Sharif Digital Repository

We study the competition between spin and charge fluctuations of the extended Hubbard model with on-site and dipole-dipole interactions in a one-dimensional lattice. Using the extended two-particle self consistent (ETPSC) method, we find the corresponding expressions for spin and charge response functions. In this approach, the irreducible spin and charge vertices are a function of inter-particle distance (r) and wave-number (q). This theory allows us to determine the crossover temperatures and the dominant instability as a function of U and V. The phase diagrams are obtained for several effective particle densities: n = 0.5, n = 1 and n = 4/3. Each phase diagram (U − V − T space)... 

Cavity quantum electrodynamics of multipartite systems are studied in depth, which consists of an arbitrary number of emitters in interaction with an arbitrary number of cavity modes. The governing model is obtained by taking the full field-dipole and dipole-dipole interactions into account, and is solved in the Schrödinger picture with assumption of vanishing field and dipole interactions at high energies. An extensive code is developed that is able to solve the system and track its evolution in time, while maintaining sufficient degrees of arbitrariness in setting up the initial conditions and interacting partitions. Using this code, we have been able to numerically evaluate various... 

The authors propose a novel solution for decorrelating closely spaced antennas with identical radiation patterns. By proper steering of the directional elements' patterns, the decorrelation distance is reduced to 0.25λ in comparison with the well-established 0.4λ result based on the Bessel-shaped correlation expression. In addition, by proposing the definition of Decorrelation Factor of an antenna pattern, a framework for comparison of correlation behaviour of different antennas is provided. Subsequently, a performance metric characterised by the Jensen upper bound for the capacity of closely spaced antennas is proposed. Finally, we apply our results to two practical directional antennas,... 

The cavity quantum electrodynamics of various complex systems is here analyzed using a general versatile code developed in this research. Such quantum multi-partite systems normally consist of an arbitrary number of quantum dots in interaction with an arbitrary number of cavity modes. As an example, a nine-partition system is simulated under different coupling regimes, consisting of eight emitters interacting with one cavity mode. Two-level emitters (e.g. quantum dots) are assumed to have an arrangement in the form of a linear chain, defining the mutual dipole-dipole interactions. It was observed that plotting the system trajectory in the phase space reveals a chaotic behavior in the... 

A compact planar wideband microstrip antenna is introduced. The antenna is a log-periodic antenna with 7 dipole elements. The antenna is designed to function in the frequency range of 500 to 700 MHz. It has a dimension of 268 mm by 145 mm by 3 mm. The matching is better than -10 dB in the working bandwidth. The proposed antenna generates linear polarization. Its maximum gain is 4.5 dB at the central frequency. It provides a low-cost alternative to the conventional Yagi-Uda TV antennas. © 2007 IEEE  

MIMO system's performance depends greatly on the correlation of employed antennas. The more the correlation is, the worse the performance will be. We propose a solution to the problem of decorrelating closely spaced antennas assuming the antennas have identical radiation patterns with the same steering angles. In fact, we will show that by proper steering of the antenna patterns, the decorrelation distance is reduced to 0.25λ which is a great improvement comparing with the well-established 0.4λ result coming from the Bessel-shaped correlation expression. Also, we analyze this result in the regime of very closely placed antennas and by proposing the definition of Decorrelation Factor of an... 

The noncommutative dipole QED is studied in detail for the matter fields in the adjoint representation. The axial anomaly of this theory is calculated in two and four dimensions using various regularization methods. The Ward-Takahashi identity is proved by making use of a nonperturbative path integral method. The one-loop β-function of the theory is calculated explicitly. It turns out that the value of the β-function depends on the direction of the dipole length L, which defines the noncommutativity. Finally using a semiclassical approximation a nonperturbative definition of the form factors is presented and the anomalous magnetic moment of this theory at one-loop order is computed  

Interpreting the complex interaction of nanostructured fluid flow with a dipole in a duct, with peripherally uniform temperature distribution, is the main focus of the current work. This paper also sheds light on the changes in the Nusselt number, temperature profiles, and velocity distributions for the fully developed nanofluid flow in a vertical rectangular duct due to a dipole placed near a corner of the duct. A finite volume approach has been incorporated for the numerical study of the problem. It is interesting to note the unusually lower values of the Nusselt number for the higher values of the ratio Gr/Re. Due to the nanostructure in the fluid, an enhancement in the Nusselt number has... 

