Sharif Digital Repository

The electrodynamics of Weyl semimetals is an extension of Maxwell's theory where in addition to field strength tensor Fμν, an axion field enters the theory which is parametrized by a four-vector bμ=(b0,b). In tilted Weyl materials (TWMs) an additional set of parameters ζ=(ζx,ζy,ζz) enters the theory that can be encoded into the metric of the spacetime felt by electrons in TWMs. This allows an extension of Maxwell's electrodynamics that describes electric and magnetic fields in TWMs, and tilted Dirac materials (TDMs) that correspond to bμ=0. The tilt parameter ζ appearing as an off-diagonal metric matrix element, mixing time and space components, which mingles E and B fields, whereby the... 

Katsnelson has shown that, within the Fermi-liquid approach, the optical conductivity of Dirac electrons in graphene is not affected by many-body interactions [M. I. Katsnelson, Eur. Phys. Lett. 84, 37001 (2008)EULEEJ0295-507510.1209/0295-5075/84/37001]. We show that, when the Dirac cone is tilted, the Fermi-liquid corrections arise in the optical conductivity in a manner that the correction depends on the angle between the electric field of the incident light and the tilt direction. Therefore the mapping of the optical conductivity for various directions of the incident light enables a determination of the many-body effect in the optical conductivity spectrum of the two-dimensional tilted... 

A method for joining metals in the semisolid state is presented. A model alloy Sn-15%Pb was used to demonstrate the concept. By presented process, dendritic microstructure of the weld zone can be avoided. Moreover, near-weld zone of the cold worked substrates which is affected by heat would have a globular structure due to a thermomechanical treatment. The two substrates were heated up locally in the joint line to the semisolid temperature range. At this point a stirrer was introduced into the weld seam in order to mix the two sides into a single uniform joint. Localized mechanical properties of different zones were examined using Shear Punch Test (SPT), showing a good strength in the weld... 

A method for joining metals in the semisolid state is presented. A model alloy Sn-15%Pb was used to demonstrate the concept. By presented process, dendritic microstructure of the weld zone can be avoided. Moreover, near-weld zone of the cold worked substrates which is affected by heat would have a globular structure due to a thermomechanical treatment. The two substrates were heated up locally in the joint line to the semisolid temperature range. At this point a stirrer was introduced into the weld seam in order to mix the two sides into a single uniform joint. Localized mechanical properties of different zones were examined using Shear Punch Test (SPT), showing a good strength in the weld... 

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... 

The rich structure of solid state physics provides us with Dirac materials the effective theory of which enjoys the Lorentz symmetry. In nonsymmorphic lattices, the Lorentz symmetry can be deformed in a way that the null energy-momentum vectors will correspond to the on-shell condition for tilted Dirac cone dispersion. In this sense, tilted Dirac/Weyl materials can be viewed as solid state systems where the effective spacetime is non-Minkowski. In this work, we show that the polarization tensor for tilted Dirac cone systems acquires a covariant form only when the spacetime is considered to be an appropriate deformation of the Minkowski spacetime that is compatible with the dispersion. As a... 

In systems with substantial spin fluctuations, dressing the polarization function by ladder diagram of Stoner (spin-flip) excitations can drastically modify the electromagnetic response. As a case study, we provide the detailed analysis of the corrections to the non-local optical conductivity of both doped and undoped graphene. While the resummation of ladder diagram of Stoner excitations does not affect the TE mode in doped graphene, it allows for a new undamped TM mode in undoped graphene. This is the sole effect of corrections arising from ladder diagrams and is dominated by Stoner excitations along the ladder rung which goes away by turning off the source of spin-flip interactions. In... 

Back extrusion technique was employed to characterize phase segregation tendency of mechanically stirred ZA27 alloy at different deformation rates. Variation of segregation intensity with ram diameter was found to follow opposite trends at low and high ram speeds. At sufficiently high ram speeds, small rams are of better performance in minimizing segregation whereas at low ram speeds, large rams result in less pronounced segregation. In addition, increasing ram speed invariably decreases segregation degree. Back extrusion at very high shear rates provided via a Drop Extruder Apparatus capable of displacing ram at speeds in excess of 1m/s results in production of very homogeneous products in... 

Effort was made to characterise segregation tendency of mechanically stirred ZA27 alloy with back extrusion as a thixoforming process. At sufficiently high ram speeds, at which liquid phase flows forcefully, pooling of the heavy liquid phase owing to gravitational segregation contributes to homogeneity of back extruded products in terms of solid phase distribution, The amount of this contribution depends on the contact time of this liquid with solid skeleton during liquid exertion to the clearance. Hence, at high ram speeds, small rams which demand taller initial slugs are more efficient in minimising the segregation. However, in the case of low speed back extrusion tests, liquid impotency... 

The insect circulatory system contains an open hemocoel, in which the mechanism of hemolymph flow control is ambiguous. As a continuous fluidic structure, this cavity should exhibit pressure changes that propagate quickly. Narrow-waisted insects create sustained pressure differences across segments, but their constricted waist provides an evident mechanism for compartmentalization. Insects with no obvious constrictions between segments may be capable of functionally compartmentalizing the body, which could explain complex hemolymph flows. Here, we test the hypothesis of functional compartmentalization by measuring pressures in a beetle and recording abdominal movements. We found that the... 

