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We theoretically investigate the electronic and thermoelectric properties of Mn2MnAl using first-principles calculations and linearized Boltzmann transport formalism with constant relaxation time approximation. As a full-Heusler alloy, Mn2MnAl possesses a stoichiometry of X2YZ, where X and Y represent transition metal elements while Z is the main group element. Within our choice of pseudopotentials and other first-principles calculation parameters, we find that Mn2MnAl is a metallic ferromagnet with large spin polarization. Despite its metallic character, from the calculation of Seebeck coefficient and electrical conductivity, we find that Mn2MnAl exhibits a relatively large power factor compared to other common thermoelectric materials. [ABSTRACT FROM AUTHOR]

Semiconductors have long been the main materials used in the manufacture of charge-based as well as photonic-based electronic devices. GaN, owing to its wide-band gap, is often used for light-emitting diodes, and other optoelectronic applications. To adjust to the desired energy band gap, GaN is often combined with Al in form of AlxGa1-xN, which is expected to have a wider band gap than that of pure GaN. The increase of aluminum content yields an increase of electron activation energy. Here, we theoretically investigate how the band structure and optical spectrum evolve as the aluminum content is varied, by performing Density Functional Theory (DFT) calculation on pure wurtzite GaN and AlxGa1-xN (with x = 0.125 and 0.25). Simple band structures without rigorous treatment of the electron-electron interactionss are obtained through Plane-Wave Self-Consistent Field (PWSCF) calculation of DFT. The correction to the band gap due to electron-electron interactions is expected from the implementation of GW method. While, excitonic signal in optical spectrum is expected to arise upon the implementation of Bethe-Salpeter equation. Our calculation results show that the band gap increases with x in the region of x values we study. [ABSTRACT FROM AUTHOR]

Heusler alloys have been known for about a century, and predictions of magnetic moment values using Slater-Pauling rule have been successful for many such materials. However, such a simple counting rule has been found not to always work for all Heusler alloys. For instance, Fe2CuAl has been found to have magnetic moment of 3.30 µB per formula unit although the Slater-Pauling rule suggests the value of 2 µB. On the other hand, a recent experiment shows that a non-stoichiometric Heusler compound Fe2Mn0.5Cu0.5Al possesses magnetic moment of ~4 µB, closer to the Slater-Pauling prediction for the stoichiometric compound. Such discrepancies signify that the theory to predict the magnetic moment of Heusler alloys in general is still far from being complete. Motivated by this issue, we propose to do a theoretical study on a full-Heusler alloy Fe2MnAl to understand the formation of magnetic moment microscopically. We model the system by constructing a density-functional-theory-based tight-binding Hamiltonian and incorporating Hubbard repulsive as well as spin-spin interactions for the electrons occupying the d-orbitals. Then, we solve the model using Green's function approach, and treat the interaction terms within the mean-field approximation. At this stage, we aim to formulate the computational algorithm for the overall calculation process. Our final goal is to compute the total magnetic moment per unit cell of this system and compare it with the experimental data. [ABSTRACT FROM AUTHOR]

A theory of high-energy optical conductivity of La0.7Ca0.3MnO3 has been proposed previously. The proposed theory works to explain the temperature-dependence of the optical conductivity for the photon energy region above ~0.5 eV for up to ~22 eV, but fails to capture the correct physics close to the dc limit in which metal-insulator transition occurs. The missing physics at the low energy has been acknowledged as mainly due to not incorporating phonon degree of freedom and electron-phonon interactions. In this study, we aim to complete the above theory by proposing a more complete Hamiltonian incorporating additional terms such as crystal field, two modes of Jahn-Teller vibrations, and coupling between electrons and the two Jahn-Teller vibrational modes. We solve the model by means of dynamical mean-field theory. At this stage, we aim to derive the analytical formulae involved in the calculation, and formulate the algorithmic implementation for the self-consistent calculation process. Our final goal is to compute the density of states and the optical conductivity for the complete photon energy range from 0 to 22 eV at various temperatures, and compare them with the experimental data. We expect that the improved model preserves the correct temperature-dependent physics at high photon energies, as already captured by the previous model, while it would also reveal ferromagnetic metal - paramagnetic insulator transition at the dc limit. [ABSTRACT FROM AUTHOR]

Heusler compounds are families of magnetic materials with general stoichiometry of either X2YZ (full-Heusler compound) or XYZ (half-Heusler compound), with X and Y being transition metal elements, and Z a main-group element. Their various potentials for technology development make them be still relevant as a subject of both experimental and theoretical studies. Half-Heusler compounds are generally crystallized in the C1b-type structure. The magnetic moments of such materials may be predicted using Slater-Pauling rule, giving m = (Nvalence electrons - 18)µB per formula unit. However, this simple counting rule does not always work for all compounds in this group. This motivates us to perform a theoretical study to investigate the mechanism of magnetic moment formation microscopically. As a case study, we focus on NiMnSb, a particular half-Heusler compound, for which comparison between existing experimental results and theoretical predictions of its magnetic moment has not yet been quite convincing. We model the system by constructing a tight-binding-based Hamiltonian, incorporating Hubbard repulsive as well as spin-spin interactions for the electrons occupying the d-orbitals. We solve the model using Green's function approach, and treat the interaction terms within the mean-field approximation. At this stage, we aim to formulate the computational algorithm for the overall calculation process. Our final goal is to compute the total magnetic moment per unit cell of this system and compare it with available experimental data. [ABSTRACT FROM AUTHOR]