Sharif Digital Repository

We use the Dissipative Particle Dynamics (DPD) method to simulate the variation in the configuration of a polymer chain, whose two ends are fixed in a nanochannel, subject to bulk motion of fluid particles. We aim to find suitable relations between the polymer chain curvature and the mean flow velocity magnitudes. In this regard, we introduce some important parameters, which can effectively affect the polymer chain curvature and we examine their magnitudes on the amount of polymer chain deflection. In fact, our main objective is to quantify the relation between some major flow parameters, possibly with different polymer chain properties. For example, we show that different flow velocity... 

In this work, a hybrid finite element volume FEV method is further extended to simulate soot nanoparticles formation and oxidation in a heavily sooting co-flow methane diffusion flame at elevated pressure. In this regard, two-equation soot model is used and soot oxidation due to O2 is further taken into account. Considering full soot oxidation and respecting the physics of the flow, physical upwinding influence scheme PIS for approximation of soot mass fraction fluxes over cell faces is further extended. To describe soot nucleation process, phenyl-route, based on soot inception from polycyclic aromatic hydrocarbons PAHs, is used and a further kinetics scheme, which consists of 80 chemical... 

Dirac electrons in clean graphene can mediate the interactions between two localized magnetic moments. The functional form of the RKKY interaction in pristine graphene is specified by two main features: (i) an atomic-scale oscillatory part determined by a wavevector → connecting the two valleys; with doping another longer range oscillation appears which arises from the existence of an extended Fermi surface characterized by a momentum scale kF; (ii) an algebraic Rα decay in large distances where the exponent α=-3 is a distinct feature of undoped Dirac sea in two dimensions. In this work, we investigate the effect of a few per cent vacancies on the above properties. Depending on the doping... 

An accurate molecular dynamics simulation of the nanocavity flow cannot be achieved without considering correct thermal treatments for the molecules both distributed in the flow and located at the cavity walls and without including their interactions correctly. In this study, we specify constant temperature at the nanocavity vertical walls; however, we examine three different thermal wall models, including a rigid wall, a controlled-temperature flexible wall, and a noncontrolled-temperature flexible wall, to model the horizontal wall behaviors. Comparing the results of these three models with each other, it is possible to evaluate the effect of wall model on the resulting temperature and... 

Tight-binding electrons on a honeycomb lattice are described by an effective Dirac theory at low energies. Lowering symmetry by an alternate ionic potential (Δ) generates a single-particle gap in the spectrum. We employ the dynamical mean-field theory (DMFT) technique to study the effect of on-site electron correlation (U) on massive Dirac fermions. For a fixed mass parameter Δ, we find that beyond a critical value U c1(Δ) massive Dirac fermions become massless. Further increasing U beyond U c2(Δ), there will be another phase transition to the Mott insulating state. Therefore, the competition between the single-particle gap parameter, Δ, and the Hubbard U restores the semi-metallic nature of... 

Within the tunneling Hamiltonian formulation for the eight-component spinors, the Josephson critical supercurrent has been calculated in a planar superconductor-normal graphene-superconductor junction. Coupling between superconductor regions and graphene is taken into account by a tunneling Hamiltonian which contains two types of tunneling, intravalley and intervalley tunneling. Within the present tunneling approach, we find that the contributions of two kinds of tunneling to the critical supercurrent are completely separable. Therefore, it is possible to consider the effect of the intervalley tunnelings in the critical supercurrent. The incorporation of these type of processes into the... 

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... 

Bismuth and its alloys provide a paradigm to realize three dimensional materials whose low-energy effective theory is given by Dirac equation in 3+1 dimensions. We study the quantum transport properties of three dimensional Dirac materials within the framework of Landauer-Büttiker formalism. Charge carriers in normal metal satisfying the Schrödinger equation, can be split into four-component with appropriate matching conditions at the boundary with the three dimensional Dirac material (3DDM). We calculate the conductance and the Fano factor of an interface separating 3DDM from a normal metal, as well as the conductance through a slab of 3DDM. Under certain circumstances the 3DDM appears... 

The two-sublattice nature of graphene lattice in conjunction with three-fold rotational symmetry, allows for the p-wave hybridization of the impurity state with the Bloch states of carbon atoms. Such an opportunity is not available in normal metals where the wave function is scalar. The p-wave hybridization function V (-k) appears when dealing with vacancies, substitutional adatoms and the hollow site impurities while the s-wave mixing on graphene lattice pertains only to the top site impurities. In this work, we compare the local moment formation in these two cases and find that the local moments formed by p-wave mixing compared to the s-wave one are robust against the changes in the... 

We theoretically study the quasi bound state of Dirac electrons in cylindrically symmetric quantum dots with sharp boundary. According to the existing picture, due to Klein tunneling "relativistic electrons" can not be localized by any confinement potential. We show however that despite of Klein tunneling, interference effects can cause the trapping of electron in quantum dots. Considering the quasi bound state as the state with complex energy, to find the energy of this state we solve the wave-equation with outgoing boundary condition at infinity. The imaginary part of complex energy determines the trapping time of electron within the quantum dot. We show that for any finite confining... 

