Sharif Digital Repository

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  

The theory of silicon optical modulators of ring resonators based on PN diode in reverse bias is primarily discussed. It secondarily provides a full-featured simulator to investigate the behavior of such modulators. Wave equation for ring structure will be solved by using the conformal transformation method and the matrix method as it was used to analyze bent planar optical waveguides. Power coupling between ring and straight waveguides will be calculated by coupled theory of nonparallel waveguides based on experimental results. The time response demonstrates the capability of this device to operate correctly at up to 10 Gbs-1 bitrate, and the frequency spectrum analysis of device shows a >... 

Automatic brain tissue segmentation is a crucial task in diagnosis and treatment of medical images. This paper presents a new algorithm to segment different brain tissues, such as white matter (WM), gray matter (GM), cerebral spinal fluid (CSF), background (BKG), and tumor tissues. The proposed technique uses the modified intraframe coding yielded from H.264/(AVC), for feature extraction. Extracted features are then imposed to an artificial back propagation neural network (BPN) classifier to assign each block to its appropriate class. Since the newest coding standard, H.264/AVC, has the highest compression ratio, it decreases the dimension of extracted features and thus yields to a more... 

This paper presents a neural network-based method for automatic classification of magnetic resonance images (MRI) of brain under three categories of normal, lesion benign, and malignant. The proposed technique consists of six subsequent stages; namely, preprocessing, seeded region growing segmentation, connected component labeling (CCL), feature extraction, feature Dimension Reduction, and classification. In the preprocessing stage, the enhancement and restoration techniques are used to provide a more appropriate image for the subsequent automated stages. In the second stage, the seeded region growing segmentation is used for partitioning the image into meaningful regions. In the third... 

A new Voronoi diagram in Laguerre geometry based on closed-pack non-overlapping circles was proposed. This diagram was used to simulate microstructure of severely deformed materials at different applied strains. Grains size and their fractions were introduced by controlling the size and distribution of nuclei. Edge number distribution and neighboring cells edge number along with area distribution of the simulated Voronoi cells were determined. The edge number distribution was observed to fit gamma distribution more accurately. However, due to high inhomogeneity in the microstructure of the deformed samples at low strains, edge number distribution could not be matched by any distribution... 

We have studied the symmetric and non-symmetric pairwise ground state and thermal entanglement in three-qubits system. We have considered the anisotropic Heisenberg (XXZ) model in the presence of DzyaloshinskiiMoriya (DM) interaction in addition to the Ising model in a magnetic field with DM interaction. We have found that the increment of DM interaction and magnetic field can enhance and reduce the entanglement of the system. We have shown that the non-symmetric pairwise state has higher value concurrence and critical temperature (above which the entanglement vanishes) than the symmetric pairwise one. For the negative anisotropy, the non-symmetric entanglement is a monotonic function of DM... 

In this article, two optical-fibre probes used in displacement measurements are reported. In one arrangement, two optical fibres with protective buffer jackets are placed together, while in the second arrangement, the protective buffer jackets are removed and the two fibres are placed with the claddings in contact, with a concomitant smaller centrecentre distance. As the core and cladding diameters are the same in both probes, the effect of the centrecentre distance on double-fibre designs can be investigated. The reflected output powers are recorded for the probes as a function of the axial distance from a reflective surface. A theoretical model is also implemented, which considers the... 

This paper aims at applying H.264/AVC in medical video compression applications and improving its compression performance with higher perceptual quality and lower coding complexity. We propose a new method that uses lossless compression in the region of interest (ROI) and very high rate lossy compression in other regions. The propose method achieves a fast intra- and interprediction mode decision that is based on encountering coarse MBs for intra- and inter- prediction mode decision of the background region and fine MBs for the ROI region. The MBs of the background region are encoded with the maximum quantization parameter allowed by H.264/AVC in order to maximize the number of null... 

In this study simulation results for three double-fiber optical designs for the reflection measurements are reported. Modeling is perused for three cases namely Equal Fibers (EF), transmitter fiber shorter (TS) and receiver fiber shorter (RS) designs. By proper modeling and written programs the operations of such symmetric double-fiber probes are simulated and the role of different crucial parameters such as fiber-to-fiber distance (t), fiber core radius (r) and fiber Numerical Aperture (NA) are investigated. In the second study simulation results for the transmitter fiber shorter and receiver fiber shorter designs are investigated for different fiber length differences (w). Finally... 

Polymer concrete (PC) has been widely used for quick repairing of concrete pavement and structures in recent years. This paper studies the mechanical behavior of the rubberized polymer concrete. Crumb and chipped rubber were used to replace fine and coarse aggregates in PC, respectively. A complete series of destructive tests including impact test, compression and splitting tensile tests and non-destructive methods including ultrasonic test, digital signal processing, electrical conductivity, and microstructure analysis was performed to demonstrate the various potential applications of the rubberized PC. X-ray diffraction (XRD) also provided information regarding the chemical composition and... 

Field analysis of ring resonator modulators based on p-i-n diodes has been dissected in this paper. This analysis is performed in time and frequency domains. The conformal transformation method has been used for solving 3-D wave equation. Coupling coefficient between the ring and straight waveguides are obtained by developing the coupled-mode assumption. In the resonant wavelength of 1573.91 nm, a drop of more than 15 dB in frequency spectrum of the device has been observed. Time domain simulation shows that this modulator could support up to 0.4 Gb/s and up to 1.5 Gb/s for NRZ and RZ signals, respectively. Obtained simulation results in both domains have been properly complied with... 

For integers n, k, and i, the generalized Kneser graph K(n, k, i), is a graph whose vertices are subsets of size k of the set {1, 2, ..., n} and two vertices F and F are connected if and only if their intersection has less than i elements. In this paper we study the chromatic number of this graph. Some new bounds and properties for this chromatic number are derived. © 2017 University of Calgary  

Sliding friction between a pavement and tire is an important concern in traffic safety. The British pendulum test has been used worldwide to evaluate friction characteristics of pavement surfaces. However, because of considerable differences in apparatus, procedures, and operation, significant variability can occur for evaluations of British pendulum number (BPN). In this investigation, a direct shear test (DST) device is modified and proposed to determine the sliding friction coefficient between tire and pavement surface under dry and wet conditions. Dissipated energy values obtained from DST were compared with evaluations of BPN, and a high correlation was obtained. Tests were done using... 

In this paper we generalize Tannakian formalism to fiber functors over general tensor categories. We will show that (under some technical conditions) if the fiber functor has a section, then the source category is equivalent to the category of comodules over a Hopf algebra in the target category. We will also give a description of this Hopf algebra using the notion of framed objects. © 2017 Akadémiai Kiadó, Budapest, Hungary