Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W20: Surface Structural and Electronic Properties |
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Sponsoring Units: DCMP Chair: Qiming Zhang, University of Texas-Arlington Room: LACC 407 |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W20.00001: Ab initio studies of the cubic boron nitride (110) surface Newton Ooi, James Adams The surface structure, surface energy, and work function of the cubic boron nitride (110) surface were determined using density functional theory calculations. The surface energy calculated using the LDA (GGA) is 3.41 (2.81) J/m$^{2}$ and the work function is 7.75 (8.9) eV. The surface energy values correspond well to published measured and calculated values of other binary compound ceramics, whereas the work function values are higher than that seen in other binary compound ceramics. The density of states of the surface was analyzed to determine the electron transitions involved in the work function. Geometry minimization of the stoichiometric BN (110) surface results in a simple reconstruction where pairs of adjacent B and N atoms reduce their bond length by $\sim $ 0.12 {\AA}, compared to the perfect crystal. Relaxations occur as small oscillations in the (110) inter-planar spacings that decay to zero after five atomic layers. Oscillations are more pronounced for B atoms than N atoms, and the final surface has the N atoms about 0.2 {\AA} higher than the B atoms. These reconstructions and relaxations are similar to that found in other III -- V zincblende materials. Adjacent B and N atoms at the surface are more covalently bonded than in the bulk. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W20.00002: Theory of Auger decay for fine surface structure features. Gian Paolo Brivio, Mario Italo Trioni, Sebastiano Caravati We calculate the core-core-valence (CCV) Auger profiles of an adatom on a periodic surface within the DFT framework by using a method based on the local density of states (LDOS), with a core hole on the atom, in the neighbourhood of the impurity. We investigate the CCV lineshapes of Na on Al(111) for the two ordered phases observed at room temperature. From a detailed comparison of the calculated spectra with specific recent experiments for the KL$_{23}$ lineshape of Na, we prove that a theoretical effort (based on DFT) can allow for determining the fine details of the two surface geometries of the adsorbate system, and also for unravelling components from inequivalent adatoms coexisting in the signal. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W20.00003: A method for calculating surface electronic structures using semi-infinite boundary conditions Yonas Abraham, N. A. W. Holzwarth We have developed a new formalism for solving the Kohn-Sham equations in the layer geometry appropriate for studying equilibrium and transport properties of surfaces and interfaces. The formalism assumes that the electron-lattice interactions are modeled by pseudopotentials containing both local contributions and non-local terms represented by separable functions, and works especially well with the projector augmented wave ``PAW'' method.\footnote{ P.~E. Bl\"{o}chl, {\em Phys. Rev. B}, {\bf{50}},17953 (1994), A.~R. Tackett, N.~A.~W. Holzwarth, and G.~E. Matthews, {\em Comput. Phys. Comm.}, {\bf{135}}, 348 (2001); Website: \urllink{http://pwpaw.wfu.edu}{http://pwpaw.wfu.edu}} Based on the Numerov algorithm, a two-point recurrence relation is used to integrate the differential equations. The recurrence formalism is used to find the generalized Bloch waves in the bulk regions of the system as well as to find the propagating and surface states in the interface regions of the system. The wavefunction is matched at the boundary between the bulk and interface regions and at intermediate points to ensure stability. The formalism is demonstrated for a simple model of a semi-infinite system and compared with a boundary matching formalism developed by Choi and Ihm.\footnote{Y. J. Choi and J. Ihm, {\em{Phys. Rev. B}} {\bf{59}}, 2267 (1999).} [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W20.00004: GW quasiparticle energy calculations for surfaces: the influence of polarization effects in a repeated-slab approach Philipp Eggert, Christoph Freysoldt, Patrick Rinke, Matthias Scheffler, Arno Schindlmayr The combination of density functional theory (DFT) and many-body perturbation theory in the \emph{GW} approximation has become an important tool for \emph{ab-initio} band structure calculations typically in good agreement with experiment. In order to treat surfaces one often employs a repeated slab geometry for computational convenience. However, electric multipole moments may occur in the slabs which then lead to a slowly decaying electrostatic interaction. If present, static dipoles must be corrected for in DFT [1]. In \emph{GW} however, dynamic dipoles are always created. We present calculations for the hydrogen-saturated silicon (001) slabs and show that slabs smaller than 10 layers are not converged fully with 10-20~\AA\ vacuum thickness. The effect of the dynamic dipoles is slowly decreasing with slab thickness, in accordance with an extension of a simple electrostatic model, that includes these polarization effects [2]. Therefore it is essential to monitor the convergence carefully and if necessary to extrapolate to infinite separation. \\ $[$1$]$ J. Neugebauer, M. Scheffler, Phys. Rev. B \textbf{46}, 16067 (1992) \\ $[$2$]$ C. Delerue, G. Allan and M. Lannoo, Phys. Rev. Lett. \textbf{90}, 076803 (2003) [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W20.00005: Using Alkali Ions to Probe the 3D Surface Potential of Cerium Oxide Surfaces Guangzhi Liu, Jory Yarmoff Charge exchange between low-energy alkali ions and cerium oxide (CeO$_{2})$ single crystal surfaces is shown to depend on the surface local electrostatic potential (LEP). CeO$_{2}$ has interesting catalytic properties, and is also a good surrogate for studies of U and Pu oxides. The surface is terminated by oxygen, and the degree of oxidation can be varied by annealing in vacuum or O$_{2}$. The final charge state of alkali ions scattered from a metal surface is determined along the exit trajectory by a resonant charge transfer process, which depends on the LEP a few {\AA}'s above the scattering site.$^{1}$ The neutral fractions of $^{23}$Na$^{+}$ ions singly scattered from the Ce sites were measured with time-of-flight. The neutralization decreases for more grazing angles and increases for higher energy, in contrast to the usual expectations. This behavior directly reflects the inhomogeneity in the surface potential, and demonstrates that this technique can provide an experimental measure of the 3D shape of the potential. $^{1}$C. B. Weare and J. A. Yarmoff, Surf. Sci. 348 (1996) 359. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W20.00006: Measurement of surface charging effects via a buried Al$_{0.24}$Ga$_{0.76}$As/GaAs quantum well structure R.D. Gann, M. Biasini, J.A. Yarmoff, A.P. Mills, Jr., B.C.D. Williams, L.N. Pfeiffer, K.W. West, X.P.A. Gao We have studied the conductivity of an Al$_{0.24}$Ga$_{0.76}$As/GaAs quantum well as a function of the surface charge generated by electron bombardment of the sample in the absence of an externally applied surface electric field. Under a suitable rate of electron irradiation we have been able to completely shut off of the conductive channel, implying a surface density $n=2.5 \times 10^{11} \mbox{el/cm}^2 $. Light illumination quenches the increase of the resistivity, apparently due to photoemission from the metastable surface states. Upon turning off the electron bombardment the conductivity reverses to the original value. We attribute this decay to the electric discharge of the surface. The lifetime ($\tau = 0.30 \pm 0.02$ s.) is almost independent of the conditions of the surface (10 nm of undoped GaAs), which was either bare or covered with $\simeq$ 100 layers of xenon and/or water. These findings suggest that electrons are trapped near the surface due to an image potential. The charging efficiency, $\mu$, defined as the ratio of the charge deposited on the surface to the beam current times $\tau$ is $\mu_0 \simeq 0.001$. We are now extending this method to investigate surface mobility in other materials, beginning with insulators such as mica. Work supported by:DOD/DARPA/DMEA(grant No.DMEA90-02-2-0216). [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W20.00007: Carbon Adsorption on Copper and Nickel Surfaces: A Comparative First-principles Study Qiming Zhang, Tu Hu, Xingao Gong, Jack C. Wells, Zhenyu Zhang The goal of this work is to understand why transition metals such as Fe, Co, and Ni are good catalysts for carbon nanotube growth while others are not. Choosing Ni and Cu as prototypes, we use first-principles total energy calculations to study the adsorption of a carbon atom on the three low-index surfaces of both metals. The adsorption energies of a carbon atom at the most stable adsorption sites on the Cu and Ni surfaces have been obtained and compared. The preference order of the adsorption sites for both Cu and Ni is the same. The (100) hollow site has the highest adsorption energy. The diffusion barriers for a C atom on the three low-index surfaces have also been obtained, with the highest mobility on the (111) surface for both Cu and Ni. Our investigation shows that the adsorption energies of the C atom on Ni are significantly higher than those on Cu for all the three surfaces. This phenomenon could be explained by their electronic structures at the Fermi level. