مجله فیزیک کاربردی
سال یازدهم شماره 4 (پیاپی 27، زمستان 1400)

In this work, superposition of spin coherent states composed of two qubits is formed and by calculating the geometric measure of mixing of the system introduced in this work, the degree of mixedness of the studied state is investigated. In the following, the changes in the entanglement of the introduced state are studied by using the negativity measure. Finally, by drawing the appropriate plots, according to the influential parameters in this system, we examine the maximum and the minimum of the changes in the mixedness and entanglement and also their relationship to the mixed system consisting of two-qubit spin coherent states. It was observed that for maximizing α, when the changes of parameters indicate the purity of the state, the entanglement of the system reaches its maximum value for two-qubit systems. Also, for all values of coherence parameter, the entanglement decreases when mixedness of the system increases.

In this work, the structural, electronic, and optical properties of bulk and monolayer of Iron dichalcogenides FeX2 (X= S, Se, Te) have been investigated using the full potential linearized augmented plane wave (FP-LAPW) in the framework of density functional theory (DFT) with Wien2k simulation package. The calculated results show that FeX2 compounds in the bulk structure are non-magnetic semiconductors with a direct gap at the Γ point, while the monolayer compounds are ferromagnetic with metallic character. The band structure and energy gap of bulk and monolayer structures of FeX2 are calculated using GGA-PBE and GGA-mbj approximations that Becke-Johnson functional gives us better results for band gaps. All-optical properties such as real and imaginary parts of the dielectric function, absorption and reflection coefficients, refractive and extinction index, conductivity, and electron energy loss spectrum have been calculated and analyzed for bulk and monolayer. The High amplitude and wide absorption coefficient in the visible and ultraviolet region make these compounds a good candidate for use in photoelectric instruments and solar cells. Since monolayer compounds show magnetic properties, all calculations for monolayer compounds are performed in the spin-polarized form.

Resistive plate chamber (RPC) is a gaseous particle detector primarily developed for particle physics experiments and found vast applications in industry. We have constructed a prototype of a single gap glass RPC, with gap width of 2 mm. For simplicity, this prototype has a single 10×10 cm2 Al pad to readout the detector’s signals. An electronic board is designed and built at our laboratory to receive, amplify and register pulses as counts per unit time at the computer. In this study, we have used the count rate (noise) as an indicator of the detector's performance. We observed that the count rate reduced in the presence of Fe shields above the detector, due to the absorption of particles related to the cosmic rays. We also studied the sensitivity of the detector to the 60 keV gammas of the Am source. Although an increase in the count rate in the presence of the 241Am source is evident, the efficiency of the detector to 60 keV gammas is very small. All of the measurements are performed at several high voltages between 1 kV and 3 kV.

In this paper, a 4 to 1 multiplexer circuit is designed with four quantum rings where each ring is threaded by a constant magnetic flux  .  The quantum rings are connected to each other in series. They are attached symmetrically to two semi-infinite one-dimensional metallic electrodes, namely, source and drain, and four gate voltages are applied to the specific atomic sites of the quantum rings as four inputs of the multiplexer and also two other gate voltages are applied as the select lines. The Hamiltonian of the full system, i.e., the quantum rings, source and drain, are approximated by the tight-binding model, and the calculations are performed by using the Green’s function formalism for the strong and weak coupling between the quantum rings and the source and drain electrodes. The drain output current is calculated by using the Landauer formula as a function of the applied bias voltage. The truth tables of the multiplexer are obtained by assigning the 0 and 1 values to zero and non-zero drain current for different values of the data inputs on the basis of the values of the selected lines. It is found that this quantum structure behaves as a binary 4 to 1 multiplexer.

In this study, the detection method of entanglement of 3-mode spin coherence states is investigated. For this purpose, by properly defining the computational codes, these states are mapped to a three qubit quantum state, and then, by using the Mermin- Kalyshko inequality, the entanglement of these states is studied and an analytical relationship to determine the entanglement region is presented. The results of numerical analysis of the extracted inequality show that the entanglement region of these states depends on the coherence parameters, the amount of spin as well as the phase of the states. For example, in the case where the values of the coherence parameters are equal but with opposite signs, by adjusting the coherence phase, the degree of entanglement can be controlled such that the maximum entanglement occurs . More important, as the value of spin increases, the allowable range of the coherence parameter for entanglement detection increases. These results are consistent with the data reported in the study of the degree of entanglement of 2-mode superposition of spin coherent states using other measures and criteria of entanglement.  The findings of this study can be used in the study of non-classical and quantum systems and quantum correlations in quantum information science.

Using a two-dimensional particle-in-cell simulation of collision-less plasmoid instability, dynamics of formation and growth of magnetic islands are investigated. In the extended nonlinear magnetic reconnection process, the electric current layer undergoes a nonlinear deformation. This perturbation shows itself in the form of a cut in the electrical current layer, which is called a point. This phenomenon creates magnetic islands or plasmoids on either side of the point. Plasmoids are structures with a high mass density that are formed from the output flow of the plasma. In this study, different properties of plasmoid instability such as temperature, electric field vector, and stages of formation and growth are investigated by particle-in-cell simulation. The effect of a constant perpendicular magnetic field (guide field) on the apparent deformation of the electric current sheet and the coalescence rate of the plasmoid is investigated. The presence of a constant guide field reduces the number of magnetic reconnections and also reduces the process rate of plasmoid coalescence. The ratio between the conduction field and the initial magnetic field determines the number of magnetic reconnections. So that reduces the number of  points to three and increases guide field up to  prevent the formation of plasmoid instability and keeps the system in the magnetic reconnection state.