Cm Chang Ming Fang

Spin-polarization in half-metals (invited)

Half-metals are defined by an electronic structure, which shows conduction by charge carriers of one spin direction exclusively. Consequently, the spin polarization of the conduction electrons should be 100%. In reality this complete spin polarization is not always observed. Since the experimental search for half-metals is tedious and the verification of the expected spin polarization is involved, electronic structure calculations have played an important role in this area. So, an important question is, how the approximations in such calculations influence the resulting spin polarization of the conduction. Another aspect is the well-known fact that bulk properties can be very different from surface and interface properties. Indeed, measurements of the spin polarization in the bulk for, e.g., NiMnSb, show results different from surface sensitive measurements. In this respect it is important to realize that the origin of half-metallic behavior is not unique. Consequently, the deviations from the bulk behavior at the surface/interface can be important. Three different categories of half-metals can be distinguished and their expected surface properties will be discussed. Finally, ways will be described to control the properties at interfaces.

Magnetic and electronic properties of strontium hexaferrite SrFe12O19 from first-principles calculations

The magnetic and electronic structure of strontium hexaferrite SrFe12O19 has been investigated using density functional theory within the local spin density approximation. The calculations show that in this ferrite all the Fe3+ ions are fully spin polarized with S = 5/2. The electronic structure of this ferrite is strongly influenced by the exchange interactions between the iron ions. The most stable form of the ferrite is a ferrimagnet with the Fe3+ ions at the 4f sites having their spin polarization anti-parallel to the rest of the Fe3+ ions, in agreement with Gorters prediction. The ferrite SrFe12O19 is calculated to be a semiconductor with both the top of the valence band and the bottom of the conduction band being dominated by Fe 3d states. A strong anisotropy was found for the conductive charge carriers. The calculated results are in good agreement with the available experimental data.

Ab initio band structure calculations of Mg3N2 and MgSiN2

Ab initio band structure calculations were performed for MgSiN2 and Mg3N2. Calculations show that both nitrides are semiconductors with direct energy gaps at . The valence bands are composed mainly of N 2p states hybridized with s and p characters of the metals. The bottom of the conduction band consists of the s characters of Mg and N for Mg3N2, as well as for MgSiN2, while the characters of Si are higher in energy. The optical diffuse spectra show an energy gap of about 2.8 eV for Mg3N2 and 4.8 eV for MgSiN2, in agreement with the calculated values.

Phonon spectrum and thermal properties of cubic Si3N4 from first-principles calculations

The phonon spectrum of cubic Si3N4 was calculated by first-principles techniques. The results permit an assessment of important mechanical and thermo-dynamical properties such as the bulk modulus, lattice specific heat, vibration energy, thermal expansion coefficient, and thermal Gruneisen parameter for this compound. The calculated phonon spectrum shows a gapless spectrum extending to a cutoff energy of ∼1030 cm−1. The calculated Raman- and infrared-active phonon frequencies are compared with measured data in the literature.

Crystal and electronic structure of the novel nitrides MYSi4N7(M = Sr, Ba) with peculiar NSi4 coordination

The new ternary nitrides MYSi4N7 (M = Sr, Ba) have been investigated using first-principles techniques and experimental methods. The crystal structure data predicted are in good agreement with our measurements on prepared powder samples. Calculations also show that the electronic structure is strongly dependent on the local chemical bonding. The top of the valence bands is dominated by the 2p states of the N atoms coordinated to two Si atoms (NSi2), while the 2p states of the N atoms coordinated to four Si atoms (NSi4) lie about 1.0 eV below the top. The bottom of the conduction band is dominated by the Y 4d states. Both MYSi4N7 compounds are calculated to be wide band-gap semiconductors, with a band gap of about 2.9 eV, in reasonable agreement with the experimentally determined values (4.0 to 4.5 eV).

Electronic structure of magnesium nitride-fluorides from first-principles calculations

Abstract Electronic structure and stability have been determined from first-principles calculations for the magnesium nitride-fluorides Mg2NF and Mg3NF3, as well as for the binaries MgF2 and Mg3N2. These calculations show that the compounds are ionic to a first approximation. In the nitride-fluorides the valence bands are mainly determined by the N 2p states while the F 2p states are well separated from the N 2p bands and lie 3.5–5.1 eV below the Fermi level. The bottom of the conduction bands is determined by the empty 3s states of the anions. The energy gap decreases steadily with increasing nitrogen content.

First-principles electronic structure calculations of BaSi7N10 with both corner- and edge-sharing SiN4 tetrahedra

Abstract First-principles band structure calculations were performed for the ternary alkaline-earth silicon nitride BaSi 7 N 10 using the density functional theory (DFT) within the pseudo-potential (PP) method. The calculations show that both the coordination number of nitrogen by silicon (N [ x ] ) as well as the way of sharing SiN 4 tetrahedra (corner vs. edge) influence the local electronic structure of these atoms. The top of the valence band is dominated by the 2p states of the N [2] atoms, while the N [3] 2p states are positioned lower in energy. It is also noted that edge-sharing N [3] atoms show N [2] character. The conduction band is determined by Ba 6s states. The compound is calculated to be a wide band gap semiconductor with an indirect energy gap of about 3.8 eV. The direct energy gap is predicted to be about 4.0 eV, in agreement with the experimental value of 4–4.5 eV.

Site preferences in β-sialon from first-principles calculations

First-principles calculations based on density-functional theory (DFT) within the pseudo-potential method have been performed to identify site preferences in β-sialon, Si6−zAlzOzN8-z. With increasing z the calculated lattice volume increases while the bulk modulus decreases; calculated values are in excellent agreement with the experimental data. The calculations also showed that the atomic arrangements with the maximum number of Al–O and Si–N bonds have the lowest energy. There are different Si(O,N)4 and Al(O,N)4 tetrahedra in the sialons, which is in line with the results of NMR experiments. Meanwhile, the calculated occupations of oxygen at 2c sites are still different from those determined by neutron diffraction, which indicates the necessity of microdomain models for these compounds.

First-principles electronic structure calculations of Ba5Si2N6 with anomalous Si2N6 dimeric units

First-principles band structure calculations were performed for the ternary alkaline-earth silicon nitride Ba5Si2N6 using the LSW method. The calculations show that both the (zero-)dimensionality of the [Si2N6]10− dimeric units present in this structure and the coordination number of nitrogen by silicon have strong influences on the local electronic structure of these atoms. The interaction between the semicore-level states Ba 5p and N 2s is significant. Finally the compound is calculated to be a semiconductor with an indirect gap of 0.7 eV. The top of the valence band is dominated by the 2p states of the N[1] atoms. The conduction bands are determined by N 3s states hybridized with Ba 6s states.

Local electronic structure of intrinsic defects and impurities at the Fe(001) surface

Ab-initio calculations for the electronic structure and magnetic properties of intrinsic defects and foreign impurities (oxygen and gold) on the Fe(001) surface using the LSW method and the super-cell approach are presented. The calculations show that the Fe intrinsic defects have different local density of states. The magnetic moment of the Fe substrate layer under the defects increases with decreasing number of nearest neighbors of the surface, while the magnetic moment of the Fe defects decreases with decreasing number of nearest neighbors on the surface. The oxygen impurities have rather narrow (2p) bands at about 6.0 eV below the Fermi energy. The influence of oxygen impurities on the Fe(001) surface states near the Fermi energy is remarkably insignificant. Both gold and oxygen impurities have small magnetic moments parallel to the surface iron.