مجله پژوهش فیزیک ایران
سال سیزدهم شماره 2 (تابستان 1392)

We studied the growth of viscous fingers as a Laplacian growth by conformal mapping. Viscous fingers grow due to Saffman-Taylor instability in the interface between two fluids، when a less viscous fluid pushes a more viscous fluid. As there was an interest in the rectangular Hele-Shaw cell، we solved the Laplacian equation with appropriate boundary conditions by means of conformal mapping techniques. The results were then visualized on a personal computer. Using these techniques، we studied singular effects of surface tension in the dynamics of the finger competition in the Saffman-Taylor problem with channel geometry. We also studied the motion of the interface between the two fluids in a pressure field. The more viscous fluid moves with a velocity proportional to the gradient of its pressure. In the two-dimensional case the interface can be described by a complex function which is analytic. Applying surface tension in the equations causes the tip-splitting at a longer finger (ahead). In zero order finger perturbation we had equal results for with and without surface tension. But for the first order perturbation، there was a difference. For limited surface tension in solutions for larger fingers (ahead)، we observed tip splitting for larger fingers، which is completely in agreement with experimental observations.

In this paper، we investigate the phonon transmission coefficient of a mass-spring in the presence of Kohn interaction by using Green’s function method within the harmonic approximation. This system is embedded between two simple phononic leads including only the nearest neighbor interactions. The results show that the presence of Kohn and the nearest neighbor interactions in the center wire makes a difference between the physics of center wire and leads. This causes some peaks and valleys to appear in the phonon transmission coefficient spectrum.

In this letter، titania nanopowder and titania-silica nanocomposite were prepared using sol-gel method. Although the size of nanocrystallites and the mass fraction percent age of rutile phase (after phase transformation) were increased by increasing calcination in the two samples، their size in titania-silica nanocomposite was smaller than that in pure titania. Moreover، the calculations of the indirect optical band gap in the two samples show ed that by increasing temperature up to the transition onset point a nd above، the optical band gap had a steady growth and the corresponding wavelength reduced. However، a remarkable red shift in the indirect optical band gap was noticed in both samples around the transition onset point. The red shift was observed more in silica-titania nano composite than in titania nanopowder. In addition، the indirect band gap of silica-titania nano composite was less than that of pure titania in the same temperature conditions.

Nowadays، heavy mesons and baryons are produced and decayed at LHC in abundance. These processes are good sources to study the QCD theory، especially to study the hadron structures. Therefore، the phenomenology of heavy hadrons fragmentation function is a basic and important subject in the Particle Physics. In the present work، we replicate the calculation heavy meson fragmentation function in the quark model and we compare our results with the well known phenomenological results and we also، for the first time، incorporate the effect of hadron mass in the calculations. We show that the mass effect not only increases the fragmentation function for the special values of the scaling variable but also it creates a threshold to produce a heavy meson.

In this paper، we study the noncommutative effect on the semileptonic transition of B Ò Dlv. We replace the weak interaction vertex in the ordinary space with its counterpart in the noncommutative space. It is shown that، more new form factors are needded to describe the hadronic part of the transition amplitude. All the form factors are obtained at the lowest order of three point QCD sum rule. Consequently، the decay rate of B Ò Dlv is calculated and a bound of the order of 4 GeV on is given for B Ò Dlv= (5. 5 ± 0. 5) × 10-2.

The widespread use of the social media has recently created a lot of interest in research on human interactions، and how ideas propagate among individuals. We review the standard rumor model adopted from the literature. These models assume that people spreading the rumor (spreaders) keep spreading it، and those who hear the rumor (ignorant) can become a spreader with a certain probability. We present a generalized model wherein the spreader may change its attitude and become an ordinary member of the society after each contact. We utilize the techniques of the dynamical systems and calculate the stability for the spread of rumor in the phase-space of our generalized model. The model possesses transitions and critical behavior. We also perform computer simulations. The simulations fully support our theoretical calculations.

In this paper، electronic properties of single-wall armchair and zigzag carbon nanotubes (CNTs) superlattices، n (12،0) /m (6،6) and n (12،0) /m (11،0) are investigated. For this reason، the topological defects of pentagon–heptagon pairs at interfaces of carbon hexagonal network appear. These defects break the symmetry of the system، and then change the electrical properties. The calculations include two parts: investigation of the structures in the absence and presence of the impurity effect، which are calculated by the nearest-neighbor tight binding model. Out numerical results can be useful in designing nanoelectronic devices based on carbon nanotubes.

Given the role of Gas-Graphite reactors as the fourth generation reactors and their recently renewed importance، in 2002 the IAEA proposed a set of Benchmarking problems. In this work، we propose a model both efficient in time and resources and exact to simulate the HTR-10 reactor using MCNP-4C code. During the present work، all of the pressing factors in PBM reactor design such as the inter-pebble leakage، fuel particle distribution and fuel pebble packing fraction effects have been taken into account to obtain an exact and easy to run model. Finally، the comparison between the results of the present work and other calculations made at INEEL proves the exactness of the proposed model.

TiO2-SnO2 thin films were coated on glass and Al / glass substrates by electron gun method. In coating process، the vacuum was 1. 5×10-5 torr. Then، films were annealed at 450، 500 and 550 ˚ C. The crystallographic structure and film morphology were investigated by means of XRD and SEM. The electrical (I-V) and optical properties were studied by the two point props system and UV/Vis/NIR spectrophotometer. The results showed the films under 550 ˚ C were crystalline. The thickness and grain size were 350 and 50 nm respectively. The electrical conductivity in the sample with Al / glass substrate under 550 ˚ C was better than the other samples. When temperature increased، the energy gap decreased from 4. 05 to 4. 03 eV for direct cases.

In this paper the effect of photonic band gap on the group velocity of reflected pulse from a dielectric slab doped with two-level or three-level atoms has been investigated. It is assumed that the slab is sandwiched between a uniform medium (like vacuum) and a one-dimensional photonic crystal. It is shown that the reflected pulse from the slab doped with two-level (three-level) atoms will be superluminal (subluminal) if the carrier frequency of the incident Gaussian pulse is in the photonic band gap. In contrast، for the incident pulse with the carrier frequency at the edge of photonic band gap، the reflected pulse from the slab doped with two-level (three-level) atoms is subluminal (superluminal).

In this paper، we calculate the spin-dependent conductance of ferromagnetic quantum wire in the presence of one or two defects by using Green''s function method at the tight-binding approach. We study the effect of rotation of defect magnetic moment on the system conductance. The results show that in the magnetic wire، independent of existence or absence of defect، the allowed energy region shifts with amount of spin exchange parameters with respect to nonmagnetic case. By creation of defect and increasing its number in a ferromagnetic wire، the conductance with (without) spin-flip decreases (increases). Moreover، the conductance strongly depends on the rotation of magnetic moments of defect and this dependence becomes more detectable by increasing the number of defects.