مجله پژوهش فیزیک ایران
سال سیزدهم شماره 3 (پاییز 1392)

Pauli (or Einstein) frame is used to study the Brans-Dicke gravity theory، minimally coupled with dilatonic Brans-Dicke scalar field، whose solutions involve degenerate metrics. Some of these solutions exhibit transitions from an Euclidean domain to a Lorentzian space-time corresponding to a spatially flat Robertson-Walker cosmology.

In this work، 137Cs activity concentration of soil in five depths and in six meteorology stations in terms of Bq/kg has been measured. For each thirty soil''s samples، parameters such as kind of soil، density and hydraulic coefficient of transmission were determined. Correlation coefficient between 137Cs activity concentration and raining average، hydraulic coefficient of transmission and soil''s density was calculated. Results for 5،15،30،45 and 60 cm depths were obtained as 0. 89، 0. 99، 0. 98، 0. 70 and 0. 66، respectively. Finally، correction coefficients for some locations with acute gradients were determined.

In this paper، optical properties of Au–Ag spherical nanoalloy are investigated by means of Generalized Lorenz–Mie Theory (GLMT) when this nanoparticle is embedded into water. Scattering، extinction and absorption cross-sections of this nanoalloy into water are calculated by changes of incident wavelengths in visible and near infra-red region. Moreover، changes of height and wavelengths of extinction and scattering cross-sections peaks (due to Particle Plasmon Resonances (PPR)) versus alloy percentage for Au–Ag spherical nanoalloy are considered.

In this simulation model، oblique angle deposition method and Monte Carlo random walks have been used. Growth model was ballistic deposition (BD). Incident particles flux comes to substrate at an angle with respect to normal. Two physical factors including self-shadowing and mobility limitation of incident particles make structures similar to independent columns with different shapes and separated with voids، grow in the incident particles direction. Some parameters such as columns angle، structural morphology، density، and surface roughness are studied. Structural morphology under oblique angle is grown columnar and with an increase in the incident angle، shadowing and porosity increase. Density and interface roughness increase by increasing the incident angle، as well.

Thermoluminescence TL materials LiF (Mg،Cu2+) and LiF (Cu2+) were produced in powder. Their 3D emission spectra were investigated and compared to the commercially available LiF (Mg،Cu2+،P) and (LiF(Mg،Ti) TL materials. Emission spectrum of LiF (Mg،Cu2+) represents a peak maximum at 450 nm with a wide shoulder ranging from 520 nm to infrared area. Emission peak of LiF (Cu2+) is situated at 380 nm and its tail also is stretched to infrared. The temperature rates of main peak in glow curves for LiF (Mg،Cu2+) and LiF (Cu2+) lie at 200 °C and 250°C، respectively. Based on the obtained results، Cu impurity in LiF (Mg،Cu،P) has a major role in removing emission frequencies which are higher than 500 nm and also UV sensitivity.

In this paper، we have investigated the spin-dependent transport properties and electron entanglement in a mesoscopic system، which consists of two semi-infinite leads (as source and drain) separated by a typical quantum wire with a given potential. The properties studied include current-voltage characteristic، electrical conductivity، Fano factor and shot noise، and concurrence. The calculations are based on the transfer matrix method within the effective mass approximation. Using the Landauer formalism and transmission coefficient، the dependence of the considered quantities on type of potential well، length and width of potential well، energy of transmitted electron، temperature and the voltage have been theoretically studied. Also، the effect of the above-mentioned factors has been investigated in the nanostructure. The application of the present results may be useful in designing spintronice devices.

In this paper، we investigate the superconducting proximity effect for a superconductor with three types of singlet، OSP and ESP triplet pairings in contact with a clean ferromagnetic region. Using the quasiclassical Green’s function approach، we calculate the superconducting pair amplitude function in terms of the characteristic parameters of the system and compare penetration of these superconducting correlations inside the ferromagnetic region. We show that the ESP triplet correlations have a long range thermal penetration compared with the singlet and OSP triplet correlations inside the ferromagnetic region.

In this paper the problem of a particle in an array of hexagons with periodic boundary condition is solved. Using the projection operators، we categorize eigenfunctions corresponding to each of the irreducible representations of the symmetry group. Based on these results، the Dirichlet and Neumann boundary conditions are discussed.

In this paper، we have presented the optimized BCS formalism using the isothermal probability distribution. The effect of statistical fluctuations on thermodynamical properties of nuclei has been investigated. The average gap parameter is calculated and then the energy، the entropy and the heat capacity are evaluated. The resulting values are compared with results of the standard BCS and the Static Path Approximation plus Random Phase Approximation، (SPA+RPA). We have shown that the resulting heat capacity versus temperature using the optimized BCS model is very similar in shape to the experiments in the other models. The peak in the heat capacity is interpreted as the transition from the paired to the normal phase.

In this paper، we investigated the effect of dipole-dipole interaction between the dust particles on the transverse oscillation of one dimensional dusty crystal. We used the Boltzmann distribution for the electrons and ions density and assumed that dust particles are negatively charged. The equation of motion for dust particles in this one dimensional chain was obtained. It is shown that the direction of dipoles plays an important role in the motion of dusts and significantly changes the oscillation frequency. Also، in the long wavelength approximation، a nonlinear Schrödinger equation for the evolution of the amplitude of the nonlinear oscillations was derived، showing that both the bright solitons and the dark solitons could exist.

