Physics

A 2d Monte Carlo Simulation of Diffusion in a Disorder Media

2026-03-12T12:44:29Z

A 2d Monte Carlo Simulation of Diffusion in a Disorder Media Migbaru Bekele Birhanu; Solomon Negash In this work, we present 2D Monte Carlo simulations of tracer in disorder media with obstacles distributed randomly. For diffusion of a particle, the mean-square displace ment of the diffusing species is linearly proportional to time for normal diffusion. But in disordered systems anomalous diffusion occurs, in which the means square displacement is proportional to a fractional power of time not equal to one.As the ob stacle concentration approaches the percolation threshold, diffusion becomes more anomalous for long times; the anomalous diffusion exponent increases.Simulation data show anomalous for short times and normal for long times below percolation threshold.Monte Carlo calculations are used to characterize anomalous diffusion for obstacle concentrations between zero and the percolation threshold.In addition,above percolation threshold anomalous for short times but for long time( t), < r2 > approx imate fixed point proportional to mean value of boundary condition.,So the slope oflog < r2 > /t versus logt goes to-1 and dw →.The intersection time from normal to crowded diffusion is the crossover time tCR.

Tuning Structural, Electronic,Magnetic and Optical Properties of 2D-WX2 under Biaxial Strain and Substitutional Doping in First Principles Method

2025-11-07T08:16:45Z

Tuning Structural, Electronic,Magnetic and Optical Properties of 2D-WX2 under Biaxial Strain and Substitutional Doping in First Principles Method Mulugeta Shiferaw Woldesenbet; Menberu Mengesha; Nebiyu Gemechu Two dimensional (2D) WX2 for X= S, Se and Te in various supercell sizes are considered in monolayer and heterobilayer. The structural, electronic, magnetic, and optical properties of 2D- WX2 are investigated using density functional theory (DFT) with respect to a plane wave ultra-soft Pseudopotentials (PW-USPPs) and normconserving pseudopotentials (NCPPs) in a generalized gradient approximation (GGA) and the Hubbard correction (GGA + U) with the implementation of quantum espresso package. The optimized lattice parameters have been investigated interactively in each type of material. The band gaps for monolayer WTe2 are investigated for unstrained, 2% biaxial compressive and tensile strain to the slab of the mono layer using GGA and GGA+U approximations. In GGA+U approximation, the energy band gap becomes slightly wider than with that of the GGA approximation. Monolayer WTe2 under biaxial compression exhibits an expanded energy band resulting in blue shift and in biaxial tension red shift by narrowing of the energy band gap in comparison to the equilibrium. Pristine WSe2 monolayer is identified as a nonmagnetic direct band gap semiconductor with a band gap of 1.55 eV. Upon substituting Mn for W in the WSe2 monolayer, the resulting structure exhibits enhanced stability, indicated by a negative formation energy. The Mn doped monolayer of WSe2 has slightly shorter bond length with 2.4294 ˚A than the pure with 2.5405 ˚A for the Mn-Se and W-Se respectively. The doped system becomes FM and total magnetic mo ment within the nearest neighboring interactions of the impurity atoms increases notably and it is attributed to the electron-correlation effect in the high spin state under the GGA+U approx imation. However, this correlation effect proves insignificant on the total magnetic moment for the second nearest neighboring interactions, yielding consistent outcomes in both GGA and GGA+U approximations. The transition of FM to AFM state occurs above room temperature for low impurity concentration, 443 k and 450 k for 8% and 12.5% substitution with Mn re spectively. This indicates long-range FM ordering and crucial for high-temperature 2D-diluted magnetic semiconductors. However, at high concentrations of impurity atoms, the temperature drops below room temperature (220 K in 22.2%) in the doped monolayer WSe2, showing weak magnetic interaction. Lastly, the optical property of a doped system is studied by applying polarization in perpendicular and parallel directions to the plane of the monolayer. For two-dimensional WS2/WSe2 hetero structure lattice constant and the vertical interlayer distance between slabs are a=3.28 ˚A andc=13.14 ˚Arespectively. The bandgapsforWS2/WSe2 hetero structure are investigated for unstrained, (0-4)% biaxial compression and tension using GGA approximations. Direct band gap of 0.51eV is obtained for unstrained heterostructure. The band gap value of the heterobilayer is affected by the application of strain. When the heterobilayer is imposed to biaxial compression the band gap value increases. However, the tensile stress results in reducing the band gap value of the heterobilayer and the system attains its metallic or semimetalic state at 4% biaxial tensile strain. Tunability of the band gap is very viii crucial in developing photovoltaic and optoelectronic devices. The strained heterostructure shows different optical property when it is under biaxial tension and compression due to the change in the values of the band gap. The heterobilayer has also high absorption coefficient in the visible light spectrum which makes it a promising material for photovoltaic applications.

Tuning Structural, Electronic,Magnetic and Optical Properties of 2D-WX2 under Biaxial Strain and Substitutional Doping in First Principles Method

2025-11-07T08:07:42Z

Electronic and structural properties of Magnesium dibo ride using density Functional theory

2025-11-06T12:55:37Z

Electronic and structural properties of Magnesium dibo ride using density Functional theory Tesfa Mosisa; Menberu Mengesha; Nebiyu Gemechu In this thesis, electronic and structural properties of Magnesium diboride (MgB2) is investi gated with the density functional theory by using Quantum EPRESSO Package. The gener alized gradient approximation (GGA) is used to compute the exchange correlation energy. The total energy of Magnesium diboride is computed as a function of cutoff energy and Monk Horst pack grid size. The results show that the total energy per cell is monotonically decreasing with increasing cutoff energy and converged at 70Ry plane wave cutoff energy and the ground state energy has its minimumat-130.58056956 Ry. Thetotal energy of MgB2 percellhasconverged at 11 × 11 ×9k-point grids with a ground state energy of-130.31353522 Ry. Besides, the opti mized lattice constants of bulk MgB2 have been determined to be a = 5.8 Bohr and hence, c = 6.669288 Bohr with respect to our computational calculation. The experimental values of bulk HCP Magnesium diboride are a = 5.808413 Bohr and c = 6.678954 Bohr. The lattice constant determined using DFT calculation is compatible with an experimental result with a relative er ror of 0.00144. Finally, the band structure and density of state of HCP magnesium diboride are computed. The band structure calculation shows that there is overlap between the conduction band and the valance band. This clearly shows that MgB2 is conductor or metal (zero band gap material). The density of state also shows that there is no discontinuity before and after the Fermi Level. The density of state is continuous and there is no an insulating regime.