Sharif Digital Repository

In this paper, we propose what we believe is a novel sensitive micro-optoelectromechanical systems (MOEMS) accelerometer based on intensity modulation by using a one-dimensional photonic crystal. The optical sensing system of the proposed structure includes an air-dielectric multilayer photonic bandgap material, a laser diode (LD) light source, a typical photodiode (1550 nm) and a set of integrated optical waveguides. The proposed sensor provides several advantages, such as a relatively wide measurement range, good linearity in the whole measurement range, integration capability, negligible cross-axis sensitivity, high reliability, and low air-damping coefficient, which results in a wider... 

Understanding of engine heat transfer is important because of its influence on engine efficiency, exhaust emissions and component thermal stresses. In this paper, the effect of various parameters such as compression ratio, equivalence ratio, spark timing, engine speed, inlet mixture temperature and swirl ratio as well as fuel type on the heat transfer through the chamber walls of a spark ignition (SI) engine is studied. For this purpose, a proper tool is developed which uses a KIVA multidimensional combustion modeling program and a finite-element heat conduction (FEHC) code iteratively. Also, an improved temperature wall function is used for the KIVA program. It was found that this iterative... 

In this investigation, polymer concrete (PC) with three different epoxy resin contents, ordinary cement concrete (OCC), lightweight concrete (LWC), and lime-mortar soil (LMS) have been studied under uniaxial and triaxial compression tests to determine their mechanical behavior by measuring axial stress-strain and volumetric strain versus axial strain curves. According to the results, PC showed higher strength, ductility, and energy absorption than that of OCC and LWC. Then, nonlinear finite element analysis (NFEA) was implemented to predict the experimental results using hierarchical single-surface (HISS) failure criterion and disturbed state concept (DSC) to capture the elastoplastic... 

In this paper, a metallic yielding damper, called piston metallic damper (PMD) is introduced for the first time. The PMD is comprised of a set of parallel hollow circular plates that interconnect inner shaft of the PMD to its outer pipe. Experiments and numerical models are used to examine its applicability as a seismic energy dissipating device. In this novel damper seismic input energy will be dissipated through flexural yielding of circular steel plates when system experiences small to medium displacements. At large displacements, tensile behavior will dominate and causes significant increase in stiffness of the system. In experimental program, different specimens were tested and the... 

We develop the real space quantum renormalization group (QRG) approach for majorana fermions. As an example we focus on the Kitaev chain to investigate the topological quantum phase transition (TQPT) in the one-dimensional spinless p-wave superconductor. Studying the behaviour of local compressibility and ground-state fidelity, show that the TQPT is signalled by the maximum of local compressibility at the quantum critical point tuned by the chemical potential. Moreover, a sudden drop of the ground-state fidelity and the divergence of fidelity susceptibility at the topological quantum critical point are used as proper indicators for the TQPT, which signals the appearance of Majorana fermions.... 

A mathematically rigorous, multiscale modeling methodology capable of coupling behaviors from the Kolmogorov turbulence scale through the full scale system in which a fiber suspension is flowing is presented. Here the key aspect is adaptive hierarchical modeling. Numerical results are presented focus of which are on fiber floe formation and destruction by hydrodynamic forces in turbulent flows. Specific consideration was given to molecular-dynamics simulations of viscoelastic fibers in which the fluid flow is predicted by a method which is a hybrid between direct numerical simulations and large eddy simulation techniques, and fluid fibrous structure interactions were taken into account. The... 

Asphaltene deposition in porous media, wellbore and surface facilities has been a severe problem in petroleum industry which causes considerable remediation costs annually. Asphaltenes are heavy and polydisperse fractions of crude oil which are insoluble in n-alkanes such as n-heptane. In this work, three Iranian crude oils were prepared for titration experiments with n-pentane, n-heptane and n-dodecane at different solvent ratios and constant temperature. The experimental data were correlated by perturbed chain form of statistical associating fluid theory (PC-SAFT). The association of asphaltene molecules has been considered in this model with adjusting the uncertain parameters (such as... 

The aim of this work is to investigate flow hydrodynamics in porous media. Random non-overlapping spherical particles in a cylindrical geometry were produced as a porous media. Navier-Stokes equations in three-dimensional porous packed bed have been solved and dimensionless pressure drop has been studied for a fluid flow through the porous media at different Reynolds numbers (based on pore permeability and interstitial fluid velocity). The numerical results are in good agreement with those reported by Macdonald (1979) in the range of Reynolds numbers studied. At higher Reynolds numbers turbulent models such as large eddy simulation (LES) and Reynolds stress model (RSM) also have been... 

In this paper, we propose a reversible Feynman gate utilizing the interference effect for optical communications and computing. The plasmonic waveguides are created by placing a suspended graphene sheet, held by two SiO2 ridges, 10 nm above the Si ribs. The Finite-difference time-domain (FDTD) method is used to simulate the proposed gate in frequency and time domains. Simulation results show that high extinction ratios as much as (15.12 dB) and 13 dB are achievable at the wavelength of 10 μm for the output bits P and Q, respectively. The device is immune against ± 20% variations in the width of the Si ribs due to fabrication errors and its performance can be controlled by setting the... 

