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green-s-function
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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...
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...
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...
Using superlattice structure in the source of GNRFET to improve its switching performance
, Article IEEE Transactions on Electron Devices ; Volume 67, Issue 3 , 2020 , Pages 1334-1339 ; Faez, R ; Shahhoseini, A ; Moravvej Farshi, M. K ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2020
Abstract
Our aim is to improve the switching performance of the graphene nanoribbon field-effect transistors (GNRFETs), exploiting the concept of energy filtering. Within the proposed scheme, a superlattice (SL) structure is used in the source of the transistor for filtering high-energy electron tail by engineering the density of states (DOS). According to simulation results, this can significantly decrease the OFF-current and the subthreshold swing (SS). A comparison of the proposed device with a conventional GNRFET and a graphene nanoribbon (GNR) tunneling field-effect transistor (GNRTFET) demonstrates a significant improvement. Therefore, a typical SL-GNRFET can reduce the average and the minimum...
Two-dimensional quantum simulation of scaling effects in ultrathin body MOSFET structure: NEGF approach
, Article 14th International Workshop on the Physics of Semiconductor Devices, IWPSD, Mumbai, 16 December 2007 through 20 December 2007 ; 2007 , Pages 240-242 ; 9781424417285 (ISBN) ; Dehdashti, N ; Faez, R ; Sharif University of Technology
2007
Abstract
For the first time, we present self-consistent solution of ultrathin body device structures to investigate the device parameters variation on the characteristics of nanoscale MOSFET. Our two dimensional (2-D) device simulator Is based on Nonequlibrium Green's Function (NEGF) forma lism. Starting from a basic structure (DG-MOSFET) with a gate length of 10 nm, variation of gate length, channel thickness, gate oxide parameters was carried out in connection with the numerical calculation of device characteristics. In this work Quantum transport equations are solved in 2-D by NEGF method in active area of the device to obtain the charge density and Poisson's equation is solved in entire domain of...
Two-dimensional numerical solution of steady withdrawal from the lens of freshwater in a tropical Island
, Article 2006 ASME Joint U.S.- European Fluids Engineering Division Summer Meeting, FEDSM2006, Miami, FL, 17 July 2006 through 20 July 2006 ; Volume 1 SYMPOSIA , 2006 , Pages 627-633 ; 0791847500 (ISBN); 9780791847503 (ISBN) ; Mohammadi, M. H ; Saeedpanah, I ; Sharif University of Technology
American Society of Mechanical Engineers
2006
Abstract
The work presented here is a study of the steady withdrawal of water from the lens of freshwater situated above the ocean's salt water and within the island. It is the aim of this paper to investigate the process of withdrawal from the lens of freshwater with a view to establishing the critical flow values for withdrawal and the effects of sink location and density differences on these values, and also to determine the effects of relative density differences.Steady solutions are found for the shape of the interface between salt and freshwater beneath a tropical island. A Green_s function approach is used and proves to be much more robust than spectral methods. Computations of the surface...
Two-dimensional numerical solution of steady withdrawal from the lens of freshwater in a tropical island
, Article 2006 2nd ASME Joint U.S.-European Fluids Engineering Summer Meeting, FEDSM 2006, Miami, FL, 17 July 2006 through 20 July 2006 ; Volume 2006 , 2006 ; 0791837831 (ISBN); 9780791837832 (ISBN) ; Haji Mohammadi, M ; Saeedpanah, I ; Sharif University of Technology
2006
Abstract
The work presented here is a study of the steady withdrawal of water from the lens of freshwater situated above the ocean's salt water and within the island. It is the aim of this paper to investigate the process of withdrawal from the lens of freshwater with a view to establishing the critical flow values for withdrawal and the effects of sink location and density differences on these values, and also to determine the effects of relative density differences. Steady solutions are found for the shape of the interface between salt and freshwater beneath a tropical island. A Green_s function approach is used and proves to be much more robust than spectral methods. Computations of the surface...
Three-dimensional dynamic Green's functions in transversely isotropic tri-materials
, Article Applied Mathematical Modelling ; Volume 37, Issue 5 , March , 2013 , Pages 3164-3180 ; 0307904X (ISSN) ; Rahimian, M ; Eskandari, M ; Sharif University of Technology
2013
Abstract
An analytical derivation of the elastodynamic fundamental solutions for a transversely isotropic tri-material full-space is presented by means of a complete representation using two displacement potentials. The complete set of three-dimensional point-load, patch-load, and ring-load Green's functions for stresses and displacements are given, for the first time, in the complex-plane line-integral representations. The formulation includes a complete set of transformed stress-potential and displacement-potential relations in the framework of Fourier expansions and Hankel integral transforms, that is useful in a variety of elastodynamic as well as elastostatic problems. For the numerical...
Three-dimensional dynamic Green's functions for a multilayered transversely isotropic half-space
, Article International Journal of Solids and Structures ; Volume 48, Issue 9 , 2011 , Pages 1349-1361 ; 00207683 (ISSN) ; Rahimian, M ; Eskandari, M ; Pak, R. Y. S ; Sharif University of Technology
2011
Abstract
With the aid of a method of displacement potentials, an efficient and accurate analytical derivation of the three-dimensional dynamic Green's functions for a transversely isotropic multilayered half-space is presented. Constituted by proper algebraic factorizations, a set of generalized transmission-reflection matrices and internal source fields that are free of any numerically unstable exponential terms are proposed for effective computations of the potential solution. Three-dimensional point-load Green's functions for stresses and displacements are given, for the first time, in the complex-plane line-integral representations. The present formulations and solutions are analytically in exact...
