Tunneling anisotropic magnetoresistance driven by resonant surface states: first-principles calculations on an Fe(001) surface

Fully relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states.     

氧在Nb(110)表面吸附的第一性原理研究

通过第一性原理赝势平面波方法研究了氧在Nb(110)表面的吸附性质随覆盖度变化规律. O在Nb(110)表面最稳定吸附位是洞位,次稳定吸附位是长桥位. 在长桥位吸附时, O诱导Nb(110)表面功函数随覆盖度的增加而几乎线性增加;但当O在洞位吸附时, 与干净Nb表面相比, 覆盖度为0.75 ML和1.0 ML时功函数增加, 而覆盖度为0.25 ML和0.5 ML时功函数减小.通过对面平均电荷密度分布和偶极矩变化的讨论, 解释了由吸附导致功函数复杂变化的原因.通过对表面原子结构和态密度分析, 讨论了O在Nb表面吸附时引起表面原子结构变化以及O和Nb(110)表面原子的相互作用.     

Thermal desorption of molecular oxygen from SnO2 (110) surface: Insights from first-principles calculations

First-principles density functional theory calculations in the generalized gradient approximation, with plane wave basis set and pseudopotentials, have been used to investigate the desorption pathways of molecular oxygen species adsorbed on the SnO₂ (110) surface. Energetics of the thermodynamically favored precursors is studied in dependence on the surface charge provided either by surface defects or by donor type impurities from the near-surface region. The resonant desorption modes of O₂ molecules are examined in the framework of ab initio atomic thermodynamics and relationship of these results to experimental observations is discussed.     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

Surface states of GaAs (110): Pseudopotential calculations

In this letter we present calculations for surface states in high symmetry points Γ, M, X and X' of Jones' zone of GaAs (110), using a step barrier potential. It is found that only four surface states appear in the thermal gap; two in Γ point at 0.38 and 1.2 eV from the top of the valence band considered as origin of energies, one at 0.8 eV in M point and another in X′ point at 0.3 eV. Also, we obtain a low band of surface states between ?3 and ?5.5 eV.     

Electronic states induced by surface defects on GaSb(110)

Electronic state calculations for point defects on the GaSb(110) surface are presented using a cluster, in order to indicate theoretically the usefulness of the defect model as a mechanism of the Fermi level pinning in Schottky barriers. The results demonstrate that the presence of atomic Ga at surface Sb vacancy sites in addition to surface Ga vacancies gives electronic states localized near the top of the valence band which can be responsible for the pinning observed experimentally.     

Adsorption of O on Mo(110) surface from first-principles calculation

First-principles calculations based on density functional theory (DFT) have been performed to study O adsorption in on-surface and subsurface sites. For different coverages, hollow site is found to be the most stable on-surface adsorption site. For the subsurface adsorption at the bare Mo surface, the adsorption energies are found to be higher than those at the on-surface sites, suggesting that these sites are less stable. However, the presence of preadsorbed O overlayer enhances the binding energy of subsurface adsorption, particularly for the adsorption of O at octahedral site. Further, vibrational frequencies, work-function and density of states are presented for O adsorption in on-surface sites.     

Optical detection of surface states in GaAs(110) and GaP(110)

We present results on the optical detection of surface states in GaAs(110)and GaP(110)by the method of the fractional change of external reflectivity. Optical transitions are observed at ～3.1 eV m GaAs(110) and ～3.4 eV in GaP. A comparison with existing theories suggests a rotational relaxation model for the surface, with partial relaxation for GaAs(110) and full relaxation for GaP(110).     

Stability of FeAl(110) alloy surface structures: a first-principles study

We have studied the stability of FeAl(110) alloy surface structures by first-principles calculations. A general method is employed to determine the allowed chemical potential range for the surface structures of alloys with several bulk ground state structures. We show that there are three stable surface structures, the Fe:Al=1:1, Fe:Al=1:2 and Fe:Al=1:3 surface structures, within the allowed chemical potential range for FeAl bulk. In the three stable surface structures, surface buckling extends deep into the bulk layers. For the Fe:Al=1:1 surface structure, the surface Al atoms displace outwards and surface Fe atoms move inwards relative to their bulk positions. The Fe:Al=1:2 and Fe:Al=1:3 surfaces show large surface rippling due to composition reconstruction of the surface.     

First-principles calculations of ammonia decomposition on Ni(110) surface

First-principles calculations based on density functional theory (DFT) have been performed to study the adsorption and decomposition of NH₃ on Ni(110). The adsorption sites, the adsorption energies, the transition states and the activation energies of the stepwise dehydrogenation of NH₃ and the associative desorption of N are determined, and the zero point energy correction is included, which makes it possible to compute the rate constants of the elementary steps in NH₃ decomposition. Combined DFT calculations and kinetic analysis at 350 K indicate that the associative desorption of N has a reaction rate lower than NH_x dehydrogenation and is therefore the rate determining step. The distinctly different rate constants over Ni(110), Ni(111) and Ni(211) imply that ammonia decomposition over Ni-based catalyst is a structure-sensitive reaction.     

