Occupied Surface States Research Articles

Screened-exchange LDA methods for films and superlattices with applications to theSi(100)2×1surface and InAs/InSb superlattices

We have developed efficient screened-exchange local-density approximation (sX-LDA) methods for films and superlattices (FLM/SL) with which to calculate self-consistent electronic structures for both occupied and unoccupied states. Considering nonuniform charge densities and local-field effects in the z direction for FLM/SL, we have employed nonlocal Thomas-Fermi wave vectors to define the screened-exchange interaction. Three methods, for bulk, superlattice, and film, have been implemented in the full-potential linearized augmented plane wave method. The film sX-LDA method was then applied to the $\mathrm{Si}(100)2\ifmmode\times\else\texttimes\fi{}1$ surface. The calculated occupied surface states show very good agreement with experiment. On the other hand, an underestimation of the correction to the unoccupied surface states, by about 0.2 eV, was estimated in comparison with available $\mathrm{GW}$ calculations. The ionization energy of Si was evaluated with the film geometry to be 5.35 eV by virtue of the quasiparticle corrections, showing good agreement with the experimental value of $5.15\ifmmode\pm\else\textpm\fi{}0.08$ eV. We also present an application of the superlattice sX-LDA method to $[001]$ ordered InAs/InSb heterojunctions and superlattices. Band gaps and band offsets under strained conditions were directly calculated by sX-LDA without any experimental data as input. Slightly larger valence-band offsets than the LDA results, by about 0.08 eV, agree with the consequence of the $\mathrm{GW}$ calculations, indicating an increase of the potential negativity in the InAs region. This potential change along with the charge redistribution at the interface is found to be crucial to evaluate accurate band gaps of the superlattices.

III–N(110) surface relaxation and its dependence on the chemical bonding

Using existing and new results obtained from first-principles pseudopotential calculations, we have studied the atomic relaxation and its dependence on the chemical bonding, on III–N(110) surfaces. It is found that the characteristics of III–N(110) surfaces differ from other III–V(110) and II–VI(110) surfaces in two important aspects: a significantly reduced surface bond rotation, and a much lower binding energy of the highest occupied surface state. Furthermore the results obtained for AlN and InN corroborate our previously proposed model of a linear relationship between the vertical buckling of the top layer and the relaxed surface bond length not only for III–N, but also for III–V and II–VI semiconductors in general.

Theory of time-resolved two-photon photoemission spectroscopy from metal surfaces

We present a theory of energy and time-resolved two-photon photoemission (TR-2PPE) spectra from metal surfaces on the basis of the density matrix formulation. A system consists of an initial occupied (bulk and surface) states, intermediate unoccupied states such as an image potential state and a hot electron state in bulk, and a final photoelectron state. A focus is placed on how the pulse duration of the excitation laser influences the transient 2PPE signal and how the relaxation time of the intermediate states is determined using laser pulses with much longer pulse duration. The present analysis is successfully applied to the recent TR-2PPE experiments for Ag and Cu surfaces. The energy-resolved 2PPE spectrum is also calculated to demonstrate that when it is measured with a ultrashort laser pulses the width of the “one” photoemission peak from the intermediate state is a function of the delay time between pump and probe pulses, in addition to the relaxation time of the system and pulse duration.

Electronic structure of the 6H–SiC(0001)-3×3 surface studied with angle-resolved inverse and direct photoemission

We have investigated the 3×3 reconstruction of the 6H–SiC(0001) surface with angle-resolved direct and inverse photoemission (ARUPS and IPES). The surface was prepared by heating the sample in a Si flux and showed an excellent 3×3 LEED pattern. In the ARUPS spectra, three occupied surface states were found at the energies 0.5 eV, 1.5 eV and 1.9 eV below E F. In the IPES spectra, an unoccupied dispersionless surface state was observed at 0.5 eV above E F. Thereby, the reconstruction has a semiconducting character with a surface bandgap of 1.0 eV. This result agrees well with recent theoretical results that predict strong electron-correlation effects, leading to a Mott–Hubbard ground state.

