Angular Resolved Photoemission Spectroscopy Research Articles

Alloy formation of Y with Pd investigated by photoemission and electron diffraction

The structure of Pd capped Y films for an application as a switchable mirror was studied. The surface structures are investigated by reflecting high energy electron diffraction and Auger electron spectroscopy. The alloy formation between Y and Pd leads to different structural and electronic properties of the surface, which are compared to the switching behavior of the underlying Y film when exposed to hydrogen. The electronic structure of the intermetallic compound YxPd1−x(0<x<0.25) was studied by angular-resolved ultraviolet photoemission spectroscopy. The samples were prepared by Y deposition on Pd (111) surfaces of epitaxial films. The electronic structure of the alloy is discussed in the framework of a qualitative tight-binding approach. The reactivity of the Y–Pd alloy surfaces is estimated using the chemisorption model of Hammer and Norskov [Nature 376, 238 (1995)]. We show that exposure to air leads to an oxygen induced surface segregation of the Y–Pd alloy, which hinders hydrogen adsorption.

Temperature dependence of the spectral weight in undoped Mott insulators

As recent angular-resolved photoemission spectroscopy experiments have shown, even an insulator can exhibit a ``remnant'' Fermi surface (FS), i.e., a boundary line between mostly occupied and mostly unoccupied states in the Brillouin zone. We derive an analytic expression for the remnant FS valid to order ${(t/U)}^{2}$ for the large-$U$ limit of the single-band Hubbard model at half-filling. The shape of the remnant FS at low temperature $(T\ensuremath{\ll}J\ensuremath{\equiv}{4t}^{2}/U)$ is found to be determined by local magnetic correlations, and is largely independent of the details of the band FS. However, miraculously, at intermediate temperatures $(J\ensuremath{\ll}T\ensuremath{\ll}U),$ where magnetic correlations are weak, the location of the remnant FS coincides with the band FS.

Photoemission study of organic and inorganic CDW-systems

We investigate by means of high resolution angular resolved photoemission spectroscopy the classical one-dimensional organic conductor TTF-TCNQ and the inorganic one-dimensional conductor (TaSe4)(2)I. We measure dispersive features (1D bands) in accordance with band structure calculations. We observe typical bulk specific properties like the band periodicities (i.e. the Fermi wavevector) and the CDW-gap, which are in agreement with other experimental techniques. Peculiar for one-dimensional systems, we do not observe a real Fermi level crossing of a quasi-particle state. Instead the spectral weight near the chemical potential is strongly suppressed, and we observe a pseudogap in the normal state in both systems. For the organic system our data suggest that electronic correlations are important. Our temperature dependent data further reveal that the pseudogap persists even above the mean field temperature, indicating that CDW-fluctuations cannot account for the pseudogap. For the inorganic compound CDW-fluctuations cannot be neglected in the measured temperature range. Nevertheless the observed energy scare of the pseudogap is much larger than expected from these fluctuations.

Heteroepitaxial graphite on6H−SiC(0001): Interface formation through conduction-band electronic structure

When annealed at elevated temperatures under vacuum, silicon carbide surfaces show a tendency towards graphitization. Using the sensitivity of empty conduction-band states dispersion towards the structural quality of the overlayer, we have used angular-resolved inverse photoemission spectroscopy (KRIPES) to monitor the progressive formation of crystalline graphite on $6H\ensuremath{-}\mathrm{SiC}(0001)$ surfaces. The KRIPES spectra obtained after annealing at 1400 \ifmmode^\circ\else\textdegree\fi{}C are characteristic of azimuthally oriented, graphite multilayers of very good single-crystalline quality. For lower annealing temperatures, the ordered interface already presents most of the fingerprints of graphite as soon as 1080 \ifmmode^\circ\else\textdegree\fi{}C. The observation of unshifted ${\ensuremath{\pi}}^{*}$ states, which reveals a very weak interaction with the substrate, is consistent with the growth of a van der Waals heteroepitaxial graphite lattice on top of silicon carbide, with a coincidence lattice of $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry. The growth of the first graphene sheet proceeds on top of adatoms characteristic of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction. These adatoms reduce the chemical reactivity of the substrate. A strong feature located at 6.5 eV above the Fermi level is attributed to states derived from Si vacancies in the C-rich subsurface layers of the SiC substrate. This strongly perturbed substrate can be viewed as a diamondlike phase which acts as a precursor to graphite formation by collapse of several layers. In this framework, previously published soft x-ray photoemission spectra find a natural explanation.

