Ali Bakhshai | Goucher College (original) (raw)
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Graduate Center of the City University of New York
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A self-consistent Hartree energy band calculation was done for the MnO crystal using the linear c... more A self-consistent Hartree energy band calculation was done for the MnO crystal using the linear combination of atomic orbitals (LCAO) method. Guassian type atomic orbitals were used in the LCAO method. This calculation was done for paramagnetic MnO with the NaCl lattice structure. The results show that the energy bands around the Fermi level of MnO are unusually flat, meaning that the electrons in this region are strongly localized. Therefore short range correlation was added to the results of this band calculation. The short range correlation effects were added by calculating atomic type corrections to the original band structure. The results of this correlation calculation show that a large amount of energy is required to excite an electron from the Mn 3d band. Therefore the lowest excitation (the one that requires the least energy) is an excitation from the top of the O 2p band to the Fermi level. This yields a fundamental band gap of 4.8 eV which is in good agreement with optical absorption experiments. This fundamental band gap of 4.8 eV implies that MnO is an insulator, in agreement with conductivity experiments. Our Hartree results for the valence bands of MnO agree very well with the results of photoemission experiments. In comparison to the photoemission data, the results of our self-consistent Hartree calculation are an order of magnitude better than the results of the only other band calculation for MnO. Comparison with band calculations for other transition metal oxides (other than MnO) imply that with a good self-consistent Hartree energy band calculation for MnO, we can get results that are similar to or even better than the results obtained with the Kunz local orbital Hartree-Fock plus long range correlation method and the Hartree-Fock-Slater method.
Thin films of high-Tc YBa2Cu3O7 have been fabricated via magnetron sputtering onto MgO substrates... more Thin films of high-Tc YBa2Cu3O7 have been fabricated via magnetron sputtering onto MgO substrates with and without Au buffer layers. In all the cases considered, Tc values greater than 80 K and metallic behavior in the normal state are found. The specimens with Au buffer layers demonstrate an improved Meissner effect and significantly higher critical current density (3.3 x 10 to the 6th A/sq m at 2 K and 3.5 x 10 to the 4th A/sq cm at 77K).
Zn and Ga, two unique dopants having definite valence states and filled 3d levels are used to pro... more Zn and Ga, two unique dopants having definite valence states and filled 3d levels are used to probe the YBa2Cu3O7 superconductor. A small doping of Ga induces an orthorhombic to tetragonal structural transition, but the values of T(c) are as high as 81 K in the tetragonal phase. YBa2(Cu/1-x/Zn/x/)3O7 retains the same orthorhombic structure as the parent compound, but with highly depressed T(c). The oxygen vacancy order, or the linear chain structure, is found to be not essential for the high T(c). Data suggest that Ga and Zn preferentially substitute Cu(1) and Cu(2) sites, which leads to the conclusion that the most important feature of the high T(c) oxide superconductors is the Cu-O2 planes.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002
The mechanochemical reduction of Cu2O with Al was induced via ball milling. A self-propagating re... more The mechanochemical reduction of Cu2O with Al was induced via ball milling. A self-propagating reaction took place in two distinctly different ways. If the mass of the reactant powder mixture was large enough, ignition happened within a few seconds after starting the mill. Neither the activation of the reactants nor the formation of a powder layer covering the surfaces of the milling tools were necessary. If the mass of the charge is smaller than a critical value, the self-propagating reaction is ignited only after several minutes of mechanical activation. A slight excess of Al promoted an immediate reaction. The mechanism of the process is determined by the balance of the reaction heat, the heat transfer within the powder, and the heat loss to the environment. Local melting plays an important role, especially by promoting the formation of partially molten aggregates from free particles.
A self-consistent Hartree energy band calculation was done for the MnO crystal using the linear c... more A self-consistent Hartree energy band calculation was done for the MnO crystal using the linear combination of atomic orbitals (LCAO) method. Guassian type atomic orbitals were used in the LCAO method. This calculation was done for paramagnetic MnO with the NaCl lattice structure. The results show that the energy bands around the Fermi level of MnO are unusually flat, meaning that the electrons in this region are strongly localized. Therefore short range correlation was added to the results of this band calculation. The short range correlation effects were added by calculating atomic type corrections to the original band structure. The results of this correlation calculation show that a large amount of energy is required to excite an electron from the Mn 3d band. Therefore the lowest excitation (the one that requires the least energy) is an excitation from the top of the O 2p band to the Fermi level. This yields a fundamental band gap of 4.8 eV which is in good agreement with optical absorption experiments. This fundamental band gap of 4.8 eV implies that MnO is an insulator, in agreement with conductivity experiments. Our Hartree results for the valence bands of MnO agree very well with the results of photoemission experiments. In comparison to the photoemission data, the results of our self-consistent Hartree calculation are an order of magnitude better than the results of the only other band calculation for MnO. Comparison with band calculations for other transition metal oxides (other than MnO) imply that with a good self-consistent Hartree energy band calculation for MnO, we can get results that are similar to or even better than the results obtained with the Kunz local orbital Hartree-Fock plus long range correlation method and the Hartree-Fock-Slater method.
Thin films of high-Tc YBa2Cu3O7 have been fabricated via magnetron sputtering onto MgO substrates... more Thin films of high-Tc YBa2Cu3O7 have been fabricated via magnetron sputtering onto MgO substrates with and without Au buffer layers. In all the cases considered, Tc values greater than 80 K and metallic behavior in the normal state are found. The specimens with Au buffer layers demonstrate an improved Meissner effect and significantly higher critical current density (3.3 x 10 to the 6th A/sq m at 2 K and 3.5 x 10 to the 4th A/sq cm at 77K).
Zn and Ga, two unique dopants having definite valence states and filled 3d levels are used to pro... more Zn and Ga, two unique dopants having definite valence states and filled 3d levels are used to probe the YBa2Cu3O7 superconductor. A small doping of Ga induces an orthorhombic to tetragonal structural transition, but the values of T(c) are as high as 81 K in the tetragonal phase. YBa2(Cu/1-x/Zn/x/)3O7 retains the same orthorhombic structure as the parent compound, but with highly depressed T(c). The oxygen vacancy order, or the linear chain structure, is found to be not essential for the high T(c). Data suggest that Ga and Zn preferentially substitute Cu(1) and Cu(2) sites, which leads to the conclusion that the most important feature of the high T(c) oxide superconductors is the Cu-O2 planes.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002
The mechanochemical reduction of Cu2O with Al was induced via ball milling. A self-propagating re... more The mechanochemical reduction of Cu2O with Al was induced via ball milling. A self-propagating reaction took place in two distinctly different ways. If the mass of the reactant powder mixture was large enough, ignition happened within a few seconds after starting the mill. Neither the activation of the reactants nor the formation of a powder layer covering the surfaces of the milling tools were necessary. If the mass of the charge is smaller than a critical value, the self-propagating reaction is ignited only after several minutes of mechanical activation. A slight excess of Al promoted an immediate reaction. The mechanism of the process is determined by the balance of the reaction heat, the heat transfer within the powder, and the heat loss to the environment. Local melting plays an important role, especially by promoting the formation of partially molten aggregates from free particles.