We use perturbative continuous unitary transformations (PCUT) to study the one dimensional extended ionic Hubbard model (EIHM) at half-filling in the band insulator region. The extended ionic Hubbard model, in addition to the usual ionic Hubbard model, includes an inter-site nearest-neighbor (n.n.) repulsion, V. We consider the ionic potential as unperturbed part of the Hamiltonian, while the hopping and interaction (quartic) terms are treated as perturbation. We calculate total energy and ionicity in the ground state. Above the ground state, (i) we calculate the single particle excitation spectrum by adding an electron or a hole to the system; (ii) the coherence-length and spectrum of... 

This paper deals with a new fractional calculus based method to stabilize fixed points of single-input 3D systems. In the proposed method, the control signal is determined by fractional order integration of a linear combination of the system linearized model states. The tuning rule for this method is based on the stability theorems in the incommensurate fractional order systems. The introduced technique can be used in suppression of chaotic oscillations. To evaluate the performance of the proposed technique in practical applications, it has been experimentally applied to control chaos in two chaotic circuits  

Remarkable inter-band nesting in FeO Fermi surface, hints to examine energy of orbitally ordered states, which turn out to be more stable. We propose this method as a general scheme to check for possibility of orbital ordering in first principle calculations, and demonstrate it for known examples, LiVO2 and LaMnO3  

This paper presents the fuzzy logic computing for the design and implementation of fuel controller for gas turbine engines. For this purpose, a fuzz controller is designed in order to be implemented on an electronic control unit where it is used to drive a servo operated fuel control valve. The fuzzy logic computing approach for different parts of the controller including fuzzification, rule base, inference engine and defuzzification are then described. Finally, computer simulation of the fuzzy controllers integrated with the engine model is performed to investigate the effectiveness of the proposed fuzzy controller on the performance of a turbojet engine. The results are provided to show... 

The iterated perturbation theory (IPT) equations of the dynamical mean field theory (DMFT) for the half-filled Hubbard model are solved on nearly real frequencies at various values of the Hubbard parameters, U, to investigate the nature of metal-insulator transition (MIT) at finite temperatures. This method avoids the instabilities associated with the infamous Padé analytic continuation and reveals fine structures across the MIT at finite temperatures, which cannot be captured by conventional methods for solving DMFT-IPT equations on Matsubara frequencies. Our method suggests that at finite temperatures, there is a crossover from a bad metal to a bad insulator in which the height of the... 

Phase transitions in the Hubbard model and ionic Hubbard model at half-filling on the honeycomb lattice are investigated in the strong-coupling perturbation theory which corresponds to an expansion in powers of the hopping t around the atomic limit. Within this formulation we find analytic expressions for the single-particle spectrum, whereby the calculation of the insulating gap is reduced to a simple root finding problem. This enables high-precision determination of the insulating gap that does not require any extrapolation procedure. The critical value of Mott transition on the honeycomb lattice is obtained to be Uc≈2.38t. Studying the ionic Hubbard model at the lowest order, we find two... 

We suggest that spin-singlet pseudo-scalar s-wave superconducting pairing creates a two dimensional sea of Majorana fermions on the surface of three dimensional Dirac superconductors (3DDS). This pseudo-scalar superconducting order parameter Δ5, in competition with scalar Dirac mass m, leads to a topological phase transition due to band inversion. We find that a perfect Andreev-Klein reflection is guaranteed by presence of anomalous Andreev reflection along with the conventional one. This effect manifests itself in a resonant peak of the differential conductance. Furthermore, Josephson current of the Δ5|m|Δ5 junction in the presence of anomalous Andreev reflection is fractional with 4π... 

We find that a conventional s-wave superconductor in proximity to a three-dimensional Dirac material (3DDM), to all orders of perturbation in tunneling, induces a combination of s- and p-wave pairing only. We show that the Lorentz invariance of the superconducting pairing prevents the formation of Cooper pairs with higher orbital angular momenta in the 3DDM. This no-go theorem acquires stronger form when the probability of tunneling from the conventional superconductor to positive and negative energy states of 3DDM are equal. In this case, all the p-wave contribution except for the lowest order, identically vanish and hence we obtain an exact result for the induced p-wave superconductivity... 

The list of two-dimensional Dirac systems with a tilt in their Dirac cone spectrum is expanding, and now, in addition to the organic system α(BEDT-TTF)2I3, it includes the two-dimensional 8Pmmn-borophene sheet, which allows for controlled doping by the gate voltage. We analytically calculate the polarization function of tilted Dirac cone for an arbitrary tilt parameter, 0≤η<1, and arbitrary doping. This enables us to find two interesting plasmonic effects solely caused by the tilt. (i) In addition to the standard plasmon oscillations, a strong enough tilt induces an additional linearly dispersing overdamped branch of plasmons, which is strongly Landau damped due to overlap with a large... 

Within the real space renormalization group we obtain the phase portrait of the anisotropic quantum XY model on square lattice in presence of Dzyaloshinskii-Moriya (DM) interaction. The model is characterized by two parameters, λ corresponding to XY anisotropy, and D corresponding to the strength of DM interaction. The flow portrait of the model is governed by two global Ising-Kitaev attractors at (λ=±1,D=0) and a repeller line, λ=0. This line itself is characterized by an attractor at D=0. Renormalization flow of concurrence suggests that the λ=0 line corresponds to a topological phase transition. The gap starts at zero on this repeller line corresponding to superfluid phase of underlying...