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Proper Closed-form Representation for Site-specific Radio-wave Propagation
, Ph.D. Dissertation Sharif University of Technology ; Shishegar, Amir Ahmad (Supervisor)
Abstract
In this dissertation, a proper closed-form representation has been presented for the site specific indoor radio-wave propagation. Characteristic Green’s function (CGF) technique is used to obtain the integral representation of the Green’s function of a rectangular 2D room with dielectric slab walls. Based on numerical simulations, this method provide quite accurate results for wave propagation in the interior region (away from the walls) but the error increases for source or field points near the walls. The presented integral form of Green’s function has slow convergence and computed using Real images, complex images and rational function fitting methods. An error of less than 20 percent is...
Improvement and Expansion of Characteristic Green’s Function-Complex Images Method for Extraction of Green’s Function of Finite Dielectric Structures
, Ph.D. Dissertation Sharif University of Technology ; Shishegar, Amir Ahmad (Supervisor) ; Faraji-Dana, Reza (Co-Advisor)
Abstract
Finite dielctric structures are commonly used in optical devices, Microwave Integrated Circuitc (MICs) and printed antennas. To analyze these structures, full-wave analysis methods cannot be employed easily. They need huge computer resources and are time-consuming especially for electrically large structures. On the other hand, asymptotic techniques may not be exact enough for these structures. In this thesis, MPIE technique is chosen as an accurate and efficient technique for analyzing these structres. To use this technique, the magnetic vector potential and electric scalar potential are required. Uniform and closed-form spatial Green's function for finite dielectric structures is...
Nonlocal Kernel Functions for fcc and hcp Crystals with Application to Dislocation Problems
, Ph.D. Dissertation Sharif University of Technology ; Mohammadi Shodja, Hossein (Supervisor)
Abstract
For half a century, the problem of extracting the components of the nonlocal moduli tensor of anisotropic materials has been remained unsolved. In the present work, for the first time, the solution of this problem is proposed and the components of nonlocal moduli tensor are obtained for close-packed crystals, i.e. face center cubic or hexagonal closed packed. To this end, new distinct nonlocal kernel functions which have the characteristics of discrete atomistic Green’s functions in the stress space are obtained through consideration of the nonlocal dispersion relations. Each of dispersion relations are associated with certain directions and are combined with ab initio Density Functional...
Wind and Turbulence Effects on Long-range Sound Propagation in Troposphere Layer
, M.Sc. Thesis Sharif University of Technology ; Taeibi-rahni, Mohammad (Supervisor) ; Massah, Hamid Reza (Co-Advisor)
Abstract
Considering atmospheric wave propagation as a complex phenomenon, it can be represented as a function of variety of parameters such as: properties of atmosphere, boundary conditions, surface characteristics, source related parameters and etc. the aim of this work is to study the propagation of sound mechanisms in troposphere layer and numerical simulation of sound field in order to investigate the wind effect and turbulence in wave propagation. In this manner, the Green’s function parabolic equation (GFPE) is hired to solve the governing equation for sound propagation in a moving inhomogeneous atmosphere. To achieve this, a code is generated using MATLAB software to predict the long range...
Theoretical study of the electron transport through the cysteine amino acid nanomolecular wire
, Article International Journal of Nanoscience ; Volume 7, Issue 2-3 , 2008 , Pages 95-102 ; 0219581X (ISSN) ; Aghaie, H ; Gholami, M. R ; Sharif University of Technology
World Scientific Publishing Co. Pte Ltd
2008
Abstract
In this paper, we study the electrical transport and Negative Differential Resistance (NDR) in a single molecular conductor consisting of a cysteine sandwiched between two Au(111) electrodes via the Density Functional Theory-based Nonequilibrium Green's Function (DFT-NEGF) method. We show that (surprisingly, despite their apparent simplicity, these Au/cysteine/Au nanowires are shown to be a convenient NDR device) the smallest two-terminal molecular wire can exhibit NDR behavior to date. Experiments with a conventional or novel self-assembled monolayer (SAM) are proposed to test these predictions. The projected density of states (PDOSs) and transmission coefficients T(E) under various...
