Browsing by Author "Amente, Chernet (PhD)"
Now showing 1 - 10 of 10
Results Per Page
Sort Options
Item Carbon Nanotubes(Cnts): Origin, Application, Preparation and Characterization by Transmission Electron Microscopy (Tem)(Addis Ababa University, 2018-08-04) Tariku, Demiss; Amente, Chernet (PhD)Carbon nanotubes (CNTs) are allotropes of carbon with a nanostructure that can have a length-to-diameter ratio greater than 1,000,000. These cylindrical carbon molecules have novel properties that make them potentially useful in many nanotechnology applications. Their unique surface area, sti_ness, strength and resilience have led to much excitement in the _eld of pharmacy. Carbon nanotubes are categorized as single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). Techniques have been developed to prepare nanotubes in sizeable quantities, such as: arc-discharge, laser-ablation, chemical vapor-deposition, silane-solution and ame-synthesis methods. The properties of CNTs are still being researched heavily and scientists have barely begun to tap the potential of these structures. They can pass through membranes, carrying therapeutic drugs, vaccines and nucleic acids deep into the cell to targets previously unreachable. Overall, recent studies regarding CNTs have shown a very promising glimpse of what lies ahead in the future of nano-medicines. This project presents the unique atomic structure, properties, application and problems of carbon nanotubes (CNTs). The electronic band structure of carbon nanotubes along with their small size and low dimension are responsible for their unique electrical, mechanical and thermal properties. More over the project focuses on the preparation, puri_cation and characterization of CNTs by Transmission Electron Microscope (TEM)Item Determination of Superconducting Transition Temperature Tc and Order Parameter(Addis Ababa University, 2018-08-12) Negash, Basaznew; Amente, Chernet (PhD)This project is well organized review of what is superconductivity, properties, some applications, and advantages of superconductors over ordinary conductors. The superconducting transition temperature (Tc) and the superconducting order parameter _(0) are determined employing the quantum field theory double time Green function technique and discussed using the well known BCS theory. The relation of Tc with the density of cooper pairs N(0) and interaction potencial V(0) together called electron coupling constant (_el) is shown graphically and discussed. Similarly the temperature dependence of the order parameter _(T) as a function of temperature (T) is shown graphically and discussed as well. For further or extensive studies on superconductivity, this project would be used as a base line.Item Diluted Magnetic Semiconductors (DMSs) and Their Applications(Addis Ababa University, 2017-08-04) Gurmessa, Gebremichael; Amente, Chernet (PhD)Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers (n- or p-type), practical magnetic semiconductors would also allow control of quantum spin state (up or down), which is an important property for spintronics applications. High Curie temperature (Tc above room temperature) DMS is re- quired for practical applications and it is still challenging. This work reviews, magnetism, and transport properties of diluted magnetic semiconductors for applications as storage and data processing. Hence, a brief of information on the property and mechanism of incorporating magnetic impurities into semiconductors to get a hybrid that demonstrates miniaturized system will be explained. Moreover, two e ects of impurity concentration x on magnon dispersion and fractional change of magnetization is plotted and discussed.Item Diluted Magnetic Semiconductors (DMSs) and their Applications(Addis Ababa University, 2017-08) Gurmessa, Gebremichael; Amente, Chernet (PhD)Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers (n- or p-type), practical magnetic semiconductors would also allow control of quantum spin state (up or down), which is an important property for spintronics applications. High Curie temperature (Tc above room temperature) DMS is re- quired for practical applications and it is still challenging. This work reviews, magnetism, and transport properties of diluted magnetic semiconductors for applications as storage and data processing. Hence, a brief of information on the property and mechanism of incorporating magnetic impurities into semiconductors to get a hybrid that demonstrates miniaturized system will be explained. Moreover, two e ects of impurity concentration x on magnon dispersion and fractional change of magnetization is plotted and discussedItem Electronic Structure of 2d-Gallium Arsenide as Studied Using First Principles Density Functional Theory (DFT)(Addis Ababa University, 2020-08-23) Amare, Andinet; Amente, Chernet (PhD)In this thesis we have calculated electronic structure of 2D-GaAs within the first principles using Quantum ESPRESSO pacakage. We optimized lattice constant and identified the nature and values of band gap of Zinc Blende structure gallium aresnide (GaAs). Norm-conserving pseudopotential is used for the self-consistent calculation, and the Generalized Gradient Approximation (GGA) for the exchange correlation. An optimum lattice constant of 7.4Å, 6.1Å, and 5.7Å, respectively, is obtained for 1D, 2D, and 3D, and in close agreement with the experimental value 5.65Å. 2-D GaAs is known to have a gap of 1.8 eV which is in close agreement with previous observations [45], but has 25.9 % error to the experimental value 1.43 eV [42]. This work clearly explains that lattice constant increases with decrease in crystal size. Moreover, we could identify the charge transfer between the Ga and As atoms. Generally we could make calculations and compare with its 1-D and 3-D structures results.Item First Principle Study of Electronic Structure of Gold Doped Graphene(Addis Ababa University, 2019-08-07) Zeleke, Beletu; Amente, Chernet (PhD)In this thesis _rst principle calculations with in density functional theory (DFT) of electronic structure of gold Au doped graphene is studied by using a plane wave