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W20.00008: Surface Electronic Structure of the Compositional Variants of SnO2(101) Matthias Batzill, Jimi Burst, Khabibulakh Katsiev, Anne Chaka, Bernard Delley, Ulrike Diebold Tin-dioxide finds widespread applications as a solid state gas sensing material and as a transparent conductor. Despite the importance of surfaces and interfaces in these applications the surface structure and composition of this material has not been extensively studied. In this contribution we show that the (101) surface exhibits a compositional transition from a Sn(IV)O$_{2}$-1x1 surface termination to a reduced Sn(II)O-1x1 composition if the oxygen chemical potential is lowered. ~The stability of the two surface phases is explained by the dual valency of Sn that stabilizes surfaces with Sn(II) as well as Sn(IV) surface species. ARUPS show the existence of a Sn-derived surface state for the oxygen deficient surface. ~Additionally, oxygen deficient surfaces have a $\sim $1eV lower work function than stoichiometric SnO$_{2}$ surfaces. This allows tuning of the band-alignment of hetero-layers with respect to SnO$_{2}$ electrodes in optoelectronic applications. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W20.00009: Structural and Electronic Properties of Magnesium Hydride Surfaces. Michel A. Posternak, Alfonso Baldereschi The Mg--MgH$_2$ system is of considerable interest in hydrogen technology, though the hydride forms very slowly. The microscopic mechanisms of hydriding/dehydriding Mg are not yet fully understood. In particular, transmission electron microscopy experiments\footnote{B. Bokhonov {\it et al.}, Mat. Lett. {\bf 5}, 218 (1987); A. Altmann and T. Schober, Scripta Met. {\bf 25}, 723 (1991).} have shown differences in the relative orientation between the MgH$_2$ lattice and the (hexagonal) Mg substrate during the formation process $[ \, (100)_{\rm hydride} \; \| \; (0001)_{\rm Mg} \, ]$, and the decomposition one $[ \, (110)_{\rm hydride} \; \| \; (0001)_{\rm Mg} \, ]$. In order to contribute to the understanding of the above orientation differences, and in general of the reactions occurring at the MgH$_2$ surface, we have performed a comparative study of the unrelaxed and relaxed (100) and (110) surfaces. The electronic structure has been calculated using the LAPW method, and the chemical bonding has been obtained from the corresponding Wannier functions. Bonding at the surface differs from that of the bulk, and strongly depends on surface orientation and relaxation. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W20.00010: Positron Trapping and Annihilation at Reconstructed Ge(100)-(2x1) and Ge(111)-(2x8) Surfaces J. L. Fry, N. G. Fazleev, A. H. Weiss The results of studies of Ge(100) and Ge(111) surfaces using high-resolution positron-annihilation-induced Auger electron spectroscopy are analyzed by performing quantum mechanical calculations of the image-potential surface states and annihilation characteristics for positrons trapped at the reconstructed Ge(100)-(2x1) and Ge(111)-(2x8) surfaces. Estimates of the positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction of the topmost layers of clean Ge. Comparison of theoretical positron annihilation probabilities computed for different reconstructed surfaces with experimental ones estimated from the measured Auger peak intensities permits identification of the atomic structure of the topmost layers of the reconstructed surfaces. The effects of adsorbates on the localization of positron surface state at the semiconductor surface and positron annihilation characteristics are discussed. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W20.00011: Step stiffness and surface diffusion on Au(111) studied by LEEM Michal Ondrejcek, Waclaw Swiech, Mahesh Rajappan, C. Peter Flynn Using LEEM we have examined the equilibrium step structure of Au(111) surface. The study is part of ongoing project of using step fluctuation spectroscopy on different fcc(111) and bcc(011) surfaces. From capillary wave analyses we obtained fluctuation amplitudes and relaxation times for Fourier components of the step edge displacement as functions of wave number q and temperature. Similar to Pt(111) and Pd(111) [1] the relaxation rates at $\sim $0.65 T$_{m}$ vary accurately with wavevector q as $q^{3}$, showing that relaxation is driven by surface diffusion on the terraces. Bulk vacancy diffusion becomes the dominant relaxation mechanism at high temperatures, with $\tau _{q}^{-1}\sim q^{2}$. The results yield values for the surface and bulk mass diffusion coefficient, D$_{s}$=1.7x10$^{-3}$ exp(-0.87eV/k$_{B}$T) cm$^{2}$/s; D$_{b}$=0.06 exp(-1.95eV/k$_{B}$T) cm$^{2}$/s. The orientation-dependent but only weakly temperature dependent step stiffness $\tilde {\beta }(\theta )$ was measured through the temperature range 900-1300K. The quantitative results of D$_{s}$ are compared to the other closed packed surfaces at homologous temperatures relative to T$_{m}$. Finally we discuss the behavior of double steps formed at higher temperatures, The stiffness is about 1.4 times that of a single step. This research is supported by DOE grants DEFG02-02ER46011 and DEFG02-91-ER45439. [1] M. Ondrejcek, W. Swiech and C.P. Flynn, Surf.Sci. 566-568, 160 (2004). [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W20.00012: STM study of size–selected Ag and Au nanoclusters on titania surface Xiao Tong, Lauren Benz, Paul R. Kemper, Michael T. Bowers, Horia Metiu, Steven K. Buratto The catalytic activity Au and Ag nanoclusters on oxide supports is known to be strongly dependent on the size of