Elastodynamic Green’s functions of transversely isotropic n-layer half- and full-spaces subjected to a surface or buried time-harmonic annular loading and associated material degeneracy
, Article Journal of Engineering Mathematics ; Volume 136, Issue 1 , 2022 ; 00220833 (ISSN) ; Shodja, H. M ; Rashidinejad, E ; Sharif University of Technology
Springer Science and Business Media B.V
2022
Abstract
The elastodynamic behavior of layered media consisting of an arbitrary combination of isotropic and transversely isotropic layers is of great importance for many engineering applications. In this work, some appropriate elastodynamic Green’s functions pertinent to both the displacement and the stress fields are devised so that the problems associated with the n-layer semi-infinite and infinite media with any combinations of transversely isotropic and isotropic layers subjected to surface and buried oblique time-harmonic annular loading can all be treated in a unified manner. The material degeneracy arising due to the scenarios where one or more regions are isotropic is also discussed and...
The effect of structural defects on the electron transport of MoS 2 nanoribbons based on density functional theory
, Article Journal of Theoretical and Applied Physics ; Volume 13, Issue 1 , 2019 , Pages 55-62 ; 22517227 (ISSN) ; Fathipour, M ; Faez, R ; Darvish, G ; Sharif University of Technology
SpringerOpen
2019
Abstract
Using non-equilibrium Green’s function method and density functional theory, we study the effect of line structural defects on the electron transport of zigzag molybdenum disulfide (MoS 2 ) nanoribbons. Here, the various types of non-stoichiometric line defects greatly affect the electron conductance of MoS 2 nanoribbons. Although such defects would be lead to the electron scattering, they can increase the transmission of charge carriers by creating new channels. In addition, the presence of S bridge defect in the zigzag MoS 2 nanoribbon leads to more the transmission of charge carriers in comparison with the Mo–Mo bond defect. Also, we find that the different atomic orbitals and their...
The importance of electron correlation in graphene and hydrogenated graphene
, Article European Physical Journal B ; Volume 88, Issue 10 , October , 2015 ; 14346028 (ISSN) ; Jafari, S. A ; Sharif University of Technology
springer berlin
2015
Abstract
Local density approximation (LDA) and Green function effective Coulomb (GW) calculations are performed to investigate the effect of electronic correlations on the electronic properties of both graphene and graphane. The size of band gap in graphane increases from 3.7 eV in LDA to 4.9 eV in GW approximation. By calculating maximally localized Wannier wave functions, we evaluate the necessary integrals to get the Hubbard U and the exchange J interaction from first principles for both graphene and graphane. Our ab-initio estimates indicate that in the case of graphene, in addition to the hopping amplitude t ∼ 2.8 eV giving rise to the Dirac nature of low lying excitations, the Hubbard U value...
Near-room-temperature spin caloritronics in a magnetized and defective zigzag MoS2 nanoribbon
, Article Journal of Computational Electronics ; Volume 19, Issue 1 , 2020 , Pages 137-146 ; Fathipour, M ; Faez, R ; Darvish, G ; Sharif University of Technology
Springer
2020
Abstract
Using a tight-binding approach and first-principles calculations combined with the nonequilibrium Green’s function method, the thermal spin transport in a zigzag molybdenum disulfide (MoS 2) nanoribbon in the proximity of a ferromagnetic insulator that induces a local exchange magnetic field in the center of the nanoribbon is investigated. It is found that a pure spin current and perfect spin Seebeck effect with zero charge current can be generated by applying a thermal gradient and local exchange magnetic field without a bias voltage near room temperature. Furthermore, it is shown that this nanoscale device can act as a spin Seebeck diode for the control of thermal and spin information in...
Combination of characteristic Green's function technique and rational function fitting method for computation of modal reflectivity at the optical waveguide end-facet
, Article PHOTOPTICS 2015 - 3rd International Conference on Photonics, Optics and Laser Technology, Proceedings, 12 March 2015 through 14 March 2015 ; Volume 2 , 2015 , Pages 14-21 ; 9789897580932 (ISBN) ; Shishegar, A. A ; Institute for Systems and Technologies of Information, Control and Communication (INSTICC) ; Sharif University of Technology
SciTePress
2015
Abstract
A novel method for computation of modal reflectivity at optical waveguide end-facet is presented. The method is based on the characteristic Green's function (CGF) technique. Using separability assumption of the structure and rational function fitting method (RFFM), a closed-form field expression is derived for optical planar waveguide. The uniform derived expression consists of discrete and continuous spectrum contributions which denotes guided and radiation modes effects respectively. An optimization problem is then defined to calculate the exact reflection coefficients at the end-facet for all extracted poles obtained from rational function fitting step. The proposed CGF-RFFM-optimization...
