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First Principles Studies of Mechanical, Physical, and Electronic Properties of a-Si - also, Diffusion of a Self-interstitial Atom in an Ultra-thin fcc Film Via Lattice Statics
, Ph.D. Dissertation Sharif University of Technology ; Mohammadi Shodja, Hossein (Supervisor)
Abstract
By employing first principles density functional theory-based (DFT) molecular dynamics (MD), the influences of dangling and floating bonds as well as distorted tetrahedral bonds are studied on the mechanical, physical, and electronic properties of amorphous Si (a-Si). For further examination of the effects of these geometrical defects, two distinct amorphous samples, namely as-quenched and annealed are generated and examined. To verify the validity of the representative cells, the obtained radial distribution function, pair correlation function, and cohesive energy are compared with those corresponding results presented in the literature. Moreover, the surface energy is calculated at final...
The Effect of Pressure on the Coexistence of Superconductivity and Magnetism in RuSr2GdCu2O8 and RuSr2Gd1.4Ce0.6Cu2O10-δ
, Ph.D. Dissertation Sharif University of Technology ; Akhavan, Mohammad (Supervisor)
Abstract
The coexistence of long-range magnetic order and superconductivity in the ruthenocuprate families, Ru1212 and Ru1222 has been studied both theoretically and experimentally. Historically these two different phases are incompatible with each other and in the most previous research reported on the coexistence of these phases, there have been observed separated magnetic and superconducting phases. However in the ruthenocuprate families, there is a single phase with both magnetic and superconducting phase which coexist with each other. It has been determined that superconductivity arises in CuO2 planes, and magnetic orders occur in RuO2. From experimental point of view, we have investigated the...
First-principle electronic structure study of Ni-doped BaFe2-xNixAs2 (x = 0, 1, 2) superconductor
, Article Physica C: Superconductivity and its Applications ; Vol. 506, issue , 2014 , p. 151-153 ; Khosroabadi, H ; Abolhassani, M. R ; Akhavan, M ; Sharif University of Technology
Abstract
The electronic structure of BaFe2-xNixAs2 (x = 0, 1, 2) as a function of Ni doping has been investigated. Electronic density of states and the band structures are calculated within the first-principle density functional theory for non-magnetic phase. Pseudopotential quantum espresso code in the generalized gradient approximation has been used. Lattice and ionic position parameters of the system have been taken from the experimental data and have been optimized to find the equilibrium structure parameters. The electronic structure is characterized by a sharp Fe/Ni3d peak close to the Fermi level and is dominated by Fe/Ni3d and As4p hybridized states similar to the other Fe-based...
Sparsity and infinite divisibility
, Article IEEE Transactions on Information Theory ; Volume 60, Issue 4 , 2014 , Pages 2346-2358 ; ISSN: 00189448 ; Unser, M ; Sharif University of Technology
Abstract
We adopt an innovation-driven framework and investigate the sparse/compressible distributions obtained by linearly measuring or expanding continuous-domain stochastic models. Starting from the first principles, we show that all such distributions are necessarily infinitely divisible. This property is satisfied by many distributions used in statistical learning, such as Gaussian, Laplace, and a wide range of fat-tailed distributions, such as student's-t and α-stable laws. However, it excludes some popular distributions used in compressed sensing, such as the Bernoulli-Gaussian distribution and distributions, that decay like exp (-O(|x|p)) for 1 < p < 2. We further explore the implications of...
A comparative study on hydrogen interaction with defective graphene structures doped by transition metals
, Article Physica E: Low-Dimensional Systems and Nanostructures ; Vol. 60 , June , 2014 , pp. 104-111 ; ISSN: 13869477 ; Saboohi, Y ; Sharif University of Technology
Abstract
In the present work, the interaction of hydrogen molecules with defective graphene structures doped by transition metal (TM) atoms is investigated by using first principles density functional theory (DFT). Defective graphene structures include Stone-Wales (SW), 585 and 555-777 and transition metals include early TMs, i.e. scandium (Sc), titanium (Ti) and vanadium (V). It is found that in comparison with the pristine graphene, presence of defects significantly enhances the metal binding. Among three defects, 585 divacancy leads to the strongest binding between graphene and metal. Hydrogen adsorption is then evaluated by sequential addition of hydrogen molecules to the system. The results...