Renewable energy research has created a push for new materials; one of the most attractive material in this field is quantum confined hybrid silicon nano-structures (nc-Si: H) embedded in hydrogenated amorphous silicon (a-Si: H). The essential step for this investigation is studying a-Si and its ability to produce quantum confinement (QC) in nc-Si: H. Increasing the gap of a-Si system causes solar cell efficiency to increase. By computational calculations based on Density Functional Theory (DFT)، we calculated a special localization factor، [G Allan et al.، Phys. Rev. B 57 (1997) 6933.]، for the states close to HOMO and LUMO in a-Si، and found most weak-bond Si atoms. By removing these silicon atoms and passivating the system with hydrogen، we were able to increase the gap in the a-Si system. As more than 8% hydrogenate was not experimentally available، we removed about 2% of the most localized Si atoms in the almost tetrahedral a-Si system. After removing localized Si atoms in the system with 1000 Si atoms، and adding 8% H، the gap increased about 0. 24 eV. Variation of the gap as a function of hydrogen percentage was in good agreement with the Tight –Binding results، but about 2 times more than its experimental value. This might come from the fact that in the experimental conditions، it does not have the chance to remove the most localized states. However، by improving the experimental conditions and technology، this value can be improved.

The knowledge of nuclear reaction rates is important for studying energy production and nucleosynthesis، especially in reactions including nuclei far from stability line، which are not accessible experimentally so far and thus it is necessary to be able to predict reaction cross sections in low excitation energies. Nuclear level density is one of the important key quantities in many nuclear physics fields for both evaluations of reactions cross sections and statistical calculations. Statistical properties of atomic nuclei can be described by considering the excited nuclei as a Fermi gas with non interacting particles consisting of two kinds of particles، protons and neutrons. Back shifted Fermi gas model is a modified form of the original Bethe analytical formula that both the energy shifted parameter E1 and the level density parameter are considered as adjustable parameters، which yield a reasonable fit to the experimental level densities over a wide range of excitation energies. In this article، the results of and E1 from fitting the number of levels versus energy with the corresponding experimental values for 468 nuclei are reported. The dependence of nuclear level density parameter on the mass number A for even- even، odd-A and odd- odd nuclei is given by، and، respectively. Large variations of this parameter in Z=82 and N=126 indicate its strong dependence on shell effects. General behavior of this parameter versus mass number A obeys the relation. These relations are reliable facilities for extrapolating the nuclei far from stability which are not accessible experimentally.

In this paper، we investigate the electrical conductance and density of states of a nanocrystal including an electrical charge or dipole located at cross section of nanocrystal by using Green’s function method in the nearest neighbor tight-binding approach. The results show that moving the electrical charge from center to the edge of the nanocrystal increases the system transmission coefficient. In contrast، shifting the electrical dipole from the center to the edge of the nanocrystal، decreases of the system conductance.

Thin film samples of Ti-doped ZnO were grown on sapphire (0001) substrates by pulsed laser deposition (PLD). The magnetic moments were measured by SQUID magnetometry and the films were ferromagnetic at room temperature. The Faraday rotations and magnetic circular dichroisms (MCD) were measured as a function of energy at the range of 1. 5-4 eV، and carrier concentrations were obtained from Hall effect measurement. The samples exhibited a band-edge shift، which varied with carrier concentration. Effective-mass of carriers were obtained by the Burstein-Moss effect and the band-gap shrinkage

Nµ/Ne، muon to electron population ratio in extensive air showers at high altitudes has been shown to be a suitable estimator of primary cosmic rays mass composition. This study is based on simulated extensive air showers. The Nµ/Ne ratio has been obtained in 100 depths from the top of the atmosphere to the sea level for different primary particle masses and energies. An empirical relation between cosmic ray atomic mass and Nµ/Ne has been obtained. The relation has then been used for estimation of atomic masses of progenitors of another set of simulated showers. Although the estimated masses are rough، the accuracy of the estimation improves with observation altitude.

Effect of Al3+ ions on the optical quality، growth rate، shape، and structure of potassium dihydrogen phosphate (KDP) crystal was investigated. Growth method was based on the “point seed” method in a solution with optimum concentration، following spontaneous growth at room temperature. FTIR spectrum showed neutralization of O-H group in KDP crystal by Al3+ ions. UV-Visible spectrum of the crystal containing Al3+ displayed an undesirable increase of the optical absorption in the 200-300 nm region with a maximum at 270 nm. It seems that the absorption in the 200-300 nm region is due to substitution of phosphorus by Al ions. In fact، (AlO4) 2- ions are present in the KDP crystals and give rise to ultraviolet absorption.

Gravitational waves are considered in thermal vacuum state. The amplitude and spectral energy density of gravitational waves are found enhanced in thermal vacuum state compared to its zero temperature counterpart. Therefore، the allowed amount of enhancement depends on the upper bound of WMAP-5 and WMAP-7 for the amplitude and spectral energy density of gravitational waves. The enhancement of amplitude and spectral energy density of the waves in thermal vacuum state is consistent with current accelerating phase of the universe. The enhancement feature of amplitude and spectral energy density of the waves is independent of the expansion model of the universe and hence the thermal effect accounts for it. Therefore، existence of thermal gravitational waves is not ruled out

Calcium fluoride nanoparticles doped with thulium and cerium were synthesized as a novel nanostructure by using the hydrothermal method. The prepared samples were characterized by X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The crystallite size of about 36 nm was estimated employing Scherer’s formula. Its shape and size were also observed by scanning electron microscope (SEM). Computerized glow curve deconvolution (CGCD) technique revealed two overlapping thermoluminescence glow peaks at around 402 and 457 K in the complex thermoluminescence glow curve of this phosphor. Other thermoluminescence characteristics of this phosphor such as fading، reuseability and dose response recommend it as a good candidate for high dose dosimetry.