Decoherence effects in propagation of optically generated electron–hole pairs in image potential states

We discuss some many-body aspects of optically induced electronic transitions taking place in the first step of two-photon photoemission from Cu(111) surfaces. Transient and decoherence effects associated with the creation and spatio-temporal propagation of an electron–hole pair generated in the Shockley occupied surface state and image potential bands of the surface image potential (IP–SS ‘exciton’) are assessed by combining the effective model Hamiltonian approach with the formalism of nonadiabatic response.

High-resolution photoemission study of the Ȳ surface state on Ag(110)

We report a high-resolution study of the ‘Shockley-type’ occupied surface state residing in the projected L 2′–L 1 gap around the Ȳ point of the surface Brillouin zone. Along ΓY on Ag(110) this band shows a parabolic dispersion with k ‖, the surface wave vector. The effective mass is m ∗/ m=0.26±0.02 and the bottom of the parabola at Ȳ shows a linear dependence on sample temperature according to E 0=−(106±5)meV+(0.17±0.01 meV/K) T. In consequence, the surface band gets empty above T=624±37 K. These numbers give a quantitative interpretation of strong temperature effects observed earlier by other authors in second-harmonic spectra from Ag(110), where the fundamental light couples the occupied surface state to an unoccupied surface state within the same projected gap. From our data we derive a more precise value of its bottom energy E f=1.66±0.02 eV above E F at Ȳ.

Two-Photon Photoemission Study of Heterogeneous Electron Transfer: C6F6on Cu(111)

Photoinduced heterogeneous electron transfer (ET) constitutes the key step in a number of chemical processes, such as solar energy conversion, photocatalysis, and surface photochemistry. We present a general strategy to investigate photoinduced heterogeneous ET using two-photon photoemission (2PPE) spectroscopy. In this report, we focus on the energetics in photoinduced heterogeneous ET in monolayer and bilayers of hexafluorobenzene on Cu(111). We found that for monolayer C6F6, which likely adsorbs flat on the surface, the ET resonance is not visible in 2PPE spectrum. However, for second-layer C6F6, a strong molecular resonance is observed. Photon energy dependence shows that the shift in electron kinetic energy (ΔEkin) from the molecular resonance scales with Δhν, unlike that from the occupied surface state, which scales with 2Δhν. This provides evidence that the observed molecular resonance results from electron transfer to an unoccupied molecular orbital located at 2.9 eV above the Fermi level (or 1.8 ...

Surface state-derived electronic transitions of SiC(001)

We report on collective surface excitations at Si-terminated (2×1) and C-terminated c(2×2) surfaces of SiC(001) by using high-resolution electron energy loss spectroscopy (HREELS). The measured energy loss spectra are fitted by two different models based on dipole scattering theory. While the first model takes account of the Fuchs–Kliewer surface optical phonons and surface plasmons only, the second one contains, additionally, a contribution from intraband plasmons due to the presence of a partially occupied surface states band. For both surfaces much better fits are obtained by the second model, i.e. if we assume the surfaces to be metallic. For the (2×1) surface this finding is in agreement with recent STM results suggesting the metallic character of this surface at high temperatures.

Low-energy(e,2e)spectroscopy from the W(001) surface: Experiment and theory

The simultaneous ejection of two electrons from the (001) surface of W due to the collision of incident low-energy electrons with valence electrons has been studied experimentally and theoretically. Energy and momenta of the ejected electrons were measured simultaneously by a combination of coincidence and time-of-flight techniques. Calculations were performed in a relativistic distorted-wave Born approximation including exchange, in which the primary electron and the two emitted electrons are described by quasiparticle multiple scattering states. The valence electron is represented by linear combinations of Bloch waves matched at the surface. Screened Coulomb interaction matrix elements between these four states are evaluated. Experimental and calculated energy distributions from W(001) for very-low-energy primary electrons at normal and grazing incidence are in fairly good overall agreement. Although some features of one-dimensional bulk densities of states are roughly reflected, Coulomb matrix elements with low-energy-electron-diffraction-type states play a vital role. Further analysis reveals in detail the importance of elastic scattering of the primary electron and of the two ejected electrons. Some observed features can be attributed to occupied surface states.

