Browsing by Author "Senbeta, Teshome (PhD)"
Item Electron Scattering in Graphene by Non-Symmetric Potential with Magnetic Dipole Moment in the Frame of Born Approximation(Addis Ababa University, 2016-08) Feyisa, Getachew; Senbeta, Teshome (PhD)The thesis is designed to study the scattering process of electron in graphene by spherically non-symmetric potentials within the Born approximation. The scattering process with electric and magnetic dipoles parallel and transverse to the graphene plane were studied and the obtained numerical result shows the scattering process by electric dipole is dominant in comparison with the magnetic dipole. In addition, we obtained non-zero backscattering for non-symmetric potentialsItem Investigation of Plasmon Enhanced Optical Properties and Photocatalytic Applications of Composite Nanostructures(Addis Ababa University, 2021-03-07) Beyene, Gashaw; Senbeta, Teshome (PhD); Mesfi n, Belyaneh (PhD); Zhang, Qinfang (Professor)In this dissertation, we investigated plasmon enhanced optical properties and photocatalytic applications of composite nanostructures by using noble materials from theoretical and experimental perspectives. The theoretical investigation mainly focused on the optical and plasmonic properties of spherical and spheroidal ZnO@Ag core-shell nanocomposite embedded in nonabsorptive medium. The e_ective dielectrics function, polarizability, refractive index and optical absorbance for spherical as well as absorption cross-section and radiation e_ciency for spheroidal nanocomposite are studied. These optical and plasmonic properties are tuned when size of core and/or composite, shell thickness, shape of core and/or shell, and refractive index of the host-medium is changed. In spherical nanocomposite, there are two resonances associated with interfaces while in spheroidal nanocomposite four plasmonic resonances are appeared that correspond to interfaces and the bonding/antibonding pairs. The tunable of plasmonic resonances from UV to infrared spectral regions of such composite enables it to exhibit very interesting material properties in a variety of applications. Experimentally, we emphasized the optical properties and photocatalytic applications of ZnO nanoparticles, triple layered ZnO@SiO2@Ag core-shell composite nanostructures and Bi doped/undoped TiO2 nanostructures. In particular, we synthesized ZnO nanoparticles using three di_erent methods as presented in the experimental section of the study. Due to di_erent synthesis methods and procedures as well as di_erent surfactant/stablizing agent, the synthesized ZnO nanoparticles have di_erent morphology, band gap energy, varying quantum size e_ect and photocatalytic responses. The photocatalytic performance of ZnO nanoparticles were evaluated by using methyl orange dye as a pollutant under UV-light irradiation. Triple layered ZnO@SiO2@Ag core-shell nanocomposites are also fabricated at 400 oC for di_erent calcination time. The photocatalytic activity and stability of the as-synthesized samples were evaluated by photo-degradation of organic methylene blue under ultraviolet light irradiation. When ZnO@SiO2 is coated with Ag, the stability and photocatalytic performance of the composite ZnO@SiO2@Ag photocatalysts were highly enhanced. In addition, Bi- doped/undoped TiO2 nanoparticles are synthesized by green synthesis method using zingiber o_cinale extract solution. After doping Bi, the band gap energy is decreased from 3:00 eV to 2:76 eV , absorption region is shifted to visible range of solar spectrum, and surface function is modi_ed. The photocatalytic activity of the sample is analyzed by UV-Vis spectroscopy using trypan blue dye. The photo-degradation e_ciency of chemical synthesis TiO2 nanoparticles is enhanced to 98:34 % by doping basic metal Bi and using ginger extraction solution. The synthesized samples were analyzed using a characterization techniques such as X-ray di_raction (XRD) spectroscopy, scanning electron microscope (SEM), Transmission electron microscope (TEM), Ultraviolet visible (UV-Vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS).Item Quantum hall Effect in Graphene(Addis Ababa University, 2015-10) Eticha, Moreda; Senbeta, Teshome (PhD)We studied Quantum Hall E ect with and without magnetic eld in graphene. The quan- tum Hall e ect is a quantum-mechanical version of the Hall e ect, observed in graphene, in which the Hall conductance ( H) takes on the quantized values ( e2/h) with = 4(n+1 2 ) an integer (integer quantized Hall E ect in graphene). Our study con rmed that there is a zero energy Landau level in graphene with magnetic elds. The fractional quantum Hall e ect in graphene is observed as the result of coulomb interaction and correlation of electronsItem Rashba Spin-Orbit interaction in Two-Dimension Electron Gas(Addis Ababa University, 2016-08) Gemechu, Belay; Senbeta, Teshome (PhD)We have studied the spin orbit-coupling from the Dirac equation, Understanding the properties of electronic states in quantum wires is a central issue in nanoscience. In the ballistic region, with low electronic densities, electron-electron interactions are not very important, and wire conductance is determined by the transmission of electrons as independent particles and the effects of spin-orbit interaction (SOI) on the energy bands,a system of two dimensional equations with respect to the spinor components "#(t), in the case of = 0, 6= 0 and ballistic conductanceItem Review of Classical and Quantum Hall Effect(Addis Ababa University, 2016-02) Alem, Lakech; Senbeta, Teshome (PhD)In this project we review the Classical and Quantum Hall Effects. We discuss these two effects theoretically based on the available literature. In classical Hall Effect when a strong magnetic field is applied perpendicular to the electrons plane of movement, the electrons execute tiny cyclotron orbits around the flux lines. If in addition, an electric field applied in transverse direction to the induced electric field, the electrons will tend to drift in a direction perpendicular to both fields, generating the Hall Effect. For the case of Quantum Hall Effect, energy associated with the cyclotron motion is quantized giving rise to the Landau levels and at low temperatures all the electrons are in the lowest Landau level. The filling factor can be changed by varying the magnetic field B for a fixed carrier density. This leads to the sitation that the Hall conductivity takes the values equal to e2/h as discussed in Chapter three. The Quantum Hall Effect can be Integral or Fractional. The Integer Quantum Hall Effect can be understood in an independent-particle model, without taking into account the electron-electron interactions, in the Fractional Quantum Hall Effect, where the filling factors take fractional values (1/2,1/3,1/5,...) the electron-electron interactions play an essential role. The Coulomb interaction produces incompressible states of highly correlated carrier motion in high magnetic fields at specific fractional filling levels. At such magnetic fields the electrons can be treated as quasi particles called composite fermionsItem Review of Giant Magnetoresistance in Ferromagnetic Crystals(Addis Ababa University, 2016-09) Lakew, Tadesse; Senbeta, Teshome (PhD)This project contains a brief review of the giant magnetoresistance in ferromagnetic crystals, the transport properties of ferromagnets with a brief discussion of magnetic properties of crystals such as magnetization susceptibility, diamagnetism, paramag- netism, and ferromagnetism as well as the magnetization dynamics in ferromagnetic systems. Giant magnetoresistance materials have magneto transport properties which determine their suitability for applications in magnetic eld sensors, read heads, ran- dom access memories. The discovery of giant magnetoresistance has been a huge impact on our life, especially for mass data storage devices. Details of giant magne- toresistance applications, such as hard-disk read-heads, sensors and magnetic memory chips are presented. One of the important aspects of giant magnetoresistance discov- ery was that it was immediately turned into commercially available products with a giant market share. The highest storage capacity in the modern computers is at- tributed to the discovery of giant magnetoresistance. This review covered a simple model of giant magnetoresistance and dependence of it on structure and magnetic layer thickness with a brief discussion of interlayer exchange coupling and giant mag- netoresistance in magnetic multilayers including the giant magnetoresistance e ect in ferromagnets (organic semi-conductors)Item Review of Spin Dependent Hall Effect(Addis Ababa University, 2018-10-04) Defar, Addis; Senbeta, Teshome (PhD)Hall effects, in general, are transport phenomena, in which an applied field on the particles results in a motion perpendicular to the field. In traditional Hall effect and its quantum versions charges are transported by the action of a Lorentz force. The spin Hall effect(SHE) and its quantum versions are a relativistic spin orbit coupling phenomenon allow to create and manipulate the spin, and generate a spin current. It is of particular importance to the development of transistor like devices. The spin orbit interaction responsible for the SHE is also expected to cause the inverse process of the SHE. The inverse spin Hall effect (ISHE) is a process that converts a spin current into an electric current. The quantum spin Hall effect allows for the existence of an unusual type of material called a topological insulator without external magnetic field which conducts electricity on the surface but not through the bulk of the material and my finding shows the plot of resistivity versus quantized magnetic field is plateau.Item Review of Spin Dependent Transport(Addis Ababa University, 2017-08) Geleta, Abdisa; Senbeta, Teshome (PhD)Spin based electronics, is based on the use of spin orbit coupling in convectional semi- conductors materials. We regard the prediction and discovery of intrinsic spin Hall e ect as an important step toward developing integrated spin logic devices and achiev- ing lower energy consumption. To develop and review the three potential means of dissipationless spin transport in semiconductors with and without spin-orbit coupling are the use of spin currents, propagating modes, and orbital currents. Dissipation- less spin currents occur in materials with strong spin-orbit coupling, such as GaAs, while orbital currents occur in materials with weak spin-orbit coupling, such as Si, but with degenerate bands characterized by an atomic orbital index. The potential means for achieving dissipationless spin transport constitute a theoretical and asso- ciated potential experimental directions with technological implications. A number of alternatives to the current semiconductor technology have recently been proposed, including technologies based on carbon nanotubes, molecular electronics, and dilute magnetic semiconductors. In materials without spin-orbit coupling, one of the current e orts should be to try to detect the orbital Hall e ect and to investigate whether the orbital moment can be transferred to spin and hence be used for spin injection. Moreover, experimental techniques should be devised to enhance the amount of spin polarization arising from spin currentItem Studies on Thermoluminescence Peak Intensities of Germanium Quantum Dots(Addis Ababa University, 2020-08-18) Eshetu, Alemayehu; Senbeta, Teshome (PhD)In this project work I studied the TL properties of germanium Quantum dots in the one trap one recombination model for fist order kinetics. I simulated the TL glow curve for different parameters like the activation energy, the frequency factor, the heating rate and the initial concentration of the electrons in the conduction bands. The result shows that as the activation energy E is increased, the TL glow curve shifts towards higher temperatures, but the curve maintains its overall shape. Moreover, the initial concentration of filled traps has no effects on both the maximum height of the TL glow curve and the temperature of maximum TL intensity Tmax, but leaves the overall shape unchanged in the case of the Garlick-Gibson model and May-Partridge model. I also showed that as the frequency factor s is increased, the TL glow curve shifts towards lower temperatures, but the curve maintains its overall shape for the three models. At the same time the effect of initial concentration of filled traps studied and showed that the initial concentration has no effects on the maximum height of the TL glow curve, and leaves the shape of TL glow curve unchanged.