Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P20: Surface Localized States |
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Sponsoring Units: DCMP Chair: Eric Fullerton, Hitachi Global Storage Technologies Room: LACC 407 |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P20.00001: Chemical Tuning of Metal-Semiconductor Interfaces Dominic Ricci, Thomas Miller, Tai-Chang Chiang We report a study of the Schottky barrier for Pb films grown on Si surfaces terminated by various metals (Ag, In, Au, and Pb) to explore the atomic-scale physics of the interface barrier and a means to control the barrier height. Electronic confinement by the Schottky barrier results in quantum well states in the Pb films, which are measured by angle-resolved photoemission. The barrier height is determined from the atomic-layer-resolved energy levels and the line widths. A calculation based on the known interface chemistry and the electronegativity yields predicted barrier heights in good agreement with the experiment. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P20.00002: Band structure calculations of the surface linear optical response of the clean and hydrogenated Si(100) surface B. Mendoza, N. Arzate, F. Nastos, J. Sipe We calculate the reflectance anisotropy (RA) and the reflectance difference (RD) spectra for a clean Si(100) surface and two H covered Si(100) surfaces. The clean surface we consider is a $2\times 1$ surface reconstruction characterized by a tilted dimer formed between the two top-most Si atoms. One of the H covered surfaces is a monohydride surface in which the two dangling bonds of the dimer are H saturated (rendering a flat dimer), and the other is a dihydride surface in which the H saturates each of the two dangling bonds leading to a bulk ideally terminated surface. This dihydride surface is thought to be created if enough H is added to the surface. The optical response is calculated both with pseudopotential and all-electron LAPW band structures including a scissors shift, and compared. In the pseudopotential we neglect the non-local contribution to the momentum matrix elements. We contrast the two methods and trace their differences to the physics involved in each one. Finally, we compare our results with available experimental measurements on this surface.\footnote{Yves Borensztein, private communication} [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P20.00003: Effects of one-dimensional multiband electronic structure on indirect interaction between Pb atoms adsorbed on the In(4x1)-Si(111) surface M. Hupalo, T.-L. Chan, C. Z. Wang, K. M. Ho, M. Tringides As predicted theoretically indirect interactions between adatoms on a metal surface can be mediated by the exchange of electrons trough the conduction band of the substrate. The long-range oscillating character of these interactions is attributed to the Friedel oscillations of the conducting electron density which screen adatoms and lattice distortions caused by adatoms. The period of the oscillations is determined by the Fermi wave-length. We have studied the growth of Pb submonolayer films on the quasi-one-dimensional In(4x1)-Si(111) which as known from spectroscopic experiments has a triple-band metallic electronic structure. A novel ordered phase of Pb is discovered with a non-primitive unit cell. Islands of this phase form well below its stoichiometric concentration ($\theta $=6/32ML). The period of the structure is 8 times the substrate lattice constant $a$ and its unit cell includes three nonequivalent Pb dimers. The average distance between the dimers is 2.66$a$.The origin of this structure is the long-range indirect interaction between Pb atoms and the observed periodicity is explained by the contribution of the three surface electronic bands with different Fermi wave-lengths. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P20.00004: Unusual Band Dispersion in Pb Films on Si(111) M.H. Upton, T. Miller, T.-C. Chiang Uncommon effects are observed in thin films, in part because the influence of the substrate is measurable. Atomically uniform Pb films are prepared on Si(111), and the quantum well states corresponding to confined valence electrons in the film are probed by angle-resolved photoemission. The subband structure shows a free-electron-like dispersion near the zone center, but the dispersion turns sharply toward higher binding energies at larger in-plane momenta. The effective mass at the zone center shows large variations of up to a factor of seven, and in one instance, the sign of the effective mass is reversed. These anomalous results are explained in terms of the bulk band structures of Pb and Si and an anticrossing coupling. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P20.00005: Bimodal electronic structure of isolated Co atoms on Pt(111) Yossi Yayon, Xinghua Lu, Michael Crommie Co atoms deposited onto Pt(111) have been found to have a giant magnetic anisotropy energy ($\sim $9meV), and therefore constitute a promising system for nanomagnetism and quantum information based applications [1]. We have used low temperature (5K) scanning tunneling microscopy and spectroscopy to probe the local electronic structure of Co adatoms on the Pt(111) surface. We observe two varieties of Co adatoms (with equal probability) that differ in their dI/dV spectra at energies around 80meV below the Fermi energy. We find that this contrast in spectral density depends on the binding site of the Co adatoms. Atoms at different surface lattice sites (i.e., fcc versus hcp sites) exhibit different local density of states (LDOS). Manipulation of a Co atom from one kind of lattice site to the other results in the expected change in the LDOS behavior. dI/dV spectra measured in this study were normalized using a new method that compensates for differences in tunneling current that occur at different surface sites. Such normalized spectra are quite useful in predicting the spatial contrast of dI/dV maps. [1] P. Gambardella \textit{et al.