Many-body calculations for the relaxed (110) surface of GaAs

Quantum Monte Carlo methods are a stochastic approach to directly tackle the manybody problem in solids. They have proven to describe virutually exactly the ground state of correlated bulk systems, like the homogeneous electron gas or solids of C, Ge, Si and GaAs. Especially Variational quantum Monte Carlo calculations using nonlocal ab initio pseudopotentials offer a way to study systematically many-body effects at solid surfaces, safely founded on the variational principle “the lower the energy, the better the wave function”. Here we report on first attempts for the relaxed (110) surface of GaAs, serving as a prototype of semiconductor surfaces. A finite layer geometry is chosen as the boundary condition of the multidimensional stochastic integration scheme. The exact many-body Hamiltonian is cast in a form allowing for rapid evaluation. New parameters in the correlated trial wave function increase the variational freedom necessary to take into account the influence of the surface. Their physical meaning and their statistical significance are discussed in detail. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by the Deutsche Forschungsgemeinschaft under Grant No. Scha 360/17-1. Our calculations were performed on the Cray-T3E at the Zentralinstitut für Angewandte Mathematik, Forschungszentrum Jülich, and on the Cray-T3E at the Konrad-Zuse-Zentrum für Informationstechnik, Berlin.     

Tight-binding calculations of surface states of Si(111)

Semi-empirical tight-binding calculations are reported for Si(111) surfaces. The calculations reproduce quantitatively the results obtained both experimentally and by more elaborate theoretical methods.     

Zero temperature calculations of the states of a monoatomic two-dimensional solid adsorbed on an fcc (110) surface

Calculations of the energy of centered rectangular monolayer lattices in the periodic potential of an fcc(110) solid surface are reported. The parameters are chosen to model the systems Xe/Ag(110), Kr/Cu(110) and Xe/Cu(110). The critical values of Fourier amplitudes of the adatom-substrate potential required in order to stabilize uniaxial registry lattices of the adlayers are estimated and the variation of the adlayer energy with uniaxial compression is calculated. The calculation of the modulation of the adlayer by the rigid periodic substrate potential includes effects of anharmonicity of the adlayer response and of the dispersion relation of adlayer vibrations. The critical Fourier amplitudes to stabilize the registry structures can be estimated with 20% accuracy by using a comparison of the lateral potential energies of the registry lattice and of the intrinsic two-dimensional close-packed triangular lattice.     

Density of states of the clean Mo (110) surface

The electronic structure of the (110) surface of Mo is calculated using transfer matrix method. The Hamiltonian is of tight-binding type with a basis of nine orbitals per atom. Spectral densities of states for special points of the 2-D Brillouin zone are presented on the surface and in the three consecutive layers. The band structure in the σ direction and the local density of states are also calculated. The results are compared with the few experimental data available in the literature.     

Electronic superstructures on the graphite surface studied by first-principles calculations

We present results of a first-principles study of the graphite surface in the presence of defects. Our calculations, based on density functional theory, show superstructures of periodicity ( ) in the electronic structure of the surface. In good agreement with STM experiments, these superstructures show a variety of patterns with their intensity decaying away from the defect. Two kind of defects were considered: metallic adatoms absorbed on graphite, and vacancies in the surface lattice. Similar results were found in both cases. Our results give strong support to the idea that these superstructures are due to purely electronic effects, and do not correspond to any atomic reconstruction of the graphite surface.     

Molecular SCF calculations to model (111) surface states and relaxation of diamond

Ab-initio SCF-MO computations in an STO-3G Gaussian basis on a molecular system, designed to model a carbon atom on the (111) surface of diamond, predict that the danglingbond carbon relaxes about 0.10 Å toward the bulk from its position on an extension of the diamond lattice. The dangling bond carbon cation relaxes Inward 0.30 Å, while the anion relaxes outward 0.10 A. Tentative ways to describe the dangling bond in terms of computed quantities are presented.     

Defect states at the TiO2(110) surface probed by resonant photoelectron diffraction

The charge distribution of the defect states at the reduced TiO(2)(110) surface is studied via a new method, the resonant photoelectron diffraction. The diffraction pattern from the defect state, excited at the Ti-2p-3d resonance, is analyzed in the forward scattering approach and on the basis of multiple scattering calculations. The defect charge is found to be shared by several surface and subsurface Ti sites with the dominant contribution on a specific subsurface site in agreement with density functional theory calculations.