Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

The rumpled relaxation and the core-level shift of full-relaxed BaTiO₃ (0 0 1) surface have been investigated by first-principles calculation. Based on the work function and the electric-field gradient, the right size of vacuum and the slab have been evaluated. The large displacements of ions deviated from their crystalline sites to lead to the formation of the surface rumples have been found. Some fully occupied surface oxygen p states at the top M point of the valance band and the empty surface titanium d states at the edge of the bulk conduction band are observed on the TiO₂-terminated surface. In contrast, on the BaO-terminated surface, two different core levels of the Ba 5p states shifted about 1.29 eV are induced by the bulk perovskite Ba atoms and the relaxation of surface Ba atoms, respectively. Our calculations are consistent with the experimental data.     

Influence of surface relaxation on the surface vibrational modes of some ionic crystals

In this paper we evaluate the effects of surface relaxation on the surface vibrational modes of the (001) surfaces of NaCl, NaI and MgO, using bulk shell models which have been modified to allow for distance dependence of the short-range interactions. It is found that these dynamical shell models do not lead to convergent results for the static surface relaxation. In spite of this inconsistency, it is found that surface relaxation gives rise to a new structure of relaxationinduced surface modes, which is common to the three crystals and independent of the details of the relaxation. By using assumed (convergent) relaxation configurations for the MgO (001) surface it is shown that relaxation can account for the high-frequency peak in the surface-excess density of vibrational states of MgO, as found in inelastic neutron scattering experiments on MgO microcrystals. In general, surface relaxation has a profound effect on surface vibrational phenomena, and better shell models will have to be developed to deal with the static and dynamical surface phenomena in a consistent fashion.     

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).     

Atomic and electronic structure of the corundum (0001) surface: comparison with surface spectroscopies

The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 Å, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.     

Effects of surface adhesion of nonequilibrium charge carriers in the photoconductivity spectra of CdS crystals

The relaxation of low-temperature (T = 77 K) edge photoconductivity (PC) spectra of CdS crystals of the first group1 caused by the removal of the transverse electric field is investigated. A clearly manifested inversion of the fine (exciton) structure is found in the spectra during their relaxation to the initial form (existing before the application of the field). It is shown that the observed relaxation changes in the PC spectra of CdS are due to relaxation of the nonequilibrium surface charge appearing as a result of trapping of a part of the charge, induced by the transverse field into the sample, by slow surface states (adhesion levels). The experimental data on the effect of preliminary illumination by self-radiation on the edge PC spectra of the first-group CdS crystals indicating the existence of surface adhesion levels with a high density in these crystals are presented.     

First-principles study for the atomic structures and electronic properties of PbTiO3 oxygen-vacancies (0 0 1) surface

The structural and electronic properties of fully-relaxed PbTiO₃ (0 0 1) oxygen-vacancy surface with PbO and TiO₂ terminations are investigated by first-principles calculations. In contrast to the perfect surface, the smaller surface rumples and interlayer distances have been found. The largest relaxation occurs on the second layer atoms not on the surface layer ones, and some in-gap Ti 3d states at about −1.1 eV below the Fermi-level are observed in the TiO₂-terminated surface caused by oxygen-vacancies. For the PbO-terminated surface, some in-gap Ti 3d states and Pb 6p states also move into the bulk midgap region to become partially occupied, and two different chemical states of the Pb 6s states were found. One is attributed to the bulk perovskite Pb atoms and another one is caused by the relaxation of surface Pb atoms. These theoretical results would give a good reference for the future experimental studies.     

Ion-stimulated desorption from a semiconductor surface containing metal nanodots

A model of the mechanism of ion-stimulated desorption of atoms and molecules pre-adsorbed from the surface of wide-band gap samples with a system of metal nanodots is developed. The relaxation processes of vibration-exciting states in adsorbed layer via electronic channel are considered. It is found in the model that the presence of nanosized metal clusters can affect the relaxation rate of the vibration excitations on the surface and the velocity of ion-stimulated desorption.     

Electronic surface properties of group V semimetals

We have studied the electronic surface properties of the (111) surface of group V semimetals (As, Sb, Bi) using a Green's function formalism in the tight-binding representation. We find that no surface states appear unless the surface layer relaxes, but any relaxation is enough to produce bona fide surface states.     

Surface relaxation influenced by surface states

A detailed theoretical investigation of the relaxation of the simple metal surface Al(100) is presented. We show the influence of electronic surface states in this context. The sign and magnitude of the relaxation of the topmost atomic layers is mainly determined by the rearrangements of the surface state charge. The degree of surface relaxation convergence, with respect to the number of slab layers, is determined by the location of the surface state band relative to bulk bands.     

Impurity scattering on the Fermi surface of aluminum

A calculation is described for the anisotropic relaxation time due to impurity scattering on the Fermi surface of aluminum. The Bloch states and the Fermi surface are obtained from Ashcroft's 4-OPW model, while the scattering potentials are obtained from locally re-screened form factors. Numerical results reveal that strong anisotropicsin the electronic lifetime arise for s-like impurities such as Si and Ge. A phase shift model is introduced to explain these anisotropies. The predicted anisotropies appear to be in reasonable agreement with experimental values for the Dingle temperature. The relaxation time for d.c. conductivity is obtained from iterative solution of the Boltzmann equation.     

