NMR shieldings from density functional perturbation theory: GIPAW versus all-electron calculations.

We present a benchmark of the density functional linear response calculation of NMR shieldings within the gauge-including projector-augmented-wave method against all-electron augmented-plane-wave+local-orbital and uncontracted Gaussian basis set results for NMR shieldings in molecular and solid state systems. In general, excellent agreement between the aforementioned methods is obtained. Scalar relativistic effects are shown to be quite large for nuclei in molecules in the deshielded limit. The small component makes up a substantial part of the relativistic corrections.

A multi-nuclear magnetic resonance and density functional theory investigation of epitaxially grown InGaP2.

In this paper the short and long range order in In0.483Ga0.517P thin films is investigated by solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. To this end two samples were grown on a GaAs substrate by metal-organic vapor phase epitaxy at two different growth-pressures. From band gap energy measurements, CuPt long range order parameters of SCuPt = 0.22 and 0.39 were deduced, respectively. In the (31)P spectrum five resonances are observed corresponding to the five possible P(GanIn4-n), n = 0-4, coordinations whose relative intensities correspond to the order in the material, but the intensity variations for order parameters between 0 and 0.5 are minimal. (69)Ga, (71)Ga and (115)In (MQ)MAS spectra were acquired to analyze the quadrupolar and chemical shift distributions related to the (dis)order in these materials in more detail. All these spectra clearly reflect the disorder in the sample and do not show the presence of highly ordered domains. The difference in the order parameter in the sample is not clearly reflected in the spectra. (31)P chemical shifts were calculated using Density Functional Theory. The experimentally observed shifts are well reproduced with a simple random model of the disorder, thus confirming the assignment of the resonances. The (31)P chemical shifts are very sensitive to changes in the lattice parameter and chemical surroundings. These effects nearly compensate and explain why the (31)P chemical shifts in pure InP and GaP are nearly identical whereas a large difference would be expected based on the observed shift difference for the P[In4] and P[Ga4] coordinations in In0.483Ga0.517P.

Interactions of adsorbed CO2 on water ice at low temperatures.

We present a computational study into the adsorption properties of CO2 on amorphous and crystalline water surfaces under astrophysically relevant conditions. Water and carbon dioxide are two of the most dominant species in the icy mantles of interstellar dust grains and a thorough understanding of their solid phase interactions at low temperatures is crucial for understanding the structural evolution of the ices due to thermal segregation. In this paper, a new H2O-CO2 interaction potential is proposed and used to model the ballistic deposition of CO2 layers on water ice surfaces, and to study the individual binding sites at low coverages. Contrary to recent experimental results, we do not observe CO2 island formation on any type of water substrate. Additionally, density functional theory calculations are performed to assess the importance of induced electrostatic interactions.

The electronic structure of organic-inorganic hybrid compounds: (NH4)2CuCl4, (CH3NH3)2CuCl4 and (C2H5NH3)2CuCl4.

Hybrid organic-inorganic compounds are an intriguing class of materials that have been experimentally studied over the past few years because of a potential broad range of applications. The electronic and magnetic properties of three organic-inorganic hybrid compounds with compositions (NH4)2CuCl4, (CH3NH3)2CuCl4 and (C2H5NH3)2CuCl4 are investigated for the first time with density functional theory plus on-site Coulomb interaction. A strong Coulomb interaction on the copper causes a relatively weak exchange coupling within the layers of the octahedral network, in good agreement with experiment. The character of the exchange interaction (responsible for magnetic behavior) is analyzed. The calculations reveal that (C2H5NH3)2CuCl4 has the strongest Jahn-Teller (JT) distortion in comparison with the two other compounds. The easy axis of magnetization is investigated, showing a weak anisotropic interaction between inter-layer Cu(2+) ions in the (C2H5NH3)2CuCl4 structure. Orbital ordering is concluded from our partial density of states calculations: a cooperation of the JT distortion with an antiferro-distortive pattern.

Intrinsic defects and dopants in LiNH2: a first-principles study.

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ∼0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.

Spin tunneling in junctions with disordered ferromagnets.

We provide compelling evidence to establish that, contrary to one's elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of fcc CoFeB. First-principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlO(x), but they also portray the key concepts that demand primary consideration in such complex systems.

Electronic band structure of tetracene-TCNQ and perylene-TCNQ compounds.

The relationship between the crystal structures, band structures, and electronic properties of acene-TCNQ complexes has been investigated. We focus on the newly synthesized crystals of the charge-transfer salt tetracene-TCNQ and similar to it perylene-TCNQ, potentially interesting for realization of ambipolar transport. The band structures were calculated from first principles using density-functional theory (DFT). Despite the similarity in the crystal structures of the acene-TCNQ complexes studied here, the band structures are very different. Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCNQ, which has good transport properties for electrons only. The estimated degree of charge transfer for tetracene-TCNQ is 0.13e and for perylene-TCNQ 0.46e.

Optimizing performance of half-metals at finite temperature.

Several aspects of half-metallic magnetism at finite temperature are discussed. Since NiMnSb is the simplest half-metal and the longest known it will be used as an example. Also it is a half-metal with remarkable little on-site Coulomb repulsion. Consequently it is a half-metal that is not notably corrupted by non-quasiparticle states. There exists an anomaly at 90 K, described before, that will be shown to be unrelated to the position of the Fermi level in the bandgap. Several substitutions are investigated that could shed some light on the origin of the transition. The calculated phonon spectrum is compared with experimental neutron scattering data. Finally, the spin-polarization of interfaces of NiMnSb with the transition metal based non-magnetic semiconductors NiTiSn and CoTiSb is investigated and the electronic structure of an infinite two-dimensional array of NiMnSb quantum dots embedded in NiScSb is reported.

Ab initio and work function and surface energy anisotropy of LaB6.

Lanthanum hexaboride is one of the cathode materials most used in high-power electronics technology, but the many experimental results do not provide a consistent picture of the surface properties. Therefore, we report the first ab initio calculations of the work functions and surface energies of the (001), (011), (111), (112), and (012) surfaces by considering the different surface terminations and structural relaxation. Either the (111)B- or the (001)La-terminated surface is the most stable, depending on La chemical potential. The work function of the latter is the lowest (2.07 eV) of the surfaces considered. Both the work function and surface energy decrease further when surface La is replaced by Ba and become, respectively, 1.43 and 7.7 eV/nm(2) at the chemical potentials of elemental lanthanum and barium bulk. These results compare favorably with previous work on the intermetallics BaAl(4), CaAl(4), and BaAuIn(3). Their most stable surfaces possess the lowest work function. Now, we study a compound with a decidedly different crystal type and with its constituting elements from column 3 of the periodic table, of which one is nonmetallic.