Iranian Journal of Physics Research
Year:2012 | Volume:11 | Issue:4|Papers Count:12

Interpretation of gravity as entropic force makes some corrections in Friedmann equations. According to calculations of Easson, Frampton and Smooth, the entropic force addss correction terms to Friedmann equations. It’s shown that these corrections change the second type of singularity to the third one in Friedmann equations. Considering Friedmann equations, even without entropic force at least one of the energy conditions is violated in the third kind of singularity. Adding the correction terms, the violated energy condition will change.

The discovery of graphene and its remarkable electronic and magnetic properties has initiated great research interest in this material. Furthermore, there are many derivatives in these graphene related materials among which graphene nanoribbons and graphene nanofragments are candidates for future carbon-based nanoelectronics and spintronics. Theoretical studies have shown that magnetism can arise in various situations such as point defects, disorder and reduced dimensionality. Using a mean field Hubbard model, we studied the appearance of magnetic textures in zero-dimensional graphene nanofragments and one-dimensional graphene zigzag nanoribbons. Among nanofragments, triangular shape, bowtie and coronene were studied. We explain how the shape of these materials, the imbalance in the number of atoms belonging to the graphene sublattices, the existence of zero-energy states and the total and local magnetic moments were related. At the end, we focused on the effects of a model disorder potential (Anderson-type), and illustrate how density of states of zigzag nanoribbons was affected.

In this study, we considered the self-focusing of a Gaussian laser pulse in an unmagnetized plasma. The equation of plasma density evolution that includes the electrons ponderomotive force is obtained. Then an equation for the width of laser pulse with a relativistic mass correction term is derived. This term is proportional to the electrons temperature. It is shown that in the large width limit we have an oscillatory solution, and for the small width limit this correction always helps the self-focusing of the laser pulse.

In this paper, we study the phonon and thermal properties of a nanocrystal by using transfer matrix method in the harmonic approximation. The considered system is a mass-spring nanocrystal with a square cross section. At first, for the ideal case, we obtain the phonon spectrum with free boundary conditions. Then we calculate some phonon transport properties of this system, namely, the phonon density of states (modes) and transmission coefficient under different physical and structural conditions. Finally, in the presence of a linear temperature gradient, we obtain the system’s thermal conductivity. The results show that the variation in values of physical and structural parameters causes the phonon system properties to strongly change. For example, by decreasing or increasing the typical band frequency of the system, the phonon transmission coefficient increases or decreases, respectively. Moreover, for a mass periodic (for sequential layers) nanocrystal embedded between the two ideal nanocrystals the thermal conductivity decreases with respect to the ideal case.

Cascade dynamics of kaonic nitrogen atom (K-N) in a gaseous target was simulated using a simple model. In this model the radiative transitions, internal Auger, K-electron refilling, nuclear absorption, and kaon decay processes were included. The effect of the K-electron refilling process on some of quantities of cascade dynamics such as X-ray yields, the averaged K-electron shell number, fraction of the nuclear absorption and the averaged de-excitation time was studied by Monte-Carlo method. Using a fitting method, the range of K-electron refilling rate was predicted between 0.1 to 0.6 ps-1.This result is in agreement with the theoretical value.

This study investigated the electronic properties of antiferromagnetic UBi2 metal by using ab initio calculations based on the density functional theory (DFT), employing the augmented plane waves plus local orbital method. We used the exact exchange for correlated electrons (EECE) method to calculate the exchange-correlation energy under a variety of hybrid functionals. Electric field gradients (EFGs) at the uranium site in UBi2 compound were calculated and compared with the experiment. The EFGs were predicted experimentally at the U site to be very small in this compound. The EFG calculated by the EECE functional are in agreement with the experiment. The densities of states (DOSs) show that 5f U orbital is hybrided with the other orbitals. The plotted Fermi surfaces show that there are two kinds of charges on Fermi surface of this compound.