Nature and dynamics of the OPd(110) surface bonding

The nature and dynamics of the OPd(110) surface bonding is analyzed by incorporating a compact model of bond-to-band and the potential-barrier into the outcomes of LEED, STM, XPS, UPS, angular-resolved photoemission spectroscopy (ARPS) and the thermal desorption spectroscopy (TDS). A model of buckleddipole and a description of the bonding dynamics are suggested. To form a Pd 2O tetrahedron, the precursor O −1 in a Pd 5O cluster evolves into a threefold-coordinated O −2. The hybridized-O −2 bonds to one Pd atom underneath and one Pd in the surface, and the O −2 polarizes its two surface neighbors through nonbonding lone pairs. The buckled metal dipoles yield zigzag protrusions in the STM images. It is shown that the outstanding spectral features are recognized as the binding processes and modified density-of-state. For example, the ARPS features around −2.0 and −4.5 eV below E f are identified as the Osp 3 nonbond and bond states, respectively. The work-function-change derived from the UPS relates to the antibonding dipole subband. The characteristic features in the exposure-resolved TDS correlate to individual processes of ionic OPd bond, nonbonding lone pair, and the virtual bond formation. Hence, it is essentially realistic to interpret the reaction as the dynamics of the O −1 developing into the hybridized-O −2.

Interaction of CO and NO with WC(0001)

The results of angular resolved photoemission and thermal desorption spectroscopy investigations of the adsorption of CO and NO on a WC(0001) single crystal surface are reported. Both molecules are stable only at low temperature. Similar to the case of adsorption on many transition metals, CO is co-ordinated to the surface via its carbon end, standing upright on the surface. Upon warming up, complete dissociation takes place at temperatures of ca 270 K for CO and 160 K for NO. A part of the CO molecules also desorbs between 200 and 250 K, whereas no molecular desorption is found for NO. Recombinative/reactive desorption is observed in the temperature range between 700 and 1200 K.

Phase transition of submonolayer Pb/Ge(111): α−3×3R30° ↔ 3 × 3at∼ 250 K

We have studied the low 3 × 3 and high (α−3×3R30°) temperature phases of Pb/Ge(111) at Pb coverage 1/3 of monolayer using synchrotron radiation angular resolved photoemission spectroscopy and low-energy-electron-diffraction measurements. The Ge 3d and Pb 5d core levels and valence bands have been recorded above and below the transition temperature (Tc ≈ 250K). We have observed that the 3 × 3 reconstructed surface is a semiconductor, while the α−3×3R30° phase is metallic. As the system undergoes this transition, a band gap opens in precise areas of the Brillouin zone, supporting the charge-density-wave model for the transition.

Anomalous weak bonding of Si dimers on the SiC(001) surface?

The atomic structure and electronic states of Si dimers on the silicon-terminated 3 C-SiC(001)2 × 1 surface have been investigated by first-principles calculations and photoemission spectroscopy. The structural optimization indicates weakly bonded Si dimers in contrast to previous experimental findings. The combination of band-structure calculations and angular-resolved ultraviolet photoemission spectroscopy measurements suggests a confirmation of the modeled surface reconstruction.