On the proof of the first Carnot theorem in thermodynamics
, Article European Journal of Physics ; Volume 34, Number 6 , 2013 , Pages 1581-1588 ; 01430807 (ISSN) ; Momeni, F ; Sharif University of Technology
2013
Abstract
The proof of the first Carnot theorem in classical thermodynamics is revisited in this study. The underlying conditions of a general proof of this principle presented by Senft (1978 Phys. Educ. 13 35-37) are explored and discussed. These conditions are analysed in more detail using a physical description of heat and work to present a simpler proof of the first principle prior to using the violation of the second law of thermodynamics. Finally, a new simple proof is also presented based on Gibbs relation. This discussion will benefit the teaching of classical thermodynamics and promote better understanding of the proof of the first Carnot theorem in general form
A combined first principles and analytical determination of the modulus of cohesion, surface energy, and the additional constants in the second strain gradient elasticity
, Article International Journal of Solids and Structures ; Volume 50, Issue 24 , 2013 , Pages 3967-3974 ; 00207683 (ISSN) ; Shodja, H. M ; Sharif University of Technology
2013
Abstract
Mindlin's (1965) second strain gradient theory due to its competency in capturing the effects of edges, corners, and surfaces is of particular interest. Formulation in this framework, in addition to the usual Lamé constants, requires the knowledge of sixteen additional materials constants. To date, there are no successful experimental techniques for measuring these material parameters which reflect the discrete nature of matter. The present work gives an accurate remedy for the atomistic calculations of these parameters by utilizing the first principles density functional theory (DFT) for the calculations of the atomic force constants combined with an analytical formulation. It will be shown...
Kinetics of chemical ordering in a Ag-Pt nanoalloy particle via first-principles simulations
, Article Journal of Chemical Physics ; Volume 137, Issue 19 , 2012 ; 00219606 (ISSN) ; Taherkhani, F ; Parsafar, G ; Caro, A ; Fortunelli, A ; Sharif University of Technology
2012
Abstract
The energetics and kinetic energy barriers of vacancy/atom exchange in a 37-atom truncated octahedron Ag-Pt binary cluster in the Ag-rich range of compositions are investigated via a first-principles atomistic approach. The energy of the local minima obtained considering various distributions of a single vacancy and a few Pt atoms within the cluster and the energy barriers connecting them are evaluated using accurate density-functional calculations. The effects of the simultaneous presence of a vacancy and Pt atoms are found to be simply additive when their distances are larger than first-neighbors, whereas when they can be stabilizing at low Pt content due to the release of strain by the...
First principles study of oxygen adsorption on nickel-doped graphite
, Article Molecular Physics ; Volume 110, Issue 13 , Feb , 2012 , Pages 1437-1445 ; 00268976 (ISSN) ; Gobal, F ; Sharif University of Technology
2012
Abstract
Density functional theory is used in a spin-polarized plane wave pseudopotential implementation to investigate molecular oxygen adsorption and dissociation on graphite and nickel-doped graphite surfaces. Molecular oxygen physisorbs on graphite surface retaining its magnetic property. The calculated adsorption energy is consistent with the experimental value of 0.1eV. It is found that substituting a carbon atom of the graphite surface by a single doping nickel atom (2.8% content) makes the surface active for oxygen chemisorption. It is found that the molecular oxygen never adsorbs on doping nickel atom while it adsorbs and dissociates spontaneously into atomic oxygens on the carbon atoms...
A combined first principles and analytical treatment for determination of the surface elastic constants: Application to Si(001) ideal and reconstructed surfaces
, Article Philosophical Magazine Letters ; Volume 92, Issue 1 , Sep , 2012 , Pages 7-19 ; 09500839 (ISSN) ; Shodja, H. M ; Sharif University of Technology
2012
Abstract
Behavior of nanostructures, which are characterized by a large surface-to-volume ratio, is greatly influenced by their surface parameters, such as surface elastic moduli tensor. Accurate determination of the surface elastic constants by first principles is of particular interest. To this end, through consideration of the fundamental thermodynamic arguments for free solid surfaces, an analytical formulation for the change in specific Helmholtz surface free energy is developed. Relating this formulation to the corresponding energy calculated via first principles leads to the determination of the surface elastic moduli tensor. The surface mechanical properties, namely surface energy, surface...