the cluster and its interaction with the surface. In this study, we have probed the size dependence of adsorption geometries by depositing size-selected clusters of Agn+ and Aun+ (n = 1-7) from the gas phase onto single crystal rutile TiO2 (110) (1x1) surfaces at RT under soft-landing ($<$ 2 eV/atom) conditions. We analyze the clusters on the surface using UHV-STM and compare the resulting structures with theory. In the case of Ag1+ and Ag2+ clusters deposited, we observe large, sintered clusters indicating high mobility for these species on the surface. For Agn+ (n $>$ 2) clusters deposited, however, we observe a high density of intact clusters bound to the surface and no large, sintered clusters indicating that these species have very limited mobility on the surface. In the case of Aun+ clusters deposited, we observe large, sintered clusters only from the deposition of Au1+ and a high density of intact clusters from the deposition of Aun+ (n $>$ 1). In cases where we observe intact clusters we can observe the binding site and geometry of the cluster in the STM image and compare these with structures calculated using density functional theory (DFT) as well as structures observed in the gas phase. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W20.00013: k-dependent electronic structure of NiNmSb single-crystal surfaces Juliet Correa, Christian Eibl, J\"{u}rgen Braun, Georgi Rangelov, Markus Donath The half-Heusler alloy NiMnSb is believed to be a half-metallic ferromagnet with 100{\%} spin polarization at the Fermi level. This property makes it an interesting material for spintronic applications. Detailed~understanding~of~the~band~structure~of NiMnSb,~not~only~in~the~bulk~but~also~at~surfaces~and~interfaces,~is essential~to~developing~spintronic~applications. To date, however, the only data available are of the density of states, specifically from polycrystalline samples. We present angle-resolved photoemission results from both the $\bar {\Gamma }\bar {X}$ and $\bar {\Gamma }\bar {K}$ directions of a carefully prepared and surface-characterized NiMnSb(100) single crystal. We observed clear energy dispersion of occupied bulk states as a function of the wave vector parallel to the surface. We distinguish between bulk and surface states in our one-step-model calculations. Our results are discussed along with band structure calculations. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W20.00014: Surface Characterization of Cleaved Bilayer La1.2Sr1.8M2O7 Brian Sell, Masamitsu Watanabe, Norman Mannella, Stephenie Ritchey, Mark West, Hong Zheng, Jonh Mitchell, Simon Mun, See-hun Yang, Charles Fadley The properties of the layered colossal magnetoresistive manganites are due to a complex competition between the electronic, magnetic and lattice degrees of freedom. These properties are furthermore extremely sensitive to the composition of the sample. If these materials are to be incorporated in nanoscale layered spintronic structures, the surface and interface properties may vary from that in the bulk, as suggested recently by Freeland et al., who have observed a ferromagnetically dead layer near the surface of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, with x=0.4, after exposure to atmosphere. We have studied the \textit{in situ}-cleaved surface of this material, using angle-resolved core-level photoemission. Our results suggest a segregation of Sr to the surface. We will also comment on the use of resonant Bragg scattering from such materials as a method for more precisely determining their layer-by-layer composition and magnetization. Work supported by DOE Off. of Science, Basic Energy Sciences, Mat. Sci. Div. [Preview Abstract] |
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W20.00015: The Role of Dangling Bond Diffusion in The Process of Self-Directed Growth of Molecular Assemblies on the H-Si(100)-3x1 Surface: from fabrication towards electrical transport measurements of hybrid silicon-molecular nano structures Xiao Tong, Robert A. Wolkow A chain reaction initiated at a dangling bond on a H- terminated Si(100)-3x1 surface leads to the creation of contiguous, linear multimolecular assemblies. In contrast to a similar growth process observed on the H-Si(100)-2x1 surface, the linear structures grow in the cross-row direction, rather than parallel to dimer rows. This process is enabled by both an uncommonly high rate of H atom diffusion, specifically in the cross-row direction, and a low barrier to H atom abstraction from dihydride sites. These results demonstrate that anisotropy inherent to the substrate can be imposed upon molecular assemblies formed via this ``self-directed'' growth process. In addition, unlike in the case of styrene where H-abstraction is restricted to adjacent surface sites, resulting in straight multimolecular assemblies, a assembly process follows a meandering path via ring-opening radical clock reactions, which is a new self-directed growth mechanism and kinetic insights, will also be introduced. [Preview Abstract] |
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