pseudopotential method. We used local density approximation (LDA) and generalized gradient approximations (GGA) for the exchange correlation potential. In all calculations, the geometry optimization option was employed in alloying the structure to fully relax. The convergence of the system was checked by calculating total energy versus kinetic energy cutt-o_ and total energy versus the size of k-points set. Convergence was attained for graphene starting from 41Ryd or 557:6eV , hence used to reduce computational cost. A uniform mesh 0f 13 _ 13 _ 1 k-points was used since it gives a good convergence at a reasonable computational cost providing charge density results and lattice constant of 2:485_A. From the band structure of graphene the two bands touch each other at K-point. DOS plot also gives neither band gap between conduction and valence bands nor overlapping at the Fermi energy indicating that graphene is semi-conductor. Finally, in the band structure of Au doped graphene built in 13_13_1 super cell the _=__ band cross at Fermi level. As clear evidence of the interaction between Au dopant and the graphene, the Fermi level is now moved to the conduction band, and the DOS display a sharp peak at the Fermi level. Therefore, the doping of Au in the graphene makes graphene semi-metallic.Item Prospects for Room Temperature Magnetic Semiconductors and their Applications(Addis Ababa University, 8/5/2018) Mosissa, Asfaw; Amente, Chernet (PhD)This project is a review of prospects for room temperature magnetic semiconductors and their applications. We also studied general properties of semiconductors related to and found that their conductivity increases as temperature increase. Moreover, diluted magnetic semiconductors (DMS) are understood as a class of magnetic semiconductor in which a fraction of the cations are substitutionally replaced by magnetic ions. Finally, by using Heisenberg model of magnetic interaction we demonstrated the dispersion relation of magnons, average number of magnons, density of state for three dimension or bulk system and found that magnons dispersion is a ected by magnetic spins from the magnetic dopant, the increase of density of state with energy, variation of number of magnons with temperature and lattice constant of the host material.Item Prospects for Room Temperature Magnetic Semiconductors and their Applications(Addis Ababa University, 2018-08-05) Mosissa, Asfaw; Amente, Chernet (PhD)This project is a review of prospects for room temperature magnetic semiconductors and their applications. We also studied general properties of semiconductors related to and found that their conductivity increases as temperature increase. Moreover, diluted magnetic semiconductors (DMS) are understood as a class of magnetic semiconductor in which a fraction of the cations are substitutionally replaced by magnetic ions. Finally, by using Heisenberg model of magnetic interaction we demonstrated the dispersion relation of magnons, average number of magnons, density of state for three dimension or bulk system and found that magnons dispersion is a ected by magnetic spins from the magnetic dopant, the increase of density of state with energy, variation of number of magnons with temperature and lattice constant of the host material.Item Signi_Cance of Hole-Magnon Interaction on Ferromagnetism in Diluted Magnetic Semiconductors(Addis Ababa University, 2018-05-02) Mesele, Degefa; Amente, Chernet (PhD)In this thesis we studied hole-magnon dynamics to understand the contribution of their interaction on the enhancement of ferromagnetism in diluted magnetic semiconductors (DMS). The t-J model is used as the interaction Hamiltonian with a method of per- turbation mathematical approach in which canonical transformation is used to obtain convergent values starting with canting the quantum anti-ferromagnetism from its Neel state by some angle _ with respect to the z-axis. Employing a spinless fermion represen- tation for hole operators, Holstein-Primako_ and Bogoliubou transformations are used to change the hole and spin dynamics to the many body problem. Our calculations in- dicated that motion of a hole in the anti-ferromagnetic background is dependent on the magnetic correlation of the system and the tilted angle. The motion of a hole around the anti-ferromagnetic order is shown to be characterized by high and low energy dispersion uctuation as the angle changes from 00 to 900. This uctuation in energy spectrum is attributed as arising from the pair repair spin alignment which could happen in anti- ferromagnetic(AF) background because of the hole motion e_ect. This is to mean that the motion of the hole causes a disturbance on the spin cloud of the localized electrons. This also infer that e_ect of hole motion gives a ferromagnetic (FM) order (maxima) and anti-ferromagnetic (AF) order (minima) uctuation. Making use of the obtained result, energy dispersion (Ek), spectral function (density function) (Ak) and quasiparticle weight (Zk) for the t=J << 1 and t=J >> 1 limit were numerically evaluated and plotted.Item Study of Effects of Magnon-Exciton Interaction on Ferromagnetism of Diluted Magnetic Semiconductors(Addis Ababa University, 2017-06) Fufa, Dereje; Amente, Chernet (PhD)In this thesis the e ect of exciton-magnon interaction on Diluted Magnetic Semiconduc- tors (DMS) is theoretically studied employing quantum eld theory. The double time temperature Green function technique is used to nd dispersion whereby single mode magnon number is obtained from spectral density calculations. It is understood that the existence of exciton and magnon coupling increases total number of magnons which is the reason for decrease in magnetization leading to lowering of ferromagnetic transition temperature Tc. This is perhaps due to scattering of orientations of introduced magnetic spins. The disorder is attributed as enhanced by long time coupling of electron-hole pair, reducing number of holes that mediate ferromagnetism. At lower temperatures the impu- rity concentration, x and Tc direct relation is known to violate due to the T1=2 containing factor dominance