Local moment formation in Dirac electrons
, Article Journal of Physics: Conference Series, 14 January 2015 through 15 January 2015 ; Volume 603, Issue 1 , January , 2015 ; 17426588 (ISSN) ; Mahyaeh, I ; Jafari, S. A ; Suzumura Y ; Matsuura H ; Fuseya Y ; Ogata M ; Sharif University of Technology
Institute of Physics Publishing
2015
Abstract
Elemental bismuth and its compounds host strong spin-orbit interaction which is at the heart of topologically non-trivial alloys based on bismuth. These class of materials are described in terms of 4x4 matrices at each v point where spin and orbital labels of the underlying electrons are mixed. In this work we investigate the single impurity Anderson model (SIAM) within a mean field approximation to address the nature of local magnetic moment formation in a generic Dirac Hamiltonian. Despite the spin-mixing in the Hamiltonian, within the Hartree approximation it turns out that the impuritys Green function is diagonal in spin label. In the three dimensional Dirac materials defined over a...
Electromagnetic wave scattering analysis from 2-D periodic rough surfaces using complex images technique
, Article IEEE Transactions on Geoscience and Remote Sensing ; Volume 53, Issue 2 , July , 2015 , Pages 862-868 ; 01962892 (ISSN) ; Shishegar, A. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2015
Abstract
The integral equation technique, in combination with the method of moments, is widely used for treating the electromagnetic scattering from periodic rough surfaces. This method, however, requires the computation of a kernel (a periodic Green's function) that consists of a slowly converging infinite series of conventional Green's functions. To accelerate the convergence of this series, different methods have been proposed in literature. In this paper, we use a complex image (CI) technique to find a closed-form expression for the periodic Green's function, which is then used to analyze the electromagnetic scattering from arbitrary periodic rough surfaces. The resulting CI Green's function...
Closed-form green's function of a 2-D rectangular room
, Article IEEE Transactions on Antennas and Propagation ; Volume 64, Issue 12 , 2016 , Pages 5291-5298 ; 0018926X (ISSN) ; Shishegar, A. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2016
Abstract
Closed-form enough-accurate Green's function for wave-propagation modeling in a rectangular 2-D room is presented using the characteristic Green's function technique. This Green's function is first developed in an integral form. Then, its spatial-domain closed form is obtained using three fast methods. In the first method, the integral is computed by the stationary phase method. In the second one, the integral is computed analytically by expanding the integrand into a complex exponential series. In the third method, the integrand is approximated by a series of rational functions and the integral is computed using Cauchy's residue theorem. The results of these three methods are studied and...
An efficient approach for reflectivity and coupling computation in an abruptly ended planar layered waveguide using modified formulation of CGF-RFFM
, Article German Microwave Conference, GeMiC 2016, 14 March 2016 through 16 March 2016 ; 2016 , Pages 128-131 ; 9783981266870 (ISBN) ; Shishegar, A. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2016
Abstract
An efficient method for computation of surface wave (SW) reflectivity and coupling in an abruptly ended planar layered waveguide is presented. Using characteristic Green's function (CGF) technique, the original structure is approximated by a separable structure. Then by applying rational function fitting method (RFFM) a uniform and closed-form expression of spatial Green's function for separable structure is derived. Having a full wave solution of the original structure, an optimization problem is defined to find the reflection coefficients of SW modes. Unlike the previous presented CGF based method called CGF-complex images (CI), CGF-RFFM leads to easy incorporation of radiation modes...
Spin FET based on graphene nanoribbon in the presence of surface roughness
, Article IEEE Transactions on Electron Devices ; Volume 64, Issue 8 , 2017 , Pages 3437-3442 ; 00189383 (ISSN) ; Faez, R ; Babaee Touski, S ; Pourfath, M ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2017
Abstract
In this paper, the characteristics of an armchair graphene nanoribbon spin FET (SFET) are investigated in the presence and absence of surface roughness, by employing a multiorbital tight-binding method along with the nonequilibrium Green's function approach. It is found that the bandgap monotonically decreases with increasing the vertical electric field, since Stark effect enhances spin-flip rate under a high vertical electric field. Furthermore, spin transport in the presence of a random potential, which is induced by the concurrent effect of the applied vertical electric field and surface roughness, is carefully analyzed. This random potential strongly scatters carriers and reduces spin...
