Iranian Journal of Physics Research
Year:2005 | Volume:5 | Issue:1|Papers Count:9

Chalcogenide glasses such as arseic sulfide (As2 S3) have attracted attention for applications such as all-optical switching in high speed communication. This is due to their high non-linear refractive-index. Z-scan and the Degenerate four wave mixing (DFWM) techniques can be used to measure the non-linear refractive index n2 and the two photon absorption coefficient β. A simultaneous closed-aperture and open-aperture Z-scan experimental set up was used to obtain the experimental results. The results were then fitted into a theoretical formula. Values of n2= 3x 10-17m2/W and β= 0.29 cm/GW have been obtained. DFWM measurements were made on arsenic sulfide films. A Box-cars forward geometry was used in these measurements. Experimental results based on non-phase matched signals were again fitted into a theoretical formula and a value of n 2= 3.9x 10-17m 2/W was obtained.

An experiment to search for short-period intensity oscillations in the solar corona was conducted during the total solar eclipse of August 11, 1999 in Esfahan, [ran. The intensity in the continuum, centered about 4700Å and with a passband having a half-width of 190 Å, was recorded at a counting rate of 5 Hz using six low-noise Hamamatsu R647 photomultiplier tubes. We recorded intensity values from 6 different regions on the image of the solar corona from 1.2 R to 1.5 R. The power spectrum analysis of all channels, except for a channel that failed to receive any coronal light, shows excess power in the frequency range 0.02-0.2 Hz. The results of analyzing all channels demonstrate various waves with some degree of coherence and amplitudes from 0.3 to 0.7 percent of the mean coronal brightness. There are some similarities between waves in various channels and it may show that these waves exist in a large region of the corona, of the order of a few hundred thousand kilometers, much larger than the fiber optic diaphragms. The energy flux of each wave was calculated by assuming them as the slow or fast mode waves for the active and quiet regions of the solar corona. The energy flux of the fast mode waves in the active regions is comparable with the required energy flux for heating up the solar corona. The intensity oscillations observed in this experiment are similar to those detected during the 1995 and 1997 eclipses.

We study the phase structure of two dimensional pure lattice gauge theory with a Chern term. The symmetry groups are non-Abelian, finite and disconnected sub-groups pf SU(3). Since the action is imaginary it introduces a rich phase structure compared to the originally trivial two dimensional pure gauge theory. The Z3 group is the center of these groups and the result shows that if we use one dimensional irreducible representations (irreps) for group elements the phase diagrams are similar to diagrams of Z3 group. Other irreps with different dimensionality show a little different behaviour for the phase diagram. The phase transition for the Z3 group is first order. The phase structure of the U(N) model is considered and it is proved that it has an infinite number of first order phase transitions.

One of the most interesting properties of boson gases is that under special conditions, there is a possibility of a phase transition, in a critical temperature below which all bosons condensate into the ground state. This phenomenon is called Bose - Einstein Condensation (BEC). In this paper, we investigate BEC in a harmonic oscillator trap. We conclude that, in contrast to a free boson gas, there is no critical temperature for phase transition in a harmonic oscillator trap. However, by numerical and analytical calculation, it is possible to obtain a temperature at which the heat capacity is maximum. We call this the critical temperature. Possible explanation for all these features will be explained in this paper.

In this research, magnetostriction and thermal expansion of polymer-bonded Nd4Fe77.5B18.5 nano-composites are studied using strain gage method. The effect of sintering on samples is also studied. X-ray diffraction patterns and SEM pictures of the original and annealed samples show that the grain size of α-Fe and Fe3B soft magnetic phases in amorphous matrix increases and mechanical hardness of the samples improves after annealing at 700 °C for 20 min.. Overall behavior of the thermal expansion is similar for samples in both cases, with about 20% larger coefficient for the annealed one. The magnetostriction measurements show huge magnitudes in order of 10-4 at the presence of relatively weak external magnetic fields. The magnitude of magnetostriction noticeably decreases after annealing which can originate from two different sources. Magnetostriction of the original sample mainly originates from single particle interactions of Nd-sublattice in Nd2Fe14B phase, but the situation is very different for the annealed sample. The complicate variations of the magnetostrictive isotherms of the annealed sample are discussed based on the following three factors: internal stresses, magnetocrystalline anisotropy and mechanisms of coercivity.

The incommensurate spin -density-wave magnetism of Cr has attracted great interest since its discovery via neutron scattering. Although the existence of spin- density -wave has been confirmed by experiment but the calculations which have been carried out have not been able to predict the correct ground state magnetic phase for chromium yet. To predict the magnetic hyperfine field at nucleus of different impurities in Cr host, we calculated the hyperfine field on Cd, Sn, Rh and Fe probes in the first step. Our calculations were performed within the framework of density functional theory, using the full-potential-linearized augmented plane-wave method. We used a supercell constructed from 8 bcc unit cells with impurity concentratin of 6.25 % and to analysise the supercell size effect on different magnetic quantities we repeated our calculation using a supercell with 54 atoms. The result of this effort showed that the magnetic hyperfine field and magnetic moment of nearest Cr is very little influenced by the size of supercell, so we can calculate the magnetic hyperfine field if its quantity is known in different alloys. We showed that the local properties such as hyperfine field, are calculated with acceptable accuracy by using small supercells. Meanwhile, we studied the structural and magnetic properties of different alloys and showed that the Fe alloy has two different magnetic phase.

PbS semiconductor non-crystals have been synthesized in order to study the modification of their electronic structures and optical properties in relation to their size. The synthesis has been carried out by using the techniques of colloidal chemistry. Strong quantum confinement behavior has been observed based on the analysis of optical spectra of these particles. The average particle size approximated by x-ray line width and hyperbolic band model calculation. Heterogeneous broadening of optical spectrum is studied finally.

The electronic structure and optical properties in cubic cry stal BaTio3 are studied using-full-potential linearized augmented plane wave (FP-LAPW) method in density functional theory (DFT) with the generalized gradient approximation (GGA) by WIEN2K package. The calculated results of BaTio3 are in good agreement with other theoretical and experimental results.