Charge and spin excitations of insulating lamellar copper oxides

A consistent description of low-energy charge and spin responses of the insulating ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ lamellar system is found in the framework of a one-band Hubbard model which besides U includes hoppings up to third nearest neighbors. By combining mean-field calculations, exact diagonalization results, and quantum Monte Carlo simulations, we analyze both charge and spin degrees of freedom responses as observed by optical conductivity, angular-resolved photoemission spectroscopy Raman and inelastic neutron-scattering experiments. Within this effective model, long-range hopping processes flatten the quasiparticle band around (0,\ensuremath{\pi}). We calculate also the nonresonant ${\mathrm{A}}_{1\mathrm{g}}$ and ${\mathrm{B}}_{1\mathrm{g}}$ Raman profiles and show that the latter is composed by two main features, which are attributed to two- and four-magnon scattering.

Fermi surfaces of Bi 2Sr 2CaCu 2O 8+δ thin film

We present angular resolved photoemission spectroscopy measurements about the Fermi surface of a Bi 2Sr 2CaCu 2O 8+δ (Bi2212) thin film. Dispersion curves in two directions are discussed. It is confirmed that the Fermi surface observed on the Bi2212 thin film fits bulk Fermi surface.

A relativistic description of spin- and angular-resolved core level photoemission spectroscopy for magnetic solids

A fully relativistic description of the spin- and angular-resolved core level photoemission spectroscopy of magnetic solids based on multiple scattering theory is presented. This approach describing the corresponding final state at a time reversed LEED state allows a detailed investigation of the magnetic linear and circular dichroism in core level spectroscopy. To simplify its application in practical calculations several approximations are suggested. Results for the 2p level spectra of pure Fe for linear polarisation are presented which are in rather satisfying agreement with the linear dichroism observed by experiment.

Valence band states of H:GaAs(110)

The two-dimensional surface electronic band structure of H: GaAs(110) is studied by angular resolved ultraviolet photoemission spectroscopy along the X ′,- M and X - M symmetry lines and in the high symmetry points of the surface Brillouin zone. Three surface state bands are resolved in the first 5 eV below the upper valence band edge. A comparison with theoretical band structure calculations yields a satisfactory agreement, giving evidence of hydrogen induced GaAs(110) surface derelaxation. The clean GaAs(110) surface electronic band structure is studied comparing results with available theoretical and experimental data.

Valence band spectroscopy of V(100) surface

We have determined electron energy band dispersion of the vanadium valence bands along the HGN high symmetry direction of the bulk Brillouin zone by use of angular-resolved photoemission spectroscopy of the V(100) surface. The dispersion of the states associated to the electron transitions from the bands G 1 and G 3 were found to be in very good agreement with existing self-consistent electron band structure calculations for the bulk vanadium. Contrast to expectation, no surface state on the Fermi level has been proved.

A Photoemission Investigation of the Interfacial Electronic Properties of Mo and Ni Schottky Barriers to CuInSe2(112)

ABSTRACTWe studied Mo and Ni contacts to p-type CuInSe2(112) with angular-resolved synchrotron radiation ultraviolet photoemission spectroscopy in the range 40 eV hv < 120 eV. By varying the photon energy, we determined that emission from the CuInSe2 Г-point in the Brillouin zone occurs for hv = 60 eV, and identified emission from the valence band maximum. After depositing Ni and Mo, we found that the conduction band minimum of the CuInSe2 very nearly aligned to the Fermi edges of both metals, giving Schottky barriers Vsb = 1.05 ±0.1 eV. The interfaces are not atomically abrupt, as seen by the outdiffusion of Se into the refractory metal overlayers and the formation of In-Mo and In-Ni alloys.

Ethylene oxide adsorption on K-modified Ag(110)

Ethylene oxide (Et-O) adsorption on clean and potassium precovered Ag(110) was studied using LEED, angular resolved photoemission spectroscopy (ARUPS), high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). The binding of Et-O towards clean Ag(110) is weak, complete molecular desorption occurs below 200 K. LEED and spectroscopic data suggest a binding of Et-O to Ag(110) via the oxygen atom in a short bridge adsorption site. Low K precoverages, (θ(K) < 0.3), lead to significant changes in the Et-O/Ag(110) bonding configuration. The TDS peak temperature for low Et-O coverages shifts by ΔT≈ 55 K to higher temperature. However, in the low coverage range of θ(K), Et-O adsorption is still molecular. For higher K precoverages θ(K) ⩾ 0.7, the interaction character between Et-O and K changes drastically. An Et-O + K reaction layer is formed with thermal stabilization to temperatures ⩾ 450 K. This reaction layer is suggested to be due to an Et-O ring breaking reaction at ⩽, 200 K in the presence of metallic K and a polymerization into a larger compound.