Kinetics of the surface charging of a semiconductor during adsorption

The kinetic characteristics of the surface charging of a semiconductor during adsorption are analyzed, due allowance being made for the recharging of the biographic surface states during the adsorption process. It is shown that the form of the kinetic curves depends very substantially on the relationship between the characteristic relaxation times of the adsorption and biographic surface states.     

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.     

VC(001)弛豫表面构型与电子结构第一性原理研究

用第一性原理方法对VC(001)清洁表面的构型和电子结构进行了详细研究，与TiC(001)面类似，VC(001)面弛豫后形成表面皱褶，其表层V原子和C原子分别朝体相和真空方向移动. 能带计算结果表明，过渡金属碳化物(001)面的能带结构符合刚性带理论模型. 对于VC(001)面，表面态主要处在-30eV附近，其主要成分为表层C原子的2pz轨道. 此外，以表层V原子的3d轨道成分为主的表面态出现在费米能级附近，由于这些表面态以表面法线方向的轨道(3d２z和3dxz/dyz)为主要成分，因此在表面反应中将起到重要作用，从而体现出与TiC(001)面不同的反应性质. 关键词： 过渡金属碳化物 表面态 能带结构     

Influence of gallium arsenide surface treatment in selenium vapors on subsurface defects

Influence of GaAs surface treatment in selenium vapors on the parameters of electronic states in the subsurface GaAs regions is investigated by the methods of volt-ampere and volt-farad characteristics and isothermal capacitance relaxation at temperatures in the interval 77–400 K. Electrophysical measurements of the Schottky barriers formed on the GaAs surface treated in selenium indicate a decrease in the surface electron state (SES) density and unfastening of the Fermi energy level. In this case, generation of subsurface defects is observed that causes compensation of shallow donors and refastening of the Fermi energy level typical of some structures.     

Ab-initio calculation of the electronic structure and energetics of the unreconstructed Au(001) surface

The electronic structure, surface and relaxation energies, and the electric field gradient for the unreconstructed Au(001) surface were calculated by means of the ab-initio all-electron full-potential linearized augmented plane wave slab method. The valence states were calculated within the standard semi-relativistic approach whereas the core states are treated in a fully relativistic way. The Au(001) surface was modelled by free slabs of 5, 7, and 9 layers. From the 9-layer calculation a work function of 5.39 eV was obtained. For the surface energy a value of 1.30 J/m² for the unrelaxed geometry was derived from the total energies of the 7- and the 9-layer slabs. From total energy minimization of the 7-layer slab, a negative, inward relaxation of −2.6% and a relaxation energy of 14.3 × 10⁻³ J/m² were derived. To discuss a mechanism of reconstruction, particular surface states were analyzed in detail in terms of the band structure, layer-dependent density of states and the charge density distribution. Differences of surface and central-layer charge densities show a gain of charge in z-direction localised below and also, to a smaller extent, above the surface atoms. We find a very small gain of delocalised charge in the surface plane between the nearest neighbour positions at the expense of more localised s-d hybridised states. The electric field gradient component Φ_zz was obtained in a two energy window calculation for which the Au5p states were also treated as band states. The resulting Φ_zz values are −16.50 × 10¹⁷ V/cm² surface layer, and −3.3 × 10¹⁷ V/cm² for the subsurface layer.     

Tight-binding calculations of (111) surface densities of states of Ge and GaAs

We have used the tight-binding method to calculate the local densities of states of unreconstructed Ge (111) and GaAs (111), ($\text{111}$) surfaces. In the unrelaxed surface configuration we find two types of states for each surface. The effects of relaxation on Ge surface states are also discussed.     

On the photon-drag effect of photocurrent of surface states of topological insulators

The photocurrent of surface states of topological insulator due to photon-drag effect is computed, being based on pure Dirac model of surface states. The scattering by disorder is taken into account to provide a relaxation mechanism for the photocurrent. The Keldysh–Schwinger formalism has been employed for the systematic calculation of photocurrent. The helicity dependent photocurrent of sizable magnitude transverse to the in-plane photon momentum is found, which is consistent with experimental data. Other helicity independent photocurrents with various polarization states are also calculated.     

On interpretation of intrinsic surface states on Si(111) surface

Extended Hückel theory is applied to the Si9111) ideal and relaxed surfaces. In addition to the results obtained by selfconsistent pseudopotential calculations of Schlütter et al., new surface states gave been found. The results are essentially independent of surface relaxation.     

First principles calculations of relationship between the Cu surface states and relaxations

In this paper the relationship between the surface relaxations and the electron density distributions of surface states of Cu(100), Cu(110), and Cu(111) surfaces is obtained by first-principles calculations. The calculations indicate that relaxations mainly occur in the layers at which the surface states electrons are localized, and the magnitudes of the multilayer relaxations correspond to the difference of electron density of surface states between adjacent layers. The larger the interlayer relaxation is, the larger the difference of electron density of surface states between two layers is.     

Electron states on the relaxed Cu(1 1 0) surface

The scattering-theoretical method has been used to search the (1 1 0) surface of copper for electron states. The relaxation of the surface was included by assuming Harrison scaling for the tight-binding parameters used in the bulk energy-band calculation. The inclusion of surface relaxation was found to improve agreement with experiment.