Equations of state based on the statistical associating fluid theory for potentials of variable range (SAFT-VR) and the perturbed chain statistical associating fluid theory (PC-SAFT) have been used to model the PVT behavior of ionic liquids and the solubility of H2S in six imidazolium-based ionic liquids. The studied systems included [bmim][PF6], [hmim][PF6], [bmim][BF4], [hmim][BF4], [bmim][NTF2] and [hmim][NTF2] at various temperatures and pressures.For pure components, parameters of the models have been obtained by fitting the models to experimental data on liquid densities; the average relative deviation between the calculated and experimental densities for ionic liquids is less than... 

Controlled rotation of a suspended soap water film, simply generated by applying an electric field, has been reported recently. The film rotates when the applied electric field exceeds a certain threshold. In this study, we investigate the phenomenon in films made of a number of other liquids with various physical and chemical properties. Our measurements show that the intrinsic electrical dipole moments of the liquid molecules seems to be vital for the corresponding film rotation. All the investigated rotating liquids have a molecular electric dipole moment of above 1 Debye, while weakly polar liquids do not rotate. However, the liquids investigated here cover a wide range of physical... 

This study describes the concept and design of a low-cost super broadband dipole antenna with a passive matching network. The proposed dipole antenna is designed in three steps. First, a simple planar dipole antenna with six different loads and a balanced to unbalanced (Balun) transformer is designed by the genetic algorithm optimisation. The position and value of the loads are optimised to achieve an acceptable radiation pattern and voltage standing wave ratio (VSWR) over the desired frequency bandwidth. In the next step, the shape of the printed strip dipole antenna is optimised to improve the frequency response of the designed antenna. Finally, an LC network is added to the antenna feed... 

A fuzzy controller has been suggested for attitude control of magnetic actuated satellites in order to calculate the desired mechanical torque based on the attitude error including error in the angles and their rates. The problem of selecting proper magnetic dipole based on the known desired mechanical torque has been investigated, two different methods for the purpose has been suggested and the performance of the attained fuzzy magnetic attitude controllers has been shown under two different simulated conditions. © 2009 IEEE  

We propose a dielectric structure which focuses laser light well beyond the diffraction limit and thus considerably enhances the exerted optical trapping force upon dielectric nanoparticles. Although the structure supports a Fabry–Perot resonance, it actually acts as a nanoantenna in that the role of the resonance is to funnel the laser light into the structure. In comparison with the lens illuminating the structure, the proposed structure offers roughly a 10,000-fold enhancement in the trapping force—part of this enhancement comes from an 80-fold enhancement in the field intensity, whereas the remaining comes from a 130-fold enhancement in the normalized gradient of the field intensity... 

A feasible and low cost approach to implement a nonplanar dual-polarized log-periodic dipole array antenna using printed circuit board (PCB) technology is proposed. The proposed antenna utilizes a very short transition from a coaxial cable to a balanced twin-lead transmission line to realize an ultrawide bandwidth balun, and has two distinct scaling factor values for high and low frequency elements. A prototype of the antenna operating over the frequency range of 1-13 GHz was designed, fabricated, and tested. An average gain of 7 dBi is obtained. Measured VSWR, cross-polarization and front-to-back ratio are better than 2.6, -16 dB, and 22 dB, respectively  

An efficient and low cost approach to realise a non-planar dual-polarised log-periodic dipole array antenna (LPDA) using printed circuit board technology is proposed which achieves a high bandwidth and operational frequency. The proposed antenna utilises a very short transition from a coaxial cable to a balanced four-conductor transmission line to realise an ultra-wide bandwidth balun and input impedance matching. An analysis of the utilised four-conductor transmission line alongside its comparison to two-conductor transmission line of a planar LPDA is also presented. Two prototypes of the antenna operating over the frequency ranges of 1-13 GHz and 1-18 GHz were designed, fabricated, and... 

The direction of arrival (DOA) error in dipole 3D arrays is estimated through Monte-Carlo simulations using the standard MUSIC algorithm. Novel 3D geometries are implemented which demonstrate better precision given the same lateral area. The mutual coupling effect is included by changing the search vector in the MUSIC DOA algorithm for the novel 3D geometries. A simple straight forward method is used which does not need complex electromagnetic computations. Simulations show that the proposed method can successfully account for the mutual coupling effects in two dimensional direction finding (both azimuth and elevation angle) with novel 3D array geometries. Reduced DOA estimation error is...