A fuzzy traffic assignment algorithm based on driver perceived travel time of network links
, Article Scientia Iranica ; Volume 18, Issue 2 A , 2011 , Pages 190-197 ; 10263098 (ISSN) ; Shafahi, Y ; Seyedabrishami, S. E ; Sharif University of Technology
2011
Abstract
Traffic assignment is a step of travel demand estimation. Given a trip origin-destination demand matrix, this step determines traffic flow in each link, according to assumptions based on the behavior of drivers. Conventional assignment algorithms, which are mostly based on the Wardrop first principle of user equilibrium, assume that all drivers choose the shortest path to the destination, based on the same travel time computed by travel time functions. However, in reality, driver perception of travel time varies for a specific route. This paper presents a traffic assignment algorithm which assumes that driver perception of travel time affects route choices. Fuzzy set theory is used to define...
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...
First-principles study on lattice instabilities and structural phase transitions in Ba doped La2 CuO4
,
Article
Physica C: Superconductivity and its Applications
;
Volume 517
,
October
,
2015
,
Pages 20-25
;
09214534 (ISSN)
; Akhavan, M
; Draxl, C
;
Sharif University of Technology
Elsevier
2015
Abstract
We present linear-response density-functional theory calculations for the high-Tc superconductor La2- x Bax CuO4 to study the doping dependence of phonon dispersion. Using the virtual crystal approximation, the doping range up to x = 0.20 is investigated. We find unstable phonon modes that soften around high-symmetry points of the Brillouin zone. These branches are analyzed as a function of doping and pressure. The structural distortions related to these phonons are in accordance with the observed phase transitions from the high-temperature tetragonal (HTT) phase to the low-temperature orthorhombic (LTO) and the low-temperature tetragonal...
Evolution of the electronic structure and structural properties of BaFe2 As2 at the tetragonal-collapsed tetragonal phase transition
,
Article
Physica C: Superconductivity and its Applications
;
Volume 516
,
2015
,
Pages 36-43
;
09214534 (ISSN)
; Khosroabadi, H
; Akhavan, M
;
Sharif University of Technology
Elsevier
2015
Abstract
Density functional theory generalized gradient approximation has been used to calculate the electronic and structural properties of BaFe2 As2 compound up to 56 GPa by using Quantum Espresso code. Structural properties like bulk modulus, Fe-As bond length, and As-Fe-As bond angles have been investigated by pressure especially near the tetragonal to collapsed tetragonal (T-cT) structural phase transition. This study shows considerable changes of these parameters in the cT phase, which happens near our calculated critical pressure, Pc = 24 ± 2 GPa. Electronic band structure and its orbital-resolved, total and partial density of states and Fermi surfaces have been...
First-principle electronic structure calculation of BaFe2-x Cox As2 (X = 0,1, 2) superconductor
,
Article
Journal of Superconductivity and Novel Magnetism
;
Volume 28, Issue 8
,
August
,
2015
,
Pages 2249-2254
;
15571939 (ISSN)
; Khosroabadi, H
; Akhavan, M
;
Sharif University of Technology
Springer New York LLC
2015
Abstract
The equilibrium crystal structure and electronic structure of BaFe2-x Cox As2 (x = 0,1, 2) superconductor have been investigated by using the pseudopotential Quantum Espresso code based on the ab initio density functional theory in the generalized gradient approximation. The equilibrium crystal structure for x = 1.0 has been determined by considering five different Fe/Co configurations. This study shows that the spin calculation is essential to obtain the experimental values at x = 0.0. The total and partial density of states, band structure, and Fermi surfaces of the three compounds has been calculated. Density of states calculation indicates the important...
