نشریه پژوهش سیستم های بس ذره ای
سال سیزدهم شماره 1 (پیاپی 36، بهار 1402)

In this paper, the compatibility of the Dirac cosmological model which leads to gravitational theories with a varying gravitational coupling parameter in the context of the generalized Rastall theory has been studied. Moreover, in this scenario, the problem of the current accelerated expansion of the Universe has been considered. Two different cosmic fluids have been investigated. Generalized Polytropic matter, which leads to an observationally consistent model. Also, considering the generalized Chaplygin gas, it is shown that an astronomically consistent model cannot be obtained. Briefly speaking, we came to the result that the generalized Rastall theory is capable of being matched with the Dirac hypothesis leading to the present accelerated expansion of the Universe.

In this research, using the characteristic matrix theory, the three-layer V2O5/Ag/WO3 (VAW) electrode as a transparent conductive structure is designed and its optimal structure is determined. Then, a perovskite solar cell based on a VAW three-layer electrode with Glass/V2O5/Ag/WO3/PEDOT:PSS/CH3NH3PbI3/PCBM/Al structure is considered and the optical properties of the perovskite solar cell are investigated using the transfer matrix method (TMM). In the following, it is shown that the effects of changing the thickness of the WO3 layer on the optical properties of the three-layer VAW and the perovskite solar cell based on the VAW electrode are more significant than the effects of changing the thickness of the layer V2O5. The maximum short circuit current density (19.3 mA/cm2) in the perovskite solar cell based on the VAW electrode was obtained for the thicknesses of the V2O5 layer of 40 nm and WO3 layer of 55 nm, which is 12.2% higher than the calculated value for perovskite solar cell based on ITO electrode (17.2 mA/cm2). Finally, the perovskite solar cell based on the optimized VAW electrode was fabricated and the results showed that this solar cell has better performance than solar cell based on the commercial ITO electrode.

In this study, we consider the effects of the cosmic rays and the magnetic diffusion on the thermal instability in the interstellar mediums. The dynamical effect as well as the heating effect of the cosmic rays are investigated on the thermal condensation in a medium where the cosmic rays diffuse along the magnetic field lines and the magnetic diffusion is present. Two separate profiles, i.e. constant field and force-free field, are considered for the magnetic field of the medium background. The linear perturbation analysis is used for the study of the thermal instability in a medium with a point of view on the previous works. The results show that the cosmic rays reduce the growth rate of the magnetothermal condensation mode, and increase the domains of stability. However, the magnetic diffusion shows two different behaviors which depend on a number of conditions affecting the magnetic diffusivity. In other words, the magnetic diffusion increases the domains of stability for cases less than a certain value of magnetic diffusivity, and decrease the domains of stability for cases larger than this certain value. Furthermore, we found that the dynamical effect of the magnetic diffusion dominates the dynamical effect of the cosmic rays. Finally, the results address some regions in which the fragmentation of the clouds into clumps and cores can be seen with the magnetothermal condensation effects.

We study the extended scalar-tensor Gauss-Bonnet. First, we review a model in which there is a coupling between the Gauss-Bonnet scalar and the scalar field. Then we extend the model to a hybrid model with two scalar fields. We study the conditions under which the solutions are unstable and spontaneous scalarization occurs. We show that the coupling coefficients in the hybrid state and in the presence of a scalar field are asymptotically the same but the sufficient condition of instability is different between these two models.

In the present work, we show how to visualize a many-electron Hamiltonian in the matrix form. This procedure is very important for finding eigenvalues of energy and eigenvectors. Here, the many-particle Hamiltonian in a crystal includes electron-nucleus effects and electron kinetic energy (one-point operators) as well as electron-electron mutual repulsion (two-point operators). It is thoroughly discussed how to reduce a many-particle problem to a two-particle problem. Starting from the antisymmetric wave functions as space bases, the Hamiltonian matrix was obtained in a graphical representation-based way. Each step of forming the components of the Hamiltonian matrix is explained instructively. Results show that the kinetic energy of electrons and the electron-nucleus interaction construct the diagonal components of the Hamiltonian matrix. In representing electron-electron interactions, we employ diagonal elements that do not require the exchange of wave functions over two particles, but off-diagonal components incorporate the effect of exchanging wave functions over the particles. In this study, since we were looking for an exact solution, we used a three-electron system, but the method used can be extended to larger systems. Also, the code written for the Hamiltonian matrix creation process can be provided to researchers in this field.

Graphene oxide/cobalt nanocomposite was synthesized using the electrooxidation method and its structural and magnetic properties were specified. The effect of the produced nanocomposite on the rheological properties of drilling fluids containing bentonite and natural gum was investigated. The nanocomposite was characterized using XRD, FT-IR, SEM and VSM. XRD results confirmed the formation of graphene oxide/cobalt nanocomposite. The SEM images show the scaly shape of cobalt nanoparticles substituted on the graphene oxide sheets. Based on VSM results, the saturation magnetization, remance magnetization and coercive field were determined to be 167.04 emu/g, 9.94 emu/g, and 142.5 Oe, respectively. In order to investigate the effect of graphene oxide/cobalt nanocomposite on the rheological properties of two types of fluids containing bentonite and natural gum, 0.1, 0.3, and 0.5 gr of nanocomposite were added to drilling fluids and their rheological properties were measured using an eight-speed viscometer at three temperatures of 23oC, 50oC and 70oC. Graphene oxide/cobalt nanocomposite in the bentonite samples increases the apparent viscosity, plastic viscosity, yield point, and gel strength up to 108 cp, 17 cp, 165 lb/100ft2, and 170 lb/100ft2, respectively, and decreases these parameters in the samples containing natural gum down to 70 cp, 32 cp, 42 lb/100ft2, and 5 lb/100ft2, respectively.