Analysis of the possibility of the spin-orbit origin of surface state splitting in thin Mg(0001) layers on W(110) and Mo(110)

A set of magnesium films ranging in thickness from submonolayer to a few tens of atomic layers grown on single-crystal W(110) has revealed film-thickness dependent splitting of states localized energywise close to the magnesium surface state. Literature refers to several models describing the origin of this splitting; in one case, it is treated as substrate-induced spin-orbit splitting, and in another, as due to formation of nondegenerate pairs of even and odd surface states penetrating deep into the film bulk. The proposed models draw upon studies of films more than five monolayers thick. A comparative investigation of the Mg/W(110) and Mg/Mo(110) systems has been carried out for magnesium films of different, starting from submonolayer, thicknesses, which did not substantiate the spin-orbit origin of this splitting and suggests instead formation on the substrate-film interface of hybridized states, with their variation with thickness being assigned to variation in the contribution due to the magnesium surface states. Original Russian Text ? A.M. Shikin, D.E. Marchenko, N.A. Vinogradov, G.V. Prudnikova, A.G. Rybkin, V.K. Adamchuk, O. Rader, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 572–583.     

Thermal desorption spectroscopy of Ni,Cu, Ag and Au from W(110)

Ni, Cu, Ag and Au on W(110) in the submonolayer range are studied by thermal desorption spectroscopy with the goal of obtaining information on lateral interactions and on the phase state of the adsorption layer in the temperature range in which desorption occurs. Ni, Cu and Ag are found to desorb over a wide coverage range from the two-phase (vapor-condensate) region while Au desorbs only from the single-phase vapor region. Segments of the coexistence curve are determined. The desorption energies have the following limiting values: 4.35–4.95. 3.2–3.85, 2.8–3.55 and 3.3–4.1 eV for Ni, Cu, Ag and Au, respectively.     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

Spin polarization of quantum-well and interface states of ultrathin films of Bi on w(110) with Ag interlayers

The electronic and spin structure of quantum-well and interface states, formed in the system, consisting of bilayer of Bi on 1 ML Ag/W(110) was investigated by angle- and spin- resolved photoelectron spectroscopy. It has been shown that interface states are formed in local surface-projected gap of W(110) and are characterized by spin polarization and spin-orbit splitting, corresponding to surface resonances with high density spin-polarized states near Fermi edge.     

Monte Carlo simulations of hydrogen adsorption on the W(110) and Mo(110) surfaces

Kinetics of low-temperature hydrogen and deuterium adsorption on W(110) and Mo(110) surfaces have been studied by the real-time Monte Carlo simulations. Recently reported qualitative dependence of the adsorption characteristics on variation of the H₂ flux is described in terms of the dynamical equilibrium between incident and desorption fluxes and improved conditions for accommodation for the hydrogen molecules at high incident fluxes. The role of the intrinsic precursor state in hydrogen dissociative adsorption is analyzed.Received: 16 February 2004, Published online: 28 May 2004PACS: 82.65. + r Surface and interface chemistry; heterogeneous catalysis at surfaces - 02.50.Ng Distribution theory and Monte Carlo studies - 82.20.Wt Computational modeling; simulation     

Thin film formation on chemically modified surfaces: Au on Mo(110)

The early stages of the formation of Au films on chemically modified Mo(110) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and work function change (Δφ) measurements. The surfaces were modified by saturation with oxygen and CO and by carburization. Carburization did not suppress initial two-dimensional (2D) growth but on the oxygen-saturated surface Au grew from the very beginning in small 3D crystals with oscillatory thickness dependence of the Auger signals. CO showed an intermediate behaviour. It is concluded that the bond strength and the location of the chemical modifier normal to the surface is decisive for the growth mode.     

Quantum-well states in bilayers of Ag and Au on W(1 1 0)

sp-Like quantum-well states (QWS) in thin monocrystalline bilayer films of Ag and Au on W(1 1 0) and of single Ag films were studied by angle-resolved photoemission. We find that the propagation of the electronic states in the bilayer films along [1 1 1] depends on the energy relative to the band edge of Au metal at the L point of the Brillouin zone. In particular, QWS with binding energies less than this band-edge energy (1.1 eV) are strongly confined to the Ag layer, while for higher binding energies the QWS extend across the whole bilayer film. This clearly demonstrates the weakness of the potential barrier at the Ag/Au interface in the context of QWS formation at energies where electronic states exist in both metals.     

Angle-resolved investigation of Auger electrons from Cu and Au adsorbed on W(110)

The angular distribution of Cu M_2,3VV and Au N_6,7VV Auger electrons from Cu and Au mono- and double layers on W(110) is measured with the goal of obtaining information on the contribution of the backscattered wave on the angular distribution of Auger electrons from adsorbed atoms.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Quantum size effect in electron transmission through Cu and Ag films on W(110)

The transmission coefficient of very low energy electrons ( ? 10 eV) normally incident on (111) epitaxial films of Cu and Ag on W(110) is modulated by interference between scattering from the vacuum/metal and metal/metal interfaces. Comparison with calculations of free-electron scattering from a one-dimensional potential model, in which grading of the metal/metal interface is represented by a smoothing of the potential step, indicates that this interface is abrupt within approximately one layer spacing. We obtain a value of 11.0 (8.0) ± 1.0 eV for the inner potential of Cu (Ag) and mean free path lengths of $\text{39 ± 8}\text{A}\text{?}$ at an energy of 7.0 eV relative to the Fermi energy and $\text{29 ± 11}\text{A}\text{?}$ at 9.0 eV for Cu, and $\text{25 ± 10}\text{A}\text{?}$ at 7.5 eV for Ag. Work function values are obtained by the field emission retarding potential technique. We investigate the effects of the surface potential barrier, inelastic scattering and surface roughness, and evaluate the validity of the one-dimensional model presented.     

