104 research outputs found
Atomistic models of hydrogenated amorphous silicon nitride from first principles
We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H), with equal concentrations of Si and N atoms (x=1), for two considerably different densities (2.0 and 3.0 g/cm3). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si3N4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate (0.023 K/fs) we obtain models with a modest number of defect states, where the low (high) density nitride favors undercoordinated (overcoordinated) defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from (atoms in) structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.
Ab initio study of magnesium alanate, Mg(AlH4)2
Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential
material for hydrogen storage. We apply ab initio calculations to characterize
structural, electronic and energetic properties of Mg(AlH4)2. Density
functional theory calculations within the generalized gradient approximation
(GGA) are used to optimize the geometry and obtain the electronic structure.
The latter is also studied by quasi-particle calculations at the GW level.
Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV.
The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the
valence and the conduction bands. On the basis of total energies, the formation
enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and
hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero
point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The
enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close
to zero, which impairs the potential usefulness of magnesium alanate as a
hydrogen storage material.Comment: 5 pages, 3 figure
Reversed spin polarization at the Co(001)-HfO2(001) interface
Ab initio electronic-structure calculations on the Co(001)-HfO2(001) interface are reported. The spin polarization of conduction electrons is positive at the interface, i.e., it is reversed with respect to the spin polarization in bulk Co. The electronic structure is very sensitive to the interface structure; without atomic relaxations the reversed spin polarization is not found. The possible relation with spin-polarized tunneling and magnetoresistance is discussed
Anomalous behavior of the semiconducting gap in WO3 from first-principles calculations
Several crystal structures of tungsten trioxide have been studied with a first-principles pseudopotential method. The electronic band gap increases significantly with the distortion of the octahedra that are the building blocks of the various crystal structures. Moreover, the tilting of the octahedra in the more complex structures leads to a strong increase of the gap upon compression
Surface Half-Metallicity of CrAs in the Zinc-Blende Structure
The development of new techniques such as the molecular beam epitaxy have
enabled the growth of thin films of materials presenting novel properties.
Recently it was made possible to grow a CrAs thin-film in the zinc-blende
structure. In this contribution, the full-potential screened KKR method is used
to study the electronic and magnetic properties of bulk CrAs in this novel
phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to
be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice
constants with a total spin magnetic moment of 3 . The Cr-terminated
surface retains the half-ferromagnetic character of the bulk, while in the case
of the As-termination the surface states destroy the gap in the minority-spin
band.Comment: 4 pages, 2 figures, new text, new titl
First principles simulations of direct coexistence of solid and liquid aluminium
First principles calculations based on density functional theory, with
generalised gradient corrections and ultrasoft pseudopotentials, have been used
to simulate solid and liquid aluminium in direct coexistence at zero pressure.
Simulations have been carried out on systems containing up to 1000 atoms for 15
ps. The points on the melting curve extracted from these simulations are in
very good agreement with previous calculations, which employed the same
electronic structure method but used an approach based on the explicit
calculation of free energies [L. Vo\v{c}adlo and D. Alf\`e, Phys. Rev. B, {\bf
65}, 214105 (2002).]Comment: To appear in Phys. Rev.
Structural response to O*-O' and magnetic transitions in orthorhombic perovskites
We present a temperature dependent single crystal x-ray diffraction study of
twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These
data show the evolution of the crystal structure from the ferromagnetic
insulating state to the ferromagnetic metallic state. The data are modelled in
space group Pnma with twin relations based on a distribution of the b axis over
three perpendicular cubic axes. The twin model allows full structure
determination in the presence of up to six twin fractions using the single
crystal x-ray diffraction data.Comment: 13 pages, including 13 figures and 2 table
Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys
We study the origin of the gap and the role of chemical composition in the
half-ferromagnetic Heusler alloys using the full-potential screened KKR method.
In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap.
Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are
semiconductors, but when Z_t > 18 antibonding states are also populated, thus
the paramagnetic phase becomes unstable and the half-ferromagnetic one is
stabilized. The minority occupied bands accommodate a total of nine electrons
and the total magnetic moment per unit cell in mu_B is just the difference
between Z_t and . While the substitution of the transition metal
atoms may preserve the half-ferromagnetic character, substituting the atom
results in a practically rigid shift of the bands and the loss of
half-metallicity. Finally we show that expanding or contracting the lattice
parameter by 2% preserves the minority-spin gap.Comment: 11 pages, 7 figures New figures, revised tex
Slater-Pauling Behavior of the Half-Ferromagnetic Full-Heusler Alloys
Using the full-potential screened Korringa-Kohn-Rostoker method we study the
full-Heusler alloys based on Co, Fe, Rh and Ru. We show that many of these
compounds show a half-metallic behavior, however in contrast to the
half-Heusler alloys the energy gap in the minority band is extremely small.
These full-Heusler compounds show a Slater-Pauling behavior and the total
spin-magnetic moment per unit cell (M_t) scales with the total number of
valence electrons (Z_t) following the rule: M_t=Z_t-24. We explain why the
spin-down band contains exactly 12 electrons using arguments based on the group
theory and show that this rule holds also for compounds with less than 24
valence electrons. Finally we discuss the deviations from this rule and the
differences compared to the half-Heusler alloys.Comment: 10 pages, 8 figures, revised figure 3, new text adde
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