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#### The formation and dissociation energy of vacancies in cementite: A first-principles study

, Article Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms ; Volume 502 , 2021 , Pages 157-163 ; 0168583X (ISSN) ; Amirhossein Feghhi, S ; Samadfam, M ; Sharif University of Technology
Elsevier B.V
2021

Abstract

Because of the possibility of various types of vacancies in cementite due to its crystalline structure, the focus of this paper was only on vacancies. In this regard, the formation energies of single, two, three and four vacancies of over than 120 different cases were calculated using first-principles method. For the case of single vacancy, the results were in three values of ~1.63, 1.39 and 0.78 eV according to iron vacancies at general positions, iron vacancies located on mirror planes and carbon vacancies in the interstitial positions, respectively. The results for the case of two, three and four vacancies were between from 2.10 to 3.34 eV, from 3.92 to 5.10 eV and from 4.77 to 6.33 eV,...

#### Coherence-based characterization of macroscopic quantumness

, Article Physical Review A ; Volume 103, Issue 3 , 2021 ; 24699926 (ISSN) ; Raeisi, S ; Sharif University of Technology
American Physical Society
2021

Abstract

One of the most elusive problems in quantum mechanics is the transition between classical and quantum physics. This problem can be traced back to Schrödinger's cat thought experiment. A key element that lies at the center of this problem is the lack of a clear understanding and characterization of macroscopic quantum states. Our understanding of macroscopic quantumness relies on states such as the Greenberger-Horne-Zeilinger (GHZ) or the NOON state. Here we take a first-principle approach to this problem. We start from coherence as the key quantity that captures the notion of quantumness and require the quantumness to be collective and macroscopic. To this end, we introduce macroscopic...

#### 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

#### 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...

#### 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...

#### Effects of Stone-Wales defect on the electronic and transport properties of bilayer armchair graphene nanoribbons

, Article Superlattices and Microstructures ; Volume 100 , 2016 , Pages 739-748 ; 07496036 (ISSN) ; Faez, R ; Morawej Farshi, M. K ; Sharif University of Technology
Academic Press

Abstract

We report a first principles study on the electronic and transport properties of bilayer armchair graphene nanoribbons (BLAGNRs) containing Stone-Wales (SW) defect. It is shown that in the presence of SW defect in BLAGNRs, some electron localization occurs in defect atoms and degradation of transmission is observed in specific energy regions. The strength of electron localization is dependent on the symmetry of SW defect. In case of symmetric SW defect, stronger electron localization leads to sharper dip in its transmission spectrum in comparison with the broad dip in the transmission spectrum of the BLAGNR containing asymmetric SW defect. The effect of electron localization is also evident...

#### Wardrop's first principle: Extension for capacitated networks

, Article Scientia Iranica ; Volume 28, Issue 1 , 2021 , Pages 175-191 ; 10263098 (ISSN) ; Poorzahedy, H ; Nourinejad, M ; Sharif University of Technology
Sharif University of Technology
2021

Abstract

In transportation literature, User Equilibrium (UE) has been widely studied since early 1950's, many studies of which define equilibrium flow of traffic for uncapacitated networks based on Wardrop's first principle, implying also a Nash Equilibrium (NE). Although, in general, the two equilibria (UE and NE) are not explicitly the same, they are shown to be equivalent under special conditions for uncapacitated UE, when volume delay functions are separable, continuous, non-decreasing and non-negative. A good deal of research is devoted to explain UE in capacitated networks based on Wardrop's first principle and the concept of generalized costs. However, UE for capacitated networks, even under...

#### Wardrop's first principle: Extension for capacitated networks

, Article Scientia Iranica ; Volume 28, Issue 1 , 2021 , Pages 175-191 ; 10263098 (ISSN) ; Poorzahedy, H ; Nourinejad, M ; Sharif University of Technology
Sharif University of Technology
2021

Abstract

In transportation literature, User Equilibrium (UE) has been widely studied since early 1950's, many studies of which define equilibrium flow of traffic for uncapacitated networks based on Wardrop's first principle, implying also a Nash Equilibrium (NE). Although, in general, the two equilibria (UE and NE) are not explicitly the same, they are shown to be equivalent under special conditions for uncapacitated UE, when volume delay functions are separable, continuous, non-decreasing and non-negative. A good deal of research is devoted to explain UE in capacitated networks based on Wardrop's first principle and the concept of generalized costs. However, UE for capacitated networks, even under...

#### 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...

#### Mechanical and chemical pressure effects on the AeFe 2 As 2 (Ae = Ba, Sr, Ca) compounds: Density functional theory

, Article Computational Materials Science ; Volume 160 , 2019 , Pages 233-244 ; 09270256 (ISSN) ; Hadipour, H ; Akhavan, M ; Sharif University of Technology
Elsevier B.V
2019

Abstract

We have studied the pressure-induced structural, magnetic and electronic properties of AeFe 2 As 2 (Ae = Ba, Sr, Ca) compounds in the framework of density functional theory within the GGA-PBE method. The effects of chemical pressure generated by Sr and Ca substitutions in BaFe 2 As 2 have been investigated. We have found a magnetic transition at the same primitive unit cell volume, around 81 Å 3 for the (Ba⧹Ca)Fe 2 As 2 compounds, which predicts a magnetic transition pressure of 12 GPa for SrFe 2 As 2 . The structural parameters of FeAs 4 tetrahedra are obtained after ionic relaxation and compared with the existing experimental results. The change of these internal parameters is ascribed to...

#### 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...

#### 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...

#### 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...

#### 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...

#### Elastic moduli tensors, ideal strength, and morphology of stanene based on an enhanced continuum model and first principles

, Article Mechanics of Materials ; Volume 110 , 2017 , Pages 1-15 ; 01676636 (ISSN) ; Ojaghnezhad, F ; Etehadieh, A ; Tabatabaei, M ; Sharif University of Technology
Elsevier B.V
2017

Abstract

The present work aims to provide an accurate description of the tensile behavior of the planar as well as low-buckled stanene and to capture their ideal strength in armchair (AC)- and zigzag (ZZ)-directions. For an accurate description of anisotropic response of such hyperelastic materials as stanene, consideration of a highly nonlinear constitutive model in which up to the fourth power of strains is incorporated is inevitable. By utilizing first principles calculations based on density functional theory (DFT), the second, third, fourth, and fifth order elastic moduli tensors corresponding to both planar and low-buckled states are obtained. Moreover, the morphology of the free-standing...

#### A computational study of vertical tunneling transistors based on graphene-WS2 heterostructure

, Article Journal of Applied Physics ; Volume 121, Issue 21 , 2017 ; 00218979 (ISSN) ; Faez, R ; Pourfath, M ; Darvish, G ; Sharif University of Technology
American Institute of Physics Inc
2017

Abstract

In this paper, for the first time, we present a computational study on electrical characteristics of field effect tunneling transistors based on a vertical graphene-WS2 heterostructure and vertical graphene nanoribbon (GNR)-WS2 heterostructure (VTGNRFET). Our model uses the nonequilibrium Green's function formalism along with an atomistic tight binding (TB) method. The TB parameters are extracted by fitting the bandstructure to first principles results. We show that, due to the advantage of switching between tunneling and thermionic transport regimes, an improvement can be achieved in the electrical characteristics of the device. We find that the increase of the number of WS2 layers enhances...

#### 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...

#### Molecular dynamics study of two dimensional silicon dioxides with in-plane negative poisson's ratio

, Article Computational Materials Science ; Volume 153 , 2018 , Pages 258-267 ; 09270256 (ISSN) ; Tavakoli, R ; Jafary Zadeh, M ; Sharif University of Technology
Abstract

In the present work, the mechanical properties, in particular, the Poisson's ratio of four two-dimensional silica structures, called here α,β,γ and δ are studied by means of molecular dynamics simulations. The α structure has been synthesized experimentally and the others have been reported as the most stable low-energy structures that reveal in-plane negative Poisson's ratio based on the first principles calculations. Among these structures, β-silica exhibits the largest in-plane negative Poisson's ratio which is 2–4 times higher than penta-graphene. Our results illustrate that the classical molecular dynamics simulation reproduces results in agreement with those of the first principles...