Electrochemical potential noise analysis was used to investigate corrosion behavior of copper electrodes in presence of 0.001, 0.005, 0.01 and 0.02 M benzotriazole (BTA) inhibitor. Energy distribution of the potential noise was obtained from wavelet transformation using the 4th order Daubechies pattern. Two lowest frequencies were added-up and fitted to the impulse response of the first order circuit to achieve the time-constant vs. concentration (T-C) diagram. T-C diagram can be used either as a conventional diagram of inhibition efficiency or a tool for evaluation of the surface film dielectric constant. The assumptions made in this treatment were confirmed by EIS method. © 2009 Elsevier... 

Typically, the lack of effective stakeholder participation in a project—especially in the initial planning and implementation stages—has a negative impact on the expected performance of the project. These negative consequences require attempts to encourage their effective participation. Nevertheless, there are some challenges ahead, such as conflict of interest among the stakeholders. For more accurate identification of the interests, objectives, and performance of the stakeholders, this paper proposes an accurate and organized model for the analysis of results from the stakeholder impact index. We implemented the proposed model to determine the existing barriers to renewable energy... 

We analyze the impact of loss in lattices of coupled optical waveguides and find that, in such a case, the hopping between adjacent waveguides is necessarily complex. This results not only in a transition of the light spreading from ballistic to diffusive, but also in a new kind of diffraction that is caused by loss dispersion. We prove our theoretical results with experimental observations  

In a nanoscopic thin film of a strong topological insulator (TI) the Coulomb interaction in the channel that exchanges the two electrons with the same chirality in two different planes of the slab takes advantage of the minus sign resulting from such "exchange" and gives rise to a bound state between the positive-energy states in one surface and the negative-energy states in the opposite surface. Therefore particle and hole pairs in the undoped Dirac cone of the TI thin film form an inter-surface spin-singlet state that lies below the continuum of free particle-hole pairs. This mode is similar to the excitons of semiconductors, albeit formed between the electron and hole pairs from two... 

The particle-hole continuum (PHC) for massive Dirac fermions provides an unprecedented opportunity for the formation of two collective split-off states, one in the singlet and the other in the triplet (spin-1) channel, when the short-range interactions are added to the undoped system. Both states are close in energy and are separated from the continuum of free particle-hole excitations by an energy scale of the order of the gap parameter Δ. They both disperse linearly with two different velocities, reminiscent of spin-charge separation in Luttinger liquids. When the strength of Hubbard interactions is stronger than a critical value, the velocity of singlet excitation, which we interpret as a... 

We present a formulation for the nonlinear optical response in gapped graphene, where the low-energy single-particle spectrum is modeled by massive Dirac theory. As a representative example of the formulation presented here, we obtain a closed form formula for the third harmonic generation in gapped graphene. It turns out that the covariant form of the low-energy theory gives rise to peculiar logarithmic singularities in the nonlinear optical spectra. The universal functional dependence of the response function on dimensionless quantities indicates that the optical nonlinearity can be largely enhanced by tuning the gap to smaller values  

Within the block spin renormalization group, we give a very simple derivation of the exact phase boundaries of the XYZ spin chain. First, we identify the Ising order along ◯ or ŷ as attractive renormalization group fixed points of the Kitaev chain. Then, in a global phase space composed of the anisotropy λ of the XY interaction and the coupling Δ of the Δσzσz interaction, we find that the above fixed points remain attractive in the two-dimesional parameter space. We therefore classify the gapped phases of the XYZ spin chain as: (1) either attracted to the Ising limit of the Kitaev-chain, which in turn is characterized by winding number ±1, depending on whether the Ising order parameter is... 

We show that the continuum limit of the tilted Dirac cone in materials such as 8-Pmmn borophene and layered organic conductor α-(BEDT-TTF)2I3 corresponds to deformation of the Minkowski space-time of Dirac materials. From its Killing vectors we construct an emergent tilted-Lorentz (t-Lorentz) symmetry group for such systems. As an example of the t-Lorentz transformations we obtain the exact solution of the Landau bands for a crossed configuration of electric and magnetic fields. For any given tilt parameter 0≤ζ<1, if the ratio χ=vFBz/Ey of the crossed magnetic and electric fields satisfies χ≥1+ζ, one can always find appropriate t-boosts in both valleys labeled by τ=±1 in such a way that the... 

Dirac cone can be tilted in condensed matter setting. As a result of tilt, the Lorentz symmetry is reduced to what we call tilted-Lorentz symmetry. In this paper, we derive the tilted-Lorentz transformations that leave a world with tilted Dirac cone invariant. © 2020, Isfahan University of Technology. All rights reserved  

This article presents an investigation on the durability of different glass-fibre-reinforced polymer composites when subjected to harsh outdoor conditions, including freeze/thaw cycles, ultraviolet radiation and moisture, as well as when used with seawater sea-sand concrete for construction applications. To achieve this, the effects of a number of parameters, including the environment of exposure, exposure time, profile cross-sectional configuration and orientation of fibres, on the mechanical properties of different glass-fibre-reinforced polymer composites were studied. To investigate the degradation of the mechanical properties, three-point bending, compression and tension tests were...