Luiz G. Ferreira

Approximation to density functional theory for the calculation of band gaps of semiconductors

The local-density approximation (LDA), together with the half-occupation (transition state) is notoriously successful in the calculation of atomic ionization potentials. When it comes to extended systems, such as a semiconductor infinite system, it has been very difficult to find a way to half-ionize because the hole tends to be infinitely extended (a Bloch wave). The answer to this problem lies in the LDA formalism itself. One proves that the half-occupation is equivalent to introducing the hole self-energy (electrostatic and exchange-correlation) into the Schroedinger equation. The argument then becomes simple: the eigenvalue minus the self-energy has to be minimized because the atom has a minimal energy. Then one simply proves that the hole is localized, not infinitely extended, because it must have maximal self-energy. Then one also arrives at an equation similar to the SIC equation, but corrected for the removal of just 1/2 electron. Applied to the calculation of band gaps and effective masses, we use the self-energy calculated in atoms and attain a precision similar to that of GW, but with the great advantage that it requires no more computational effort than standard LDA.

LOW-ENERGY ELECTRON SCATTERING BY H2O, H2S, H2SE, AND H2TE

We report elastic differential, integral, and momentum transfer cross-sections for H2X molecules (X: O, S, Se, and Te) obtained at the static exchange level of approximation. The energy range considered was from 2 up to 30 eV for H2O and from 5 up to 30 eV for the other molecules. Our calculations were performed with the Schwinger multichannel method with pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)], combined with a Born closure procedure in order to account for the long-range potential due to the permanent dipole moment of the targets. Our calculated cross-sections for H2O and H2S are in good agreement with other theoretical results. Agreement with available experimental data is also encouraging. It was found that molecular size plays a crucial role in the scattering process. The influence of heavy and H atoms in the collisions is also discussed. For the integral cross-sections of the heavier molecules we also investigated incident energies below 5 ...

Note on the generation of Gaussian bases for pseudopotential calculations

We present a technique to generate Cartesian Gaussian bases for electronic configuration and cross-section calculations on molecules. The technique is specially useful for pseudopotential work, when the bases cannot be tabulated because they depend on the specific choice of the pseudopotential.

Magnetic properties of MnN: Influence of strain and crystal structure

For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in [100] direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AlN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4μB per Mn atom) instead of the metallic state obtained for smaller lattice constants. ...

Elastic scattering of low-energy electrons by CF3Cl, CF3Br and CF3I

We report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by CF3X (X = Cl, Br, I) molecules. We use the Schwinger multichannel method with pseudopotentials (Bettega et al1993 Phys. Rev. A 47 1111) at the static exchange approximation. Our calculations cover the energy range between 5 and 30 eV. We compare our results with available theoretical and experimental results for CF3Cl and CF3I, and in general find good agreement. In particular, our results show the shape resonances belonging to the A1 and E representations of the C3v group that have been reported by previous work.

Calculation of elastic scattering cross sections of low‐energy electrons by PbH4 and SnH4

We report elastic integral, differential, and momentum transfer cross sections from 10 to 30 eV for electron scattering by SnH4 and PbH4, obtained using the Schwinger Multichannel Method with Pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)]. With these molecules we close the series of XH4 molecules, with X=C, Si, Ge, Sn, Pb. We compare the present results with those obtained previously for CH4, SiH4, and GeH4. We find similarities in the cross sections for SiH4, GeH4, SnH4, and PbH4 and a distinctive behavior of CH4. We discuss the role of the center atom size in the scattering process. To our knowledge this is the first ab initio calculation of the SnH4 and PbH4 electron scattering cross sections.

A comparative study of elastic scattering of low-energy electrons by boron, aluminum and gallium trihalides

In this paper we present integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by some metal-halogen molecular compounds, namely, BF3, BCl3, BBr3, BI3, AlF3, AlCl3, AlBr3, AlI3, GaF3, GaCl3, GaBr3, and GaI3. The pseudopotential based calculations were carried out with the Schwinger multichannel method at the static-exchange level of approximation. It is the purpose of this work to make a comparative study of the scattering processes involving aluminum and gallium trihalides with previous results for the boron ones [M. H. F. Bettega, Phys. Rev. A 61, 042703 (2000)]. We find through direct comparison of the elastic cross sections that, at low energies, the scattering processes are mainly dominated by the halogen atoms.

Electron collisions with isomers of C4H8 and C4H10

We report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by several isomers of the C4H8 molecules, such as isobutene, trans-2-butene, cis-2-butene, skew-1-butene and syn-1-butene, and by two isomers of C4H10, the isobutane and the butane molecules. To calculate the cross sections, we use the Schwinger multichannel method with pseudopotentials (Bettega et al 1993 Phys. Rev. A 47 1111) applied at the static exchange level of approximation for incident energies from 10 to 50 eV. Although the C4H8 isomers have different geometric structures, our results show that the integral cross sections for each of these isomers present a broad shape resonance around 10 eV; our results also show that for all C4H8 molecular targets, integral cross sections are very similar in shape and magnitude. Similarities are also found in the momentum transfer and in the differential cross sections of these isomers. The same pattern is found in the cross sections of the C4H10 isomers. Through comparison of the integral and momentum transfer cross sections of 1,3-butadiene (C4H6), trans-2-butene (C4H8) and butane (C4H10), all belonging to the C2h group and having similar structures, we discuss the role of the hydrogen atoms in the scattering process by these molecules. Further we show that the integral elastic cross sections of all simple hydrocarbons present strong similarities after a scaling. We present a simple geometric model for this scaling that works quite well for a whole family of CnHm molecules (with combinations of n = 1, 2, 3, 4 and m = 2, 4, 6, 8, 10).

Phase Separation and Ordering in Group-III Nitride Alloys

The electronic, structural, and thermodynamic properties of cubic (zinc blende) group-III nitride ternary InxGa1�xN and quaternary AlxInyGa1�xyN alloys are investigated by combining first-principles total energy calculations and cluster expansion methods. External biaxial strain on the alloys are taken into account in the calculations. While alloy fluctuations and strain effects play a minor role in the physical properties of AlGaN, due to the small lattice-mismatch, in the InGaN alloys we found a remarkable influence of strain on the phase separation. The effect of biaxial strain on the formation of ordered phases has been also investigated, and the results are used to clarify the origin of the light emission process in InxGa1�xN based optoelectronic devices. The dependence of the lattice constant and energy bandgap in quaternary AlxInyGa1�xyN alloys was obtained as function of the compositions x and y.

Experimental and theoretical elastic cross sections for electron collisions with the C3H6 isomers

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi* shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect.