Spectra of quantum states in thin metal films and their modification: Al/W(110) system

The modification of spectra of quantum well states of sp-type in thin Al films on the W(110) surface was experimentally investigated by angular-resolved photoelectron spectroscopy both during deposition and in dependence of the detection angle. Quantum well states are observed for the partially filled band of valence states in the range of binding energies from 4.4 eV to the Fermi level. An Al film with a thickness of 11 monolayers exhibits a jump of the dispersion relations of quantum well states in the local W(110) band gap in the ΓS direction and splitting of these relations due to the effect of substrate electronic structure on the formed spectrum of quantum states and their possible spin polarization.     

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.     

Ultra thin Al film on the W(110) surface

We have investigated the surface structure of the Al/W(110) surface using low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (ISS). We observe a p(2 × 1) double domain LEED image for the 0.5 ML Al/W(110) surface at annealing temperature 850 °C. We found that 0.5 ML Al atoms cover on the W(110) surface uniformly but do not form 3 or 2-dimensional islands. We also measured the Al adsorption site at the Al/W(110)-p(2 × 1) surface using ISS. We found that Al atoms adsorbed at the center of the bridge site. The height of the adsorbed Al atoms is determined to be 2.18 ± 0.15 Å above the W surface layer.     

Quantum well states in thin (110)-oriented Au films and k-space symmetry

We present electron diffraction and electron photoemission results for thin Au films grown on Nb(100) in a hexagonal close-packed stacking sequence, which is unusual for Au. Strong d-band [48] quantum size effects occur in photo-electron spectroscopy from 5-26 monolayer thick ( ) oriented Au films whose confined direction ([11.0]) is not perpendicular to any face of the bulk Brillouin zone. Also in this case, the energetics of the quantum well states can be explained by a discretisation of the bulk band structure corresponding to the quantum well. However, the bulk states corresponding to the quantum well states do not lie in the confined direction of the first bulk Brillouin zone, contrary to what is required by the quantum well. This can be remedied by the construction of a layer symmetry adapted Brillouin zone, which is consistent with the symmetry of the quantum well but different from the bulk one. We subsequently determine, for the first time, the periodicity of the quantum well states from their measured ). This periodicity proves to be consistent with the newly introduced Brillouin zone.Received: 1 December 2003, Published online: 15 March 2004PACS: 73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems - 82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.) - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.20.Gj Other metals and alloys     

Quantum well states in thin (110)-oriented Au films and k-space symmetry

We study the one- and two-dimensional extended Hubbard model by means of the Composite Operator Method within the 2-pole approximation. The fermionic propagator is computed fully self-consistently as a function of temperature, filling and Coulomb interactions. The behaviors of the chemical potential (global indicator) and of the double occupancy and nearest-neighbor density-density correlator (local indicators) are analyzed in detail as primary sources of information regarding the instability of the paramagnetic (metal and insulator) phase towards charge ordering driven by the intersite Coulomb interaction. Very rich phase diagrams (multiple first and second order phase transitions, critical points, reentrant behavior) have been found and discussed with respect to both metal-insulator and charge ordering transitions: the connections with the experimental findings relative to some manganese compounds are analyzed. Moreover, the possibility of improving the capability of describing cuprates with respect to the simple Hubbard model is discussed through the analysis of the Fermi surface and density of states features. We also report about the specific heat behavior in presence of the intersite interaction and the appearance of crossing points.Received: 2 July 2004, Published online: 12 October 2004PACS: 71.10.-w Theories and models of many-electron systems - 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27. + a Strongly correlated electron systems; heavy fermions     

Order-disorder transformations in chemisorbed layers: Oxygen on W(110)

We have investigated the order-disorder transformation in oxygen adsorbed on W(110). An analysis of the ordering at T = 0 using the lattice gas formalism shows that there must be significant three-particle interactions to break the particle-hole symmetry. This is necessary since there is a p(2 × 2) phase at three-quarter coverage which is not present at one-quarter coverage. Monte Carlo techniques are used to obtain estimates of the strength of the two and three-particle interactions by matching calculated and measured LEED intensity curves. The qualitative characteristics of the phase diagram are discussed with emphasis on the multicritical points which must be present if the transition at half coverage is second order. Evidence in support of a second order transition is reviewed.     

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.     

Interface effects on the quantum well states of Pb thin films

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.     

Substrate-induced spin-orbit splitting of quantum-well and interface states in Au,Ag, and Cu layers of different thicknesses on W(110) and Mo(110) surfaces

The substrate-induced spin-orbit splitting of interface and quantum-well states formed in Au, Ag, and Cu layers on W(110) and Mo(110) surfaces has been revealed using angle- and spin-resolved photoelectron spectroscopy. It has been shown that the magnitude of the splitting depends noticeably on the atomic number of the substrate material and is markedly larger for layers of these metals on W(110), i.e., on the surface of a metal with a larger atomic number (Z _W = 74), than on the surface of Mo(110), i.e., an element with a smaller atomic number (Z _Mo = 42), while depending only weakly on the atomic number of the adsorbed metal. Measurements of the dispersion of the formed quantum-well states have shown that the substrate-induced spin-orbit splitting increases with increasing parallel component of the photoelectron momentum (which correlates with the Rashba model) for all thicknesses of deposited films (up to 10 ML). The magnitude of induced spin-orbit splitting of the interface states evolving in monolayer Au, Ag, and Cu coatings on W(110) and Mo(110) decreases with increasing parallel component of the excited photoelectron momentum.     