In [1], [2] it is shown that the minimum broadcast rate of a linear index code over a finite field Fq is equal to an algebraic invariant of the underlying digraph, called minrankq. In [3], it is proved that for F2 and any positive integer k, minrankq(G) ≤ k if and only if there exists a homomorphism from the complement of the graph G to the complement of a particular undirected graph family called 'graph family {Gk}'. As observed in [2], by combining these two results one can relate the linear index coding problem of undirected graphs to the graph homomorphism problem. In [4], a direct connection between linear index coding problem and graph homomorphism problem is introduced. In contrast to... 

I present an in-depth review of the trends and the directions taken by researchers worldwide in optical code-division multiple-access (OCDMA) systems. I highlight those trends and features that I believe are essential to the successful introduction of various OCDMA techniques in communication systems and data networks in the near future. In particular I begin by giving a comprehensive review of the construction of optical orthogonal codes (OOCs). Specifically I discuss the recently developed algorithms that are based on matrix algebra, which simplify and enhance the efficiencies of algorithms in OOC generation. In communication systems studies I first focus on and discuss various OCDMA... 

In this paper, we analyze the worst-case delay performance of a network of fair queuing servers in case of multiple-paths between source-destination pairs. In order to develop worst-case delay analysis for the multiple-path scenario, two challenging events should be addressed: merging different sub-streams of a session as well as distributing a traffic stream among different paths (traffic partitioning). Specifically, we propose to merge incoming sub-streams of a session to a node according to a limited rate Fair Queuing (FQ) scheme. Although the merged stream is treated as a unity, this approach results in a guaranteed level of service to each sub-stream. In addition, an algorithm is... 

This paper presents an investigation of the effects of tool rake angle and nose radius on the surface quality of ultraprecision diamond-turned porous silicon. The results showed that as rake angle decreases, the high-stress field induced by the tool edge increases, causing microcracks to propagate extensively near the pore walls. As a result, the ductile-machined areas shrank under a negative tool rake angle. On the other hand, brittle fracture occurred around pores released cutting pressure significantly. These trends of rake angle effects are distinctly different from those in the cutting of non-porous silicon. Finite element simulation of stress in the cutting area agreed with the... 

This paper presents an investigation of the effects of tool rake angle and nose radius on the surface quality of ultraprecision diamond-turned porous silicon. The results showed that as rake angle decreases, the high-stress field induced by the tool edge increases, causing microcracks to propagate extensively near the pore walls. As a result, the ductile-machined areas shrank under a negative tool rake angle. On the other hand, brittle fracture occurred around pores released cutting pressure significantly. These trends of rake angle effects are distinctly different from those in the cutting of non-porous silicon. Finite element simulation of stress in the cutting area agreed with the... 

The mechanical properties of Mg-4wt.% Zn alloy single crystals along the [0001] orientation were measured through micropillar compression at 23 °C and 100 °C. Basal slip was dominant in the solution treated alloy, while pyramidal slip occurred in the precipitation hardened alloy. Pyramidal dislocations pass the precipitates by forming Orowan loops, leading to homogeneous deformation and to a strong hardening. The predictions of the yield stress based on the Orowan model were in reasonable agreement with the experimental data. The presence of rod-shape precipitates perpendicular to the basal plane leads to a strong reduction in the plastic anisotropy of Mg. © 2020 Elsevier B.V  

The mechanical properties of Mg-4wt.% Zn alloy single crystals along the [0001] orientation were measured through micropillar compression at 23 °C and 100 °C. Basal slip was dominant in the solution treated alloy, while pyramidal slip occurred in the precipitation hardened alloy. Pyramidal dislocations pass the precipitates by forming Orowan loops, leading to homogeneous deformation and to a strong hardening. The predictions of the yield stress based on the Orowan model were in reasonable agreement with the experimental data. The presence of rod-shape precipitates perpendicular to the basal plane leads to a strong reduction in the plastic anisotropy of Mg. © 2020 Elsevier B.V  

This research explored the influences of shielding gases on the appearance of weld beads and the microstructures and mechanical properties of thin-wall samples using conventional gas metal arc welding as the heat source by using 5356 aluminium alloy welding wire as the raw materials and nitrogen (N2) and argon (Ar) as the shielding gases. The results showed that under the same parameters and after mono-layer single-bead welding was performed using N2 as the shielding gas, the bead height was higher, the bead width was narrower, and the penetration depth was shallower. The grain size of the thin-wall sample protected by N2 was 43.5–47.8 % smaller than that obtained under Ar protection.... 

In this paper, we describe a carbon dioxide mineralization process and its associated solid products. These solid products include amorphous silica, iron hydroxides and magnesium carbonates. These products were subjected to various characterization tests, and the results are published here. It was found that the iron hydroxides from this process can have different crystalline properties, and their formation depended very much on the pH of the reaction conditions. Different forms of magnesium carbonate were also obtained, and the type of carbonate precipitated was found to be dependent on the carbonation temperature. Hydromagnesite was obtained mainly at low temperatures, while dypingite was...