### Browsing by Author "Dulla, Habte"

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Item The study of spin Glass state in Diluted Magnetic materials and semiconductors using monte carlo method and Theoretical Analysis(Addis Ababa University, 2013-03) Dulla, HabteShow more Spin glass system is a complex disordered system with a number of local minima separated by entropic barriers. Therefore, Parallel tempering Monte Carlo simulation was used in order to get fast thermalisation (to minimize the relaxation time). Distance dependent interaction coupling in 2D is studied in order to show how a spin glass phase transition occurs when couplings between far away spins are permitted by considering Edwards-Anderson Ising spin glass model. The interaction coupling is a quenched random variable whose probability of being non-zero decays with distance between two spin sites rij = |i−j|mod(L/2). The interaction coupling is random and its probability distribution is decaying with the distance between the spins (p(Jij) / r− ). The model is studied by changing among three different regimes ( > 2D, 4/3D < < 2D, < 4/3D) . A phase transition temperature for = 2, 3, 4 is obtained. In the present work, the possibility of existence of spin glass phase using classical Heisenberg model with Edwards-Anderson type of interactions has been explored employing Monte Carlo simulation of Binder parameter (g (L, T)). Previous experimental studies show that there is finite temperature phase transition but this study indicates that there is no finite temperature phase transition in 3D Heisenberg vector spin glass model. In the dissertation we also explore magnetic properties especially spin glass state, antiferromagnetic state and paramagnetic state of diluted magnetic semiconductors (A1−xMnxA0(A = Zn,Cd and A0 = S, Te, Se)) at critical region using classical Heisenberg spin model with high temperature series expansion extrapolated with vii pad´e approximants. The critical exponents associated with magnetic susceptibility ( ' 1.38 ± 0.1) and correlation function ( ' 0.8 ± o.1) were also obtainedShow more Item The study of upper critical field Calculetion of Superconducting magnesium Diboraide(Addis Ababa University, 2006-07) Dulla, HabteShow more Magnesium diboride(MgB2) has joined the class of high Tc superconductors following the discovery of superconductivity in this compound in 2001 below Tc 40k. A large part of the excitement is due to the vast amount of unusual properties originating from the involvement in superconductivity of two sets of bands that are of a very different nature. Because of the different dimensionality of the two sets of bands,this”two band superconductivity” leads to a complex behavior of the anisotropic superconducting state properties as a function of magnetic field and temperature. The purpose of this thesis is to study the upper critical field (Hc2) of two-band s-wave superconducting magnesium diboride(MgB2) in the vicinity of Tc using a two-band Ginzburg-Landau(GL) theory. The analytical formula of Hc2(T) involving anisotropy of order parameter and anisotropy of effective-mass are found. The parameters of the upper critical field in ab-plane(Hab c2 ) and c-axis (Hc c2) can be found by fitting to the experimental data. In addition to this we have considered the linearized two band Eilenberger equations to calculate the upper critical field of superconducting magnesium diboride. Finally by taking the ratio of the upper critical field ( the fields in ab-plane and c-axis) we tried to show the anisotropy property of superconducting magnesium diboride. We observe that the ratio of the two fields strongly depends on temperature. So, this ratio of upper critical field is in the range of experimental result and temperature dependent. Key words: S-wave superconductor,anisotropy two-band superconductors,Ginzburg-Landau approach,Eilenberger equations,Magnesium diborideShow more