Thermally induced spin-dependent current based on Zigzag Germanene Nanoribbons
, Article Physica E: Low-Dimensional Systems and Nanostructures ; Volume 86 , 2017 , Pages 175-183 ; 13869477 (ISSN) ; Faez, R ; Sharif University of Technology
Elsevier B.V
2017
Abstract
In this paper, using first principle calculation and non-equilibrium Green's function, the thermally induced spin current in Hydrogen terminated Zigzag-edge Germanene Nanoribbon (ZGeNR-H) is investigated. In this model, because of the difference between the source and the drain temperature of ZGeNR device, the spin up and spin down currents flow in the opposite direction with two different threshold temperatures (Tth). Hence, a pure spin polarized current which belongs to spin down is obtained. It is shown that, for temperatures above the threshold temperature spin down current increases with the increasing temperature up to 75 K and then decreases. But spin up current rises steadily and in...
Thermal conductivity of anisotropic spin-1/2 two leg ladder: Green's function approach
, Article European Physical Journal B ; Vol. 87, issue. 8 , Oct , 2014 ; Langari, A ; Van Loosdrecht, P. H. M ; Zotos, X ; Sharif University of Technology
Abstract
We study the thermal transport of a spin-1/2 two leg antiferromagnetic ladder in the direction of legs. The possible effect of spin-orbit coupling and crystalline electric field are investigated in terms of anisotropies in the Heisenberg interactions on both leg and rung couplings. The original spin ladder is mapped to a bosonic model via a bond-operator transformation, where an infinite hard-core repulsion is imposed to constrain one boson occupation per site. The Green's function approach is applied to obtain the energy spectrum of quasi-particle excitations responsible for thermal transport. The thermal conductivity is found to be monotonically decreasing with temperature due to increased...
Theoretical study of the electron transport through the cysteine amino acid nanomolecular wire
, Article International Journal of Nanoscience ; Volume 07, Issue 02, n 03 , April & June , 2008 , pp. 95-102 ; Aghaie, H ; Gholami, M. R.(Mohammad Reza) ; Sharif University of Technology
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...
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...
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...
The Gaussian expansion of the Green's function of an electric current in a parallel-plate waveguide
, Article 2008 IEEE International RF and Microwave Conference, RFM 2008, Kuala Lumpur, 2 December 2008 through 4 December 2008 ; April , 2008 , Pages 46-48 ; 9781424428663 (ISBN) ; Shishegar, A. A ; Sharif University of Technology
2008
Abstract
In this paper, a novel closed form expression is derived to find the Green's function of a horizontal electric current in a parallel-plate waveguide. It is achieved by expanding the Green's function into a series of Gaussian functions. This new method is called the Gaussian Green's function (GGF) method. The main advantage of the GGF method lies in its precision as well as rapid convergence. Numerical results confirm that the closed form expression yields less than 0.2% error compared to the numerical integration of the spectral integral. Furthermore, it is verified that this method can be in excellent agreement with the complex images (CI) method. © 2008 IEEE
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...
Stable semi-analytical method for analysis of plasmonic propagation on periodically patterned metal plates
, Article Proceedings of SPIE - The International Society for Optical Engineering, 26 January 2010 through 28 January 2010 ; January , 2010 ; 0277786X (ISSN) ; 9780819480057 (ISBN) ; Khorasani, S ; Karami Taheri, H ; Rashidian, B ; Hosseini, A ; Sharif University of Technology
2010
Abstract
The need for antennas with improved characteristics for communication and radar applications has resulted in an ever-increasing demand for research in the field of high impedance surfaces, which can work as an artificial magnetic conductor. One method in fabrication of these surfaces is formation of a metamaterial by patterning a metallic surface in the shape of space filling curves (e.g. Hilbert or Peanu Curves). In this paper, we present a novel semi-analytical solution to the problem of plasmonic propagation on these surfaces. The method is based on a previously presented Green's function formalism, which has been reported in an earlier paper of ours. We have modified and improved the...
Spin relaxation in graphene nanoribbons in the presence of substrate surface roughness
, Article Journal of Applied Physics ; Volume 120, Issue 5 , 2016 ; 00218979 (ISSN) ; Babaee Touski, Sh ; Pourfath, M ; Faez, R ; Sharif University of Technology
American Institute of Physics Inc
Abstract
In this work, spin transport in corrugated armchair graphene nanoribbons (AGNRs) is studied. We survey combined effects of spin-orbit interaction and surface roughness, employing the non-equilibrium Green's function formalism and multi-orbitals tight-binding model. Rough substrate surfaces have been statistically generated and the hopping parameters are modulated based on the bending and distance of corrugated carbon atoms. The effects of surface roughness parameters, such as roughness amplitude and correlation length, on spin transport in AGNRs are studied. The increase of surface roughness amplitude results in the coupling of σ and π bands in neighboring atoms, leading to larger spin...
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...