}, \textit{Science} \textbf{300}, 1130 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P20.00006: Breakup of Quasiparticles in Thin-Film Quantum Wells S.-J. Tang , L. Basile, T. Miller, T.-C. Chiang Quantum well states in thin films are commonly described in terms of a quasiparticle confined in a quantum box, but this single-particle picture can fail dramatically near a substrate band edge, as shown by this angle-resolved photoemission study. Atomically uniform Ag films are prepared on Ge(111) to facilitate accurate line shape and dispersion relation measurements. A quantum well peak is observed to split into two peaks near the Ge valence band edge. The unusual line shapes are shown to be due to many-body interactions and are quantitatively explained by a Green's function calculation. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P20.00007: Correlation between Electron Reflectivity and Quantum Bound States Observed by Scanning Tunneling Spectroscopy on Ag Thin Film Wei-Bin Su, Shin-Ming Lu, Hwa-Te Shu, Chi-Lun Jiang, Chia-Seng Chang, Tien-Tzou Tsong The transmission coefficient of a metal film for free electrons at low energy often reveals peaks that are associated with quantum well (QW) resonances above the vacuum level. We have observed that QW resonance peaks can also be observed on Ag films grown Si(111)7$\times $7 with scanning tunneling spectroscopy (STS). It indicates the transmission density of states probed by STS is equivalent to the transmission coefficient in free space. This equivalence is no longer valid for the reflectivity of the free electron. The distinct quantum bound states (QBS), which do not appear in the reflectivity spectra of free electron, are observed by STS. The total density of states of each QBS is equal to a total electron reflectivity in the energy distribution of the QBS, manifesting a correlation between electron reflectivity and QBS. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P20.00008: Unoccupied Metallic Quantum Well States and CO Adsorption on Ni/Cu(100) Hua Yao, Anthony Danese, Robert Bartynski When ultrathin metal films are grown on metal surfaces, reflection at the interface and at the film surface give rise to so-called metallic quantum well (MQW) states in the valence levels. The Cu/Ni/Cu(100) system is anomalous in that occupied MQW states in the Cu overlayer disperse upwards with increasing overlayer thickness, while the Cu-induced features seen in inverse photoemission (IPE) disperse downward. To better understand the origin of this phenomenon, we performed an IPE study of the Ni/Cu(100) and CO/Ni/Cu(100) systems as a function of Ni thickness. For thin Ni layers, the film grows pseudomorphically, while for thicker layers the Ni relaxes to its bulk lattice parameter. IPE spectra from Ni films up to 10 ML exhibit two features that increase in energy with increasing Ni thickness. CO adsorption strongly modifies the spectrum indicating that one feature is a Ni surface resonance while the other is a state confined to the Ni/Cu interface. In the range from 10 ML to 20 ML, the spectra exhibit a single strong feature, similar to Ni(100) surface but at a higher energy, which also appears dominated by the surface resonance. Above 20 ML the energy of the feature returns to the value for Ni(100) and the spectrum is only weakly affected by CO. Our results suggest that the Ni surface resonance and the Cu/Ni interface state play key roles in the anomalous dispersion of the unoccupied states in the Cu/Ni/Cu(100) system. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P20.00009: Electronic Structure and Dynamics of Quantum-Well States in thin Yb-Metal Films Daniel Wegner, Andreas Bauer, G\"unter Kaindl By low-temperature scanning tunneling spectroscopy, we have studied quantum-well states above the Fermi energy in thin Yb(111)- metal films deposited on a W(110) single crystal. These states are laterally highly localized and give rise to sharp peaks in the tunneling spectra. Due to the high lateral resolution of STS, the quantum-well states and their film-thickness dependence can be observed on rather rough films with variations of the local thickness over a range of several monolayers. A quantitative analysis of the spectra yields the bulk-band dispersion in $\Gamma - \mbox{L}$ direction as well as quasi-particle lifetimes. The quadratic energy dependence of the lifetimes is in quantitative agreement with Fermi-liquid theory. cond-mat/0411580. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P20.00010: Local potential profile and confined states on the Si(111)- $\sqrt 3$ x $\sqrt 3$-Ag surface by scanning tunneling spectroscopy Masanori Ono, Y. Nishigata, T. Nishio, T. Eguchi, Y. Hasegawa It has been known that on the Ag-covered Si surface forms two-dimensional metallic states, like the cases of (111) surface of the noble metals, in the band gap of the semiconductor substrate. Confined potential around defects and steps on the surface might, therefore, create 0D and 1D electronic states, respectively. We investigated in two-dimensional tunneling spectroscopic measurements using low-temperature (6K) STM how the electrical potential changes and what kinds of electronic states are formed around these sites. Our STS measurements reveal that the steps and Ag-adatom induced defects lower the potential around them. It is found that the potential variation around step edges spreads in a range of a few nm and can be fitted well with a 2D Thomas-Fermi screened potential. These can be explained with a model of positive charging at the step edges. Electronic states related with the varied potential were also observed and their details will be discussed in the presentation. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P20.00011: Positron Trapping at Quantum-Dot Like Particles on Metal and Semiconductor Surfaces N. G. Fazleev, J. L. Fry, A. H. Weiss The results of studies of sputtered surfaces of the Fe-Cu alloy with quantum-dot like Cu nano-particles embedded in Fe and submonolayer films of Au and Pd deposited on Cu(100) and Si(100) using Positron-Annihilation-Induced Auger-Electron Spectroscopy are analyzed by performing quantum mechanical calculations of positron surface states and annihilation characteristics. Estimates of the positron binding energy, work function and annihilation characteristics performed for studied surfaces reveal their strong sensitivity to nano-particle coverage. Trapping of positrons at nano-particles on studied surfaces is determined from calculated positron surface state wave functions and comparison of theoretical core annihilation probabilities with experimental ones estimated from the measured Auger peak intensities. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P20.00012: Effect of terrace width on site selectivity of C and S adsorption on stepped Pd surfaces Faisal Mehmood, Sergey Stolbov, Talat S. Rahman We have performed \textit{ab initio} density functional theory calculations of C and S adsorption on stepped Pd surfaces to extract the role of terrace width on adsorption characteristics. Previous calculations$^{1}$ show that both C and S prefer to sit on hollow site between step edge and corner atom on Pd(211), although C atoms penetrate deeper into the surface and form much stronger bond with the neighboring Pd atoms. We present results here for C and S on Pd(533) which consist of 4-atoms wide terraces instead of 3 atoms on Pd(211). The trend in adsorption energies for C and S and the nature of the local bonding with the Pd atoms will be presented in detail along with the comparison between Pd(533) and Pd(211). Implications will be drawn for the relevance of the results to those for the Pd nanoparticles. This work is supported in part by NSF Grant No. CHE0205064, and by DOE under Grant No. DE-FGO3-03ER15445. $^{1}$S. Stolbov, F. Mehmood, T. S. Rahman, M. Alatalo, I. Makkonen, and P. Salo, Phys. Rev. B \textbf{70}, 155410 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P20.00013: On mechanism of influence of alkali adsorbates on the interaction of co-adsorbed molecules with metal surface Sergey Stolbov, Talat S. Rahman It is well known that adsorption of alkali atoms on metal surfaces dramatically changes their properties. Alkali adsorbates promote various chemical reactions on catalyst surfaces, form quantum wells at the Cu(111) surface, and substantially reduce vibration frequencies of co-adsorbed molecules. To understand the nature of these effects, we study the electronic structure of Cu and Pd surfaces adsorbed with alkalis and reveal groove-like or plateau-like features of surface potential formed upon the adsorption. These features are found to produce high density low-energy electronic excitations strongly delocalized toward vacuum with respect to the ground states. We find this phenomenon to be responsible for strong enhancement of the electronic polarizability in the vicinity of the surface that leads to dramatic softening of the vibration frequencies of co-adsorbed molecules. We also discuss the connection of this phenomenon to the promotion of surface reactivity and unusual optical properties of quantum wells formed at Cu(111) upon alkali adsorption. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P20.00014: Contribution of Vibrational Dynamics to Adatom Diffusion on Metal Surfaces Handan Yildirim, Sondan Durukanoglu, Abdelkader Kara, Talat S. Rahman We have calculated the vibrational dynamics and thermodynamics for a series of scenarios where an adatom is adsorbed of flat and stepped surfaces for both cases of hollow site and saddle-point adsorption. We have used the embedded atom method for the interatomic potential for Cu. The local vibrational densities of states were calculated using real space Green's function formalism and the thermodynamical functions were evaluated using the harmonic approximation. Activation energies for the static systems show a strong anisotropy for Cu(110) and near a step. For Cu(110), for example, we find activation energies of 0.230 and 1.146 eV for diffusion along and perpendicular to the open channel respectively. The change in the vibrational free energy for these two cases was found to be about 50 meV which represents about 20{\%} for the first case and less than 5{\%} for the second. Trends extracted from our detailed study on the contribution of the vibrational dynamics to the diffusion prefactors will be presented and comparisons with experimental data will be made when available. [Preview Abstract] |
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P20.00015: Computational studies on small silicon clusters deposited on a graphite substrate Frank Hagelberg, Jianhua Wu The structural and electronic properties of small silicon clusters adsorbed on a graphite (0001) substrate are studied by Density Functional Theory (DFT) adopting periodic boundary conditions. Monolayer coverage of single Si atoms on the graphite surface is considered as well as the adsorption of Si$_ {n}$ clusters with n = 2, 3. By virtue of covalent bond formation between Si and C, the adsorption of Si$_{n}$ clusters deposited on graphite turns out to be distinctly more stable than that of alkali metal clusters $^{2}$ used as a standard for comparison in the present work. The structure and electronic properties of the Si$_{3}$ cluster are strongly affected by the interaction between adsorbate and substrate. As shown by Density of States (DOS) analysis, the energy gap of the free Si$_{3}$ cluster shrinks due to the influence of the graphite surface. The resulting energy gap of about 0.4 eV is within the experimental range $^{3}$. \\[4pt]$^{2}$K. Rytk\"onen, J. Akola, and M Manninen, Phys. Rev. B {\bf 69}, 205404 (2004); $^{3}$B. Marsen, M. Lonfat, P.Scheier, and K Sattler, Phys. Rev. B {\bf 62}, 6892 (2000). [Preview Abstract] |
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