Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y10: Electronic Structure of Surfaces and Interfaces |
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Sponsoring Units: DCMP Chair: Steven Erwin, Naval Research Laboratory Room: D221 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y10.00001: Interface structure and magnetic anisotropy of Fe/Pd(001) and Pd/Fe/Pd(001) monatomic films Tetsuro Ueno, Masahiro Sawada, Kazuhito Furumoto, Tetsuro Tagashira, Akio Kimura, Hirofumi Namatame, Masaki Taniguchi Fe and Pd are known to form L1$_{0}$-ordered alloy, which exhibits easy magnetization axis perpendicular to the atomic stacking plane. In order to reveal the origin of the uniaxial magnetic anisotropy in the point of view of atomic structure, we performed the experiments on bare and Pd-covered Fe monatomic films on Pd(001) surface. Interface structure analysis was done by means of intensity-voltage analysis of low-energy electron diffraction (LEED I-V), and the magnetic anisotropy was investigated by X-ray magnetic circular dichroism (XMCD). Sample fabrication and XMCD experiments were performed at HiSOR-BL14 of Hiroshima Synchrotron Radiation Center, Hiroshima University. It is revealed that the intermixing between Fe films and Pd substrate occurs at room temperature growth, and Pd-overlayer compresses the interlayer distance around Fe layer. Fe thickness dependent XMCD revealed that the spin reorientation transition from perpendicular to in-plane direction occurs in bare Fe/Pd(001) with Fe thickness increase. On the contrary, in-plane magnetic anisotropy is stable in Pd/Fe/Pd(001). We attributed the perpendicular magnetic anisotropy in Fe/Pd(001) to the L1$_{0}$-like interface structure which realized in this system. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y10.00002: Enhancement of Kondo effect through Rashba spin-orbit interactions Nancy Sandler, Mehdi Zarea, Sergio Ulloa The role of Rashba spin-orbit (RSO) interactions on the Kondo regime has been a topic of debate since resistivity measurements on Pt doped Cu:Mn compounds were interpreted as evidence for suppression of the Kondo effect by SO scattering. Subsequent theoretical and experimental activity has yielded conflicting results. Thus, the question: what is the role of SO interactions in the Kondo regime? remains open. To provide a definite answer we obtain an exact solution of an Anderson magnetic impurity model in a two-dimensional metallic host with RSO interactions. We show that the Hamiltonian reduces to an effective two-band Anderson model coupled to a S=1/2 impurity. An appropriate Schrieffer-Wolff transformation produces an effective 2-channel Kondo model plus a Dzyaloshiinski-Moriya (DM) interaction term. The exact solution reveals that the impurity couples to the bath with ferro- and antiferromagnetic couplings. DM interactions, that vanish at half-filling and at the Hubbard U-infinity limits, introduce an exponential increase in the value of the Kondo temperature. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y10.00003: Correlation induced charge ordering metal-insulator transition in a two-dimensional triangular lattice R. Cortes, A. Tejeda, J. Lobo-Checa, C. Didiot, B. Kierren, D. Malterre, J. Merino, F. Flores, E.G. Michel, A. Mascaraque Mott insulators are one of the clearest examples on how electronic correlations limit the band theory. Semiconducting surfaces offer an ideal playground to study correlation effects in two dimensions. We report here a combined experimental and theoretical analysis on correlation effects in an atomically ordered reconstruction of 1/3 ML of Sn on Ge(111). This interface exhibits a Mott metal insulator transition below 30 K [1,2]. We find a novel phase between the known metallic and insulating phases, settled by electronic correlations and characterized as a charge ordering insulator (COI) that competes with the lower temperature Mott phase. We describe here the electronic mechanism behind the stabilization of the COI-phase, the role of atomic vibrations in the process, and interpret these findings on the basis of DMFT theoretical calculations. These results explain recent controversies [3] on the interpretation of the nature of the low temperature phase. [1] A. Tejeda et al. Phys. Rev. Lett. 100 (026103) 2008 [2] R. Cortes et al. Phys. Rev. Lett. 96 (126103) 2006 [3] H. Morikawa et al. Phys. Rev. B 78 (245307), 2008; S. Colonna et al., Phys. Rev. Lett. 101 (186102) 2008 [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y10.00004: Passage from Spin-Polarized Surface States to Unpolarized Quantum Well States in Topologically Nontrivial Sb Films Guang Bian, Thomas Miller, Tai-Chang Chiang Topological insulators, which possess robust gapless surface states as a result of strong spin-orbit coupling, have attracted much interest because of their unusual surface spin structures. When such materials are reduced to ultrathin films, the spin-split surface states must connect, by analytic continuation, to quantum well states, which are spin-unpolarized in centrosymmetric systems. We report herein a combined experimental and theoretical study of this passage from polarized to unpolarized states in Sb films. Bulk Sb is semimetallic with a negative band gap; nevertheless, it shares the same topological order as Bi$_{1-x}$Sb$_{x}$ (0.07$<$ x$<$ 0.2), the first material identified as a three-dimensional topological insulator. Angle-resolved photoemission (ARPES) from Sb films, aided by first-principles calculations, shows smooth dispersion relations associated with this passage; the spin polarizations of the two states fade away, while the energy splitting is maintained through the emergence of different charge density patterns of the resulting quantum well states. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y10.00005: Electron-grain boundary scattering and the resistivity of nanometric metallic structures R.C. Munoz, M. Flores, G. Kremer, R. Henriquez, J.G. Lisoni, L. Moraga, S. Oyarzun, M.A. Suarez The resistivity of metallic structures depends on electron-grain boundary and electron-surface scattering. By tuning the grain size, we have been able to separate the contribution to the resistivity originating in electron-grain boundary scattering, from that arising in electron-surface scattering. The resistivity of gold films approximately 54 nm thick deposited onto mica substrates under high vacuum, was measured between 4 and 300 K. It exhibits a cross over