Origin of spin-orbit splitting for monolayers of au and ag on w(110) and mo(110)

Spin-orbit coupling can give rise to spin-split electronic states without a ferromagnet or an external magnetic field. We create large spin-orbit splittings in a Au and Ag monolayer on W(110) and show that the size of the splitting does not depend on the atomic number of the Au or Ag overlayer but of the W substrate. Spin- and angle-resolved photoemission and Fermi-surface scans reveal that the overlayer states acquire spin polarization through spin-dependent overlayer-substrate hybridization.     

The adsorption of CO and oxygen on Cu layers on a W(110) surface

The adsorption of CO, and to a lesser extent that of oxygen on Cu layers deposited on a W(110) surface has been investigated by thermal desorption. Auger, and XPS measurements. For CO the amount adsorbed decreases monotonically with Cu thickness from 1–5 layers. For O there is a slight increase for 1 layer, followed by a steep decrease up to 4 Cu layers where the amount adsorbed levels off. CO adsorption shifts the core levels of Cu (observed for 1 layer of Cu) to higher binding energy by 0.4 eV; the O 1s level of CO is also shifted to higher binding energy by 1.5 eV, relative to CO/W(110) suggesting that electron transfer from CO occurs but is passed on to the underlying W. For O adsorption there is very little shift in the Cu core levels or in the O 1s level, relative to O/W(110). Thermal desorption of CO at saturation coverage from Cu/W(110) shows desorption peaks at 195, 227 and 266 K, as well as small peaks associated with CO desorption from clean W, namely a peak at 363 K and β-desorption peaks at 1080 and 1180 K. As CO coverage is decreased the 195 and 227 K peaks disappear successively; the W-like peaks remain unchanged in intensity. It is argued that the latter may be due to adsorption on bare W at domain boundaries of the Cu overlayer, while the 190–266 K peaks are associated with adsorption on Cu, but probably involve reconstruction of the Cu layer. For n = 2–8 a single but composite peak is seen, shifting from 180 to 150 K as Cu thickness increases as well as a minor peak at 278 K, which virtually vanishes on annealing the Cu deposit at 850 K. The effect of tungsten electronic structure on the behavior of adsorbates on the Cu overlayers, as well as similar effects in other snadwich systems are discussed.     

Surface densities of states for normal photoemission from Mo(110) and W(111)

Tight-binding calculations of one-dimensional densities of states on Mo(110) and W(111) are presented. As reported earlier for W(001), hybridization gaps contain localized surface states or resonances, and band-edge singularities are modified near the surface. Prominent structures in directional photoemission spectra are discussed in the light of our results, and further experiments using polarized light are suggested to test our assignments.     

Virtual bound states of Pd in Cu,Ag and Au and of Pt in Ag

The positions and widths of the virtual bound states of 5 and 10% Pd in Cu, Ag and Au and of 5 and 10% Pt in Ag have been measured by XPS. The spin—orbit splitting of the Pt state in Ag is reduced from the theoretical value, but comparable to the spectroscopic atomic value. The host lattice d-band structure is perturbed in the alloys.     

Formation of spectra of quantum well states in thin Al layers on W(110)

Quantum well states of sp-type in thin metal layers of aluminum on the W(110) surface were experimentally studied by angle-resolved photoelectron spectroscopy depending on the layer thickness in a range of about 1–15 monolayers. It is shown that the aluminum layer is formed in accordance with the Kurdyumov-Sachs orientation relationship. Modification of the quantum well state spectra is observed with the increase in the layer thickness. The changes of the energy of quantum well states with the formation of each new monolayer have a stepwise character. This behavior can be used to calibrate the thickness of the deposited film with an accuracy within fractions of a monolayer. To confirm the reliability of the calibration, the thickness of the formed layers was tested using the attenuation of the W4f _7/2 peak intensity.     

Quantum-well states and resonances in thin single-crystal layers of noble metals on W(110) substrates

This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.     

Comparative study of the electronic structure of the ordered and disordered Cu3Au (100) and Cu3Au (110) surfaces

The influence of structural changes on the electronic structure has been investigated by a comparison of the angle-integrated valence and core level photoelectron spectra of ordered and disordered Cu₃Au(100) and (110) surfaces. The total width of the Au 5d and Cu 3d bands does not change with the ordering state or surface orientation. The spectra for the (100) surface are compared with selfconsistent calculations and good agreement is found, for the ordered state, however with a 0.6 eV correction of the calculated Fermi level position. We observe three Au 5d derived bands at 5.1, 6.0 and 6.9 eV, in contrast to previous experimental findings. Our results indicate the existence of shortrange order above the critical temperature.