Spin-dependent occupied surface state of Fe(001)

We have observed experimental energy band dispersions of the surface electronic states of Fe(001) by spin- and angle-resolved photoemission spectroscopy. We have shown that the majority spin surface state exists below the Fermi level throughout the Γ− Δ− X symmetry line in the surface Brillouin zone and there is no evidence of the minority spin surface states predicted by a theoretical calculation. Comparing the experimental results with the calculations, we suggest that the enhancement of the surface magnetic moment is larger than predicted.

Room temperature adsorption of aniline (C 6H 5NH 2) on Si(100)(2×1) observed with scanning tunneling microscopy

The chemisorption of aniline (C 6H 5NH 2) on Si(100)(2×1) at room temperature has been studied for the first time with scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images reveal that aniline adsorbs on Si(100)(2×1) and forms regions of moderately ordered p(2×2) or c(4×2) structures. The STS results show a quenching of the silicon surface states after adsorption and an increased energetic gap between occupied and unoccupied surface states if compared with the clean surface. The first occupied state coincides with the corresponding peak in the ultraviolet photoelectron spectrum.

Surface-state contribution to the optical anisotropy of Ag(110) surfaces: A reflectance-anisotropy-spectroscopy and photoemission study

The (110) surface of an Ag crystal was investigated by reflectance anisotropy spectroscopy and angle-resolved photoemission spectroscopy. A strong resonance in the optical spectra of the clean surface is assigned to a surface-state transition at the $\overline{Y}$ point of the surface Brillouin zone. This resonance is absent on the oxygen-covered surface. The accompanying photoemission spectra show the corresponding occupied surface state on the clean surface as well as its disappearance with oxygen coverage.

Ab initiostudy of atomic geometry, electronic states, and bonding for H2S adsorption on III-V semiconductor (110)-(1×1)surfaces

Ab initio calculations, based on pseudopotentials and the density-functional theory, have been made to investigate the equilibrium atomic geometry, electronic states, and bonding of the H${}_{2}$S molecule on the InP(110), GaAs(110), and GaP(110) surfaces within a dissociative adsorption model. In general, the adsorption of H${}_{2}$S on the three semiconductors shows similar behavior. The calculated average distances between the topmost InP, GaAs, and GaP layer and the sulfur atom are 1.87, 1.67, and 1.65 \AA{}, respectively. In all the cases studied here, the fundamental band gap is free of surface states, with an occupied surface state close to the valence-band maximum. The calculated electronic states are in agreement with reported angle-resolved photoemission data.

Two-photon Time-resolved Photoemission.

Ultrafast energy and phase relaxation of photo-excited electrons has been measured by means of interferometric twophoton time-resolved photoemission with 15 fs pulses. The phase relaxation time for the excitation from the occupied surface state on Cu(111) is found to be ∼20 fs, and is longer than for the excitation from the bulk bands. This makes it possible to demonstrate a coherent control of photo-excited electronic populations with phase-locked pulse pair and chirped pulse excitation.

Photoelectron spectroscopy at clean and hydrogenated c(2 × 2)-SiC(100) surfaces

In order to test structure models of the c(2 × 2)-reconstruction, angle-integrated valence band spectra at c(2 × 2) and (2 × 1) 3CSiC(100) surfaces are reported. The surfaces are characterised by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Two different preparation techniques for the c(2 × 2)-reconstruction are compared, and it is shown that both methods yield the same well-defined ultraviolet photoemission (UPS) spectra. On the (2 × 1)-surface, we find emission from an occupied surface state. On the c(2 × 2)-surface, no unambiguous evidence for a true surface state could be found. However, we find a surface resonance below the valence band maximum. Band bending and work function results for the two reconstructions and the effect of atomic hydrogen on the c(2 × 2)-surface are also reported.

Raman Scattering by Bulk Phonons in Microcrystalline Silver and Copper via Electronic Surface Excitations

In Raman scattering from cryocondensed metal films one observes the bulk acoustic phonons. This is not caused by the laser field which penetrates the metal, but via the scattering of optically excited intrasurface band electron hole pairs by the phonons. Phonons with wave vector $\mathbf{Q}$ contribute to the Raman signal according to the scattering probability $S(\mathbf{q})$ of the image states by surface roughness, where $\mathbf{q}$ is the projection of $\mathbf{Q}$ onto the low index facets having occupied electronic surface states, in the present case mainly of (111) orientation.

Surface electronic structure with the linear methods of band theory

We present an ab initio method for calculating electron states in a semi-infinite crystal. The complex band structure is obtained by the extended linear augmented-plane-wave method within the exact $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ formulation of the band-structure problem. We also present a variational scheme of matching the wave functions at the interface. The practical applicability of the method is demonstrated by solving the Schr\odinger equation for the (100) surface of Al. We have calculated the normal incidence low-energy electron-diffraction spectra, the occupied surface state at the \ensuremath{\Gamma} point, and the normal-emission photoelectron spectra within the one-step model. The results are in a good agreement with available measurements. We have developed a simplified procedure to assess the conducting properties of a crystal surface in terms of the ${\mathit{k}}_{\ensuremath{\parallel}}$-projected real band structure. We introduce a new quantity, the conductance index, whose energy dependence is shown to yield gross features of the exact energy dependence of transmitted current.

Image potential states on lithium, copper and silver surfaces

The electronic structure of the Li(110) surface is calculated within the self-consistent pseudopotential method. The calculation does not give any occupied surface state. At the same time, unoccupied resonance surface states have been found at the \\ ̄ qG point and along the P̄H symmetry directon. Image potential states with energies of E 1 = −0.73 eV and E 2 = −0.22 eV are predicted at the Г point. For the description of the image potential states, we propose a one-dimensional model potential. The parameters of this potential are determined with the use of experimental (or first-principles calculation) values of the width and position of the energy gap at the \\ ̄ qG point and the energies E 0 and E 1 of the surface state and first image potential state, respectively. Using this model potential, we study the binding energies, wave functions and lifetime of image potential states on the Li(110), Cu(100), Cu(111), Ag(100) and Ag(111) surfaces. We also show that using the local part of the full, screened, non-local pseudopotential can produce incorrect results for the binding energy of the image states.

Ab initiocalculations of the atomic and electronic structure of clean and hydrogenated diamond (110) surfaces

We present ab initio local-density-functional calculations of the electronic structure of clean and hydrogenated diamond (110) surfaces. The clean surface relaxes to a structure where the chains in the first two planes are straightened so that the interatomic distances are shortened and the bond angles are increased. Upon relaxation the surface remains flat and no dimerization occurs. The dangling bonds lead to surface states within the bulk gap. The surface is metallic, but with a very low density of states at the Fermi level. After the deposition of a monolayer of hydrogen, which saturates all dangling bonds, the surface relaxes back to an almost bulk-terminated structure. The occupied surface states are removed from the gap and the surface becomes semiconducting. We also compare the C(110) surface with the other two low-index diamond surfaces.

Scattering of fast Na +-ions from alkali-halide surfaces under a grazing angle of incidence

We have studied charge exchange of Na +-ions with energies ranging from 1 keV to 100 keV in the scattering from LiF(100)-, KI(100)-, and KCl(100)-surfaces under a grazing angle of incidence. In the experiments we observe generally small fractions of neutral atoms and conclude that the density of occupied surface states within the band gap of the insulator is negligible.