Inhalation of radon gas 222Rn, which is a decay product of 226Ra, and its decay products accounts for typically about half of the effective doses received by public from all natural sources of ionizing radiation. Radon alpha particles can initiate a series of molecular and cellular events that culminates in the development of lung and other cancers. Also, 226Ra in the environment is widely distributed, being present in various concentrations in waters, soils and rocks. When radium is ingested, the majority of material is rapidly excreted.However, since radium is chemically similar to calcium, a significant fraction is absorbed into the bloodstream and deposited mainly in the skeleton. So, presence of these radioactive contaminants in water is dangerous and many studies especially about radon have been done in this area.For these reasons and becauses some areas of Ramsar, a city in northern Iran in mazandaran province, have been among the highest known background radiation levels in the world we measured radon and radium concentrations in water sources of Ramsar region. In this study, Radon and radium concentrations of the 22 streams and 20 drinking water samples were measured by PRASSI system. According to the data, the arithmetic mean of radon concentration for all samples was 3.030 ± 1.122 Bq/l. Similarly, arithmetic mean of radium for all samples was 0.185 ± 0.055 Bq/l. Also 1 sample of streams and 1 sample of drinking water showed radon concentration higher than 10Bq/l as normal level. Radium-226 alone, in 11 samples of streams and 8 samples of drinking water had concentrations higher than 0.185Bq/l as normal level for the combined Radium-226 and Radium-228.

In this paper, we study the electronic conductance and density of states for a comb-like polymer with periodic hopping energies in the tight-binding approach. Electron transmission coefficient and density of states are analytically calculated by using Green’s function of the system. The results show that the electronic conductance spectrum has one energy gap in the absence of carbon-hydrogen bond’s hopping energy, which is proportional to the dimerization strength.Carbon-Hydrogen bond’s hopping energy makes the appearance three energy gaps in the conductance spectrum and the dimerization strength influences only the outer gaps.

In this letter, titanium dioxide nanoparticles (TiO2) were synthesized via a sol-gel method and combining titanium tetrachloride (TiCl4) and ethanol. The activation energy was investigated on the phase transformation from anatase to rutile in the presence and absence of ultrasound waves. The anatase nanocrystallites were only crystallized up to the calcination of 500 oC. By increasing the calcination in the region after 500 oC, rutile nanocrystallites grew in samples, and mixed-phase TiO2 nanoparticles were obtained. Our results show that applying ultrasound waves decreases both onset transition temperature and the activation energy of the phase transformation from anatase to rutile. The activation energy showed a considerable reduction at about 18.5 KJ/mol by applying the ultrasound waves.

In this paper, we calculate nuclear structure function and EMC effect of 40Ca and 56Fe nuclei. To achive the goals, we consider Fermi motion and binding energy contrbiution in the harmonic oscillator model. In this model, harmonic oscillator parameter hw related to shells root mean square radius and for free nucleon structure functions, is obtained from GRV’s free nucleon structure functions. Then, we calculate differential cross section of lepton scattering from those nuclei at the E=4.8 GeV and E=4.032 GeV. The obtained results show good agreement with available experimental data.

The ZnO thin films were prepared froms zinc acetate dihydrate as a main precursor by using sol-gel method and deposited by drainage and dip coating technique. Four different routes and coating techniques were used for the preparation of samples. The morphology, optical, and structural properties of the sol-gels made ZnO thin films were studied with respect to the preparation of sol-gel route, drainage and dip coating techniques. The microstructure of the ZnO thin films and the powders were analyzed by X-ray diffraction. The ZnO thin films prepared in this study were amorphous while its powders were polycrystalline with various diffraction peaks in the X-ray diffraction patterns. The morphology of the film was examined by using scanning electron microscopy. The surface morphology of the ZnO thin films strongly depends on preparation route and deposition technique. The optical characteristics of the samples were obtained by using UV-Visible spectrophotometer at 200–900 nm wavelength. The optical constants (refractive index, extinction coefficient etc.) of the ZnO thin films depend on preparation conditions.

Algebraic hyperstructures theory is a natural extension of the classical ‎ ‎ algebraic structures ‎. Because of importance and new viewpoints in this theory, we try to apply this theory to the elementary particle physics and nuclear physics presenting the definitions and the concepts of the algebraic hyperstructures. In this work we intend to use this new branch of mathematics to describe the interaction between leptons in the elementary particle physics and the fusion processes in the nuclear physics.