Modeling of a vertical tunneling transistor based on graphene-mos2 heterostructure
, Article IEEE Transactions on Electron Devices ; Volume 64, Issue 8 , 2017 , Pages 3459-3465 ; 00189383 (ISSN) ; Faez, R ; Pourfath, M ; Darvish, G ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2017
Abstract
In this paper, for the first time, we present a computational study on the electrical behavior of the field-effect tunneling transistor based on vertical graphene-MoS2 heterostructure and vertical graphene nanoribbon-MoS2 heterostructure. Our simulation is based on nonequilibrium Green's function formalism along with an atomistic tight-binding (TB) model. The TB parameters are obtained by fitting the bandstructure to first-principle results. By using this model, electrical characteristics of device, such as I ON/I OFF ratio, subthreshold swing, and intrinsic gate-delay time, are investigated. We show that the combination of tunneling and thermionic transport allows modulation of current by...
Propagation and scattering analysis in a 2D rectangular room using closed-form Green's function
, Article 2016 8th International Symposium on Telecommunications, IST 2016, 27 September 2016 through 29 September 2016 ; 2017 , Pages 401-404 ; 9781509034345 (ISBN) ; Shishegar, A. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2017
Abstract
Wave propagation analysis inside a 2D rectangular room in the presence of an object is studied. Closed form Green's function of the empty room is derived and the problem is solved by implementing the Method of Moments. The Green's function is obtained by employing the Characteristic Green's Function (CGF) technique in conjunction with the Complex Images (CI) method. The proposed form is independent from location of transmitter/receiver and a single run of the procedure is sufficient to completely model the empty room. The performance of the presented method is studied in an example. © 2016 IEEE
Evaluating the capacity of indoor millimeter wave MIMO system using closed-form Green's function
, Article 4th International Conference on Millimeter-Wave and Terahertz Technologies, MMWaTT 2016, 20 December 2016 through 22 December 2016 ; 2017 , Pages 103-106 ; 21570965 (ISSN); 9781509054145 (ISBN) ; Sadrearhami, M. H ; Shishegar, A. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2017
Abstract
Indoor communication systems operating at millimeter wave frequencies are getting increasing attention due to their ability of having wider bandwidth and as a result conveying information at higher bit rates. At these systems, the capacity of channel is very important especially where there is no line of sight due to an obstacle. Here we examine the use of a multiple-input multiple-output (MIMO) system in a 2D room with an obstacle at 60 GHz. We estimate the channel by using the closed-form Green's function of the room and show the ability of MIMO to increase the available capacity of the system in the room chiefly behind the obstacle. © 2016 IEEE
Engineered nanopores-based armchair graphene nanoribbon fet with resonant tunneling performance
, Article IEEE Transactions on Electron Devices ; Volume 66, Issue 12 , 2019 , Pages 5339-5346 ; 00189383 (ISSN) ; Ahmadi, V ; Faez, R ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2019
Abstract
This article presents a novel armchair graphene nanoribbon (AGNR) field-effect transistor with engineered nanopores for resonant tunneling. Two rectangular nanopores are punched to create two potential barriers and one quantum well. Channel and source and drain contacts are AGNR, indicating structure homogeneity. Nonequilibrium Green's function and Poisson's equations are used for structural analysis. The input variables are well width (WW), drain voltage (VD), and barrier width (BW). The effects of repositioning nanopores and AGNR type (i.e., semiconductor and semimetal) are also studied. The impact of the parameters on the density of states, transmission probabilities, peak current (IP)...
Recent advances in spatial analog optical computing
, Article 5th International Conference on Millimeter-Wave and Terahertz Technologies, MMWaTT 2018, 18 December 2018 through 20 December 2018 ; Volume 2018-December , 2019 , Pages 6-11 ; 21570965 (ISSN); 9781538677179 (ISBN) ; Karimi, P ; Khavasi, A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2019
Abstract
Traditional analog computers that perform mathematical operations electronically or mechanically suffer from their relatively large size and slow response. Recently, the idea of spatial analog optical computing has overcome these restrictions. The different techniques to implement spatial analog optical computation can be categorized into two fundamental approaches: (I) metasurface (MS) approach and (II) Green's function (GF) approach. In the first approach, a metasurface is designed to implement the Green's function of the desired operator in the spatial domain. This means that this approach needs two sub-blocks to perform Fourier and inverse Fourier transform. On the other hand, in the...