Gap tuning and effective electron correlation energy in amorphous silicon: A first principles density functional theory-based molecular dynamics study
, Article Computational Materials Science ; Volume 102 , May , 2015 , Pages 110-118 ; 09270256 (ISSN) ; Shodja, H. M ; Esfarjani, K ; Sharif University of Technology
Elsevier
2015
Abstract
First principles density functional theory (DFT)-based molecular dynamics (MD) is used to study some physical and electronic properties of amorphous silicon (a-Si) samples, as-quenched and annealed containing dangling and floating bonds (pertinent to the threefold- and fivefold-coordinated defects, respectively) as well as distorted tetrahedral bonds. Surprisingly, except for the work of Pantelides (1986) who gave a rough estimate for the effective electron correlation energy, Ueff of a floating bond on the fivefold-coordinated Si, to date, there are no theoretical studies in the literature for the calculation of Ueff pertinent to this type of defect. In this work, Ueff for each type of...
Simulation of vacancy diffusion in a silver nanocluster
, Article Chemical Physics Letters ; Volume 498, Issue 4-6 , 2010 , Pages 312-316 ; 00092614 (ISSN) ; Negreiros, F. R ; Parsafar, G ; Fortunelli, A ; Sharif University of Technology
Abstract
The formation and diffusion of a vacancy in a silver nanocluster are studied via a combination of first-principles and statistical mechanics simulations. A 38-atom truncated-octahedral (TO) arrangement and its homologue with 37 Ag atoms and one vacancy are considered, and density-functional calculations are performed to derive the energies of the local minima and the energy barriers connecting them. These data are then used as an input for a study of the system dynamics via a kinetic Monte Carlo algorithm, evaluating site occupancies, diffusion coefficient and equilibration time. It is found that vacancy formation and diffusion represents a viable path for atom-atom exchange in these...
Substitutional doping of Cu in diamond: Mott physics with p orbitals
, Article European Physical Journal B ; Volume 77, Issue 3 , October , 2010 , Pages 331-336 ; 14346028 (ISSN) ; Jafari, S. A ; Abolhassani, M. R ; Sharif University of Technology
2010
Abstract
Discovery of superconductivity in the impurity band formed by heavy doping of boron into diamond (C:B) as well as doping of boron into silicon (Si:B) has provided a rout for the possibility of new families of superconducting materials. Motivated by the special role played by copper atoms in high temperature superconducting materials where essentially Cu d orbitals are responsible for a variety of correlation induced phases, in this paper we investigate the effect of substitutional doping of Cu into diamond. Our extensive first principle calculations based on density functional theory which are averaged over various geometries indicate the formation of a mid-gap band, which mainly arises from...
Multi-objective minimum entropy controller design for stochastic processes
, Article Proceedings of the 2010 American Control Conference, ACC 2010, 30 June 2010 through 2 July 2010 ; 2010 , Pages 355-360 ; 9781424474264 (ISBN) ; Nobakhti, A ; Wang, H ; Chai, T ; Sharif University of Technology
Abstract
Minimum variance control is an established method in control of systems corrupted by noise. In these cases, as it is not possible to directly control the actual value of the system variables, one aims to reduce the variations instead. However, when the system noises are non-Gaussian, this approach fails because non-Gaussian noise cannot be characterised by simple measures such as variance. In these cases, the Entropy is proposed as a generalisation of the variance measure and the control objective becomes that of minimising the Entropy. Previously a limited form of this problem has been solved using first order Newtonian methods. In this paper, the control objective is first expanded to also...
Mathematical modeling of CSF pulsatile hydrodynamics based on fluid-solid interaction
, Article IEEE Transactions on Biomedical Engineering ; Volume 57, Issue 6 , 2010 , Pages 1255-1263 ; 00189294 (ISSN) ; Bastani, D ; Najarian, S ; Ganji, F ; Farmanzad, F ; Seddighi, A. S ; Sharif University of Technology
2010
Abstract
Intracranial pressure (ICP) is derived from cerebral blood pressure and cerebrospinal fluid (CSF) circulatory dynamics and can be affected in the course of many diseases. Computer analysis of the ICP time pattern plays a crucial role in the diagnosis and treatment of those diseases. This study proposes the application of Linninger et al.s [IEEE Trans. Biomed. Eng. , vol. 52, no. 4, pp. 557565, Apr. 2005] fluidsolid interaction model of CSF hydrodynamic in ventricular system based on our clinical data from a group of patients with brain parenchyma tumor. The clinical experiments include the arterial blood pressure (ABP), venous blood pressure, and ICP in the subarachnoid space (SAS). These...