Surface quantum oscillations in (110) and (111) n-type silicon inversion layers

Shubnikov-de Haas oscillations have been studied for (110) and (111) n-type silicon inversion layers. The measured cyclotron masses m_c = (0.38 ± 0.03)m₀ and m_c = (0.40 ± 0.03)m₀ for (110) and (111) planes, respectively, are larger than theoretically predicted values. The experimental valley degeneracy factor g_v = 2 ± 0.2 for both orientations is also at variance with self consistent calculations. The electronic g-factor depends on the surface carrier concentration and is enhanced over its bulk value. There was no evidence for the occupation of other subbands.     

Spin-resolved photoemission study of in situ grown epitaxial Fe layers on W(110)

Spin- and angle-resolved photoemission spectroscopy of in situ grown epitaxial Fe layers on W(110) shows bulk-like behavior for more than two atomic Fe layers. For about ten and more atomic layers of Fe we find a spin polarization to be about -100% near the Fermi energy and +80 % between 1 eV and 3 eV binding energy. For the bilayer of Fe drastic changes in the spin-resolved spectra and a 20 % enhancement of the spin polarization compared to the bulk value are observed. The monolayer of Fe is ferromagnetically ordered with a spin polarization reduced by about 50%. A switching of the easy magnetization axis from [001] to [11̄0] is observed in the spin polarization with decreasing Fe layer thickness near d = (65±5) Å.     

Wave-vector conservation upon hybridization of 4f and valence-band states observed in photoemission spectra of a Ce monolayer on W(110)

Angle-resolved resonant photoemission data for a hexagonally ordered monolayer of Ce on W(110) are presented. The spectra reveal a splitting of the 4f(0) ionization peak around a point in k space where a degeneracy with a valence-band state is expected. The phenomenon is described within a simple approach to the periodic Anderson model. It is found that the Ce 4f state forms a band and hybridization predominantly occurs between the 4f and the valence-band states at the same wave vector.     

Direct observation of strain relaxation in iron layers on W(110) by time-resolved STM

Time-resolved,in-situ-applied STM has been used to study the epitaxial growth of iron on W(110) at room temperature. By this way, sequences of STM images show directly the atomistics of the growth process on the surface. The first layer of iron on W(110) grows pseudomorphically without a preferred growth direction. Beginning with the second layer, the islands grow anisotropically with preferred growth in the [001]-direction. The generation of an ordered two-dimensional dislocation network starts at a coverage of 1.4 pseudomorphic monolayers to relax the misfit of 9.4%. A direct correlation of the creation of misfit dislocations in the second layer and the nucleation of the third-layer islands was found. Together with the onset of strain relaxation, the growth mode abruptly changes from layer-by-layer to statistical growth. A quantitative statistical analysis of the data allows to exactly determine the onset of relaxation, the vertical location of the dislocation lines, and the lateral extension of an island that is necessary to induce the formation of dislocations.     

Adsorption and electron properties of thin nickel films on W(110) surface

The evolution of the properties of ordered nickel films with thicknesses increasing from one to three atomic monolayers (ML) adsorbed on the W(110) single crystal surface is studied under ultrahigh vacuum conditions by the methods of reflection-absorption infrared spectroscopy (RAIRS) and ultraviolet photoelectron spectroscopy (UPS). The film structure corresponds to that of the Ni(111) single crystal face. The RAIRS technique is used to study the vibrational properties of the probing NO molecules adsorbed on the nickel films studied. In the course of the nickel film growth, whereby its thickness increases from 1 to 3 ML, both the vibrational and photoelectron spectra exhibit significant variation, which is indicative of a change in the adsorption and electron properties of the film. Stabilization of the IR and photoelectron spectra at a film thickness of 3 ML indicates that this thickness corresponds to the formation of the main adsorption and electron properties of the deposit. At the same time, the vibrational spectra of NO molecules adsorbed on a monoatomic nickel film exhibit features typical of adsorption on the W[110] surface of a massive tungsten crystal.     

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

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.     

Temperature dependence of the work function of Cu layers on a W(110) plane

Temperature coefficients of the work function for Cu layers in the range 0 ≤ n ≤ 6 where n is the number of monolayers were measured from 90 to 500 K by a Kelvin probe method. dφ/dT is negative at all coverages and has a strong maximum of −1.5×10⁴eV/K at n=1 at 90 K. A qualitative discussion of these findings is given. The design and operating characteristics of the Kelvin probe are also described.