Landau showed the lack of long range translational order in a two-dimensional solid. In 1968, Mermin proved that despite the absence of long range translational order, two-dimensional solids can still exhibit a different kind of long range bond orientation. During the last decade, fascinating theories were put forward to explain the role of topological defects in the melting of two-dimensional solids, starting with Kosterlitz and Thouless. Recent surge of interest in melting is also due to the theoretical ideas of Halperin, Nelson and Young. They have suggested that the transition may be fundamentally different from that observed in ordinary three-dimensional systems. Computer simulations suggest that the transition is of the usual first-order type observed in a three-dimensional system. A large body of experimental and simulation research into the two-dimensional melting followed the announcement of the KTHNY theory. In spite of all this effort, the question as to the nature of two dimensional melting remains unresolved. Recent experimental work supporting the existence of continuous melting transitions in some two dimensional systems indicates the need for further theoretical and computational work to lead to an understanding of the experimental results.In this paper we intend to summarize and clarify the current situation with regard to research in the two-dimensional melting with an emphasis on computer simulations. The paper begins with an overview of the current status of relevant theoretical, experimental and simulation research, then a two-dimensional simulation of an ionic salt system is studied in detail. This simulation has been done by using the molecular dynamics method. The most important parameters that have been determined are: The transition temperature, the total energy of the system, the mean square displacement and the bond angle distribution. The transition temperature of the system has been specified by plotting some of these parameters as a function of temperature and time. The first order transition is observed. It is difficult to distinguish a hexatic phase from a two-phase coexistence region.

UBV photometric observations of Ap star Cyg V1584 have been presented. To find the rotational period of the star, a sinusoidal wave function has been fitted to the normal points of UBV filters. Assuming that a circular hot spot located at the magnetic pole of the star is responsible for the observed light variations, both. Physical and geometrical parameters of the spot have been determined. Finally, the angle between the magnetic and the rotational axis has been calculated from combining the spectroscopic and photometric data and the magnetic structure of the star has been discussed.

Some new equations of state which are derived for dense fluids in recent years, namely the linear isotherm regularity (LIR), the dense system equation of state (DSEOS), Ihm-Song-Mason equation of state (ISM), and a newly derived semi- emperical equation of state have been used to investigate the common intersection point of isobaric expansivity (αp) in dense fluids. We have shown that the accuracy of these equations of state in predicting such a common intersection point is reduced from the new semi-imperical equation of state, DSEOS, LIR, to ISM Respectively.Form physical point of view, the van der Waals equation of state is used to investigate such an intersection point. It is shown that the van der Waals repulsion forces and temperature dependency of the effective molecular diameter are important for existence of this common point.Finally, we have shown that the common intersection points of the isotherms of thermal pressure coefficient, the isotherms of heat capacity at constant volume, and the isochors of internal pressure for a fluid are related to each other. Also, the common intersection points of the reduced bulk modulus and 1/(Tαp) for isotherms of a fluid both appear at the same density.

Although many phenomena in condensed matter Physics can be understood on the basis of a model, there are also considerable number of physical properties of solid which can not be explained except in the framework of lattice dynamics.We have calculated the phonon frequencies of NaCl, using an approach which is a combination of frozen phonon and force constants methods in the framework of density functional pseudo-potential theory. The dispersion relation curves were calculated along symmetry direction Δ, ∑ and Λ. We also calculated Grunesein parameters for all modes at X and L points in Brillion zone. The calculations are made in the framework of density functional and pseudo-potential theory, using super cell method, with the valence orbitals expanded in plane waves.