Azimuthal orientation of tetracene adsorbed on Cu(110) single crystal surfaces

Angular resolved photoemission spectroscopy using synchrotron radiation from the BESSY storage ring has been employed to investigate the geometrical arrangement of tetracene molecules adsorbed on Cu(110) surfaces. By simple symmetry considerations based on dipole selection rules we find a perpendicular orientation of the molecules with their molecular planes aligned along the [1̄10] azimuth. Additionally, a c(4 × 2) structure has been observed by LEED.

Cs adsorption on Si(100) 2 × 1

The adsorption of Cs layers on Si(100) 2 x 1 is investigated at room temperature (300 K) and low temperature (95 K) by using angular resolved photoemission spectroscopy. At 300 K, Cs are located on one type of site and the coverage at saturation is ≤ 0.5. At 95 K, two kinds of site are populated even at low coverage. Evidence for a metallization is observed for coverage above the WF minimum occuring at ϑ = 0.33. The nature of the adsorption site is discussed.

Interface and growth studies of α-Sn/CdTe(110) superlattices

Angular-resolved synchrotron radiation photoemission spectroscopy and reflection high-energy electron diffraction (RHEED) are used to study the molecular-beam epitaxy of CdTe/ α-Sn(110) interfaces and superlattices. Core level photoemission spectra indicate that both sides of the interface are stable, nonreactive, and abrupt for growth temperatures up to 100 °C. At the α-Sn/CdTe interface, the valence band maximum at Γ is at EV=1.1 eV below the Fermi level. This gives a valence band offset of ΔEV=1.1 eV, assuming zero band gap for the Sn. Stable superlattices of α-Sn/CdTe(110) have been grown at 100 °C. The surface quality of the superstructure degrades after the growth of several α-Sn/CdTe periods. After the growth of ten periods each 50-Å thick, the RHEED pattern shows mainly three-dimensional bulk diffraction, indicating increased surface, and interface roughness.

Electronic structure and orientation of tetracene on Cu(100)

Angular resolved photoemission spectroscopy using synchrotron radiation (10 eV≤ hv ≤45 eV) yields a detailed picture of the electronic structure of the valence bands and information concerning the orientation of thin films of tetracene adsorbed at room temperature on Cu(100) substrates. With LEED a well ordered 2×2 overlayer is observed. Dipole selection rules based on the molecular symmetry have been used in order to analyze the angular-dependence of the observed photoemission bands. We find a perpendicular orientation of the tetracene molecular plane with respect to the Cu(100) substrate and arrive at a detailed assignment of all valence band features in good agreement with MO calculations.

The electronic structure of cubic SiC grown by chemical vapor deposition on Si(100)

The electronic structure of chemical vapor deposition grown SiC(100) has been investigated with synchrotron radiation by angular resolved valence-band photoemission spectroscopy. SiC grown on n-type Si substrates was also n type with a Fermi level pinned at ∼2.0 eV above the Γ15 valence-band maximum. The band structure mapped along ΓX is in good agreement with theoretical predictions. A surface sensitive structure, which was nondispersive with k⊥, has been identified at ∼1.0 eV below EF. The initial reaction of Si(100) with propane at 1400 K resulted in C 2s emission at ∼7.5-eV binding energy. This emission indicates that the first reaction steps at the surface may lead to carbon bands which are typical for a ‘‘graphitic’’ material rather than a carbide. The emission of the C 2s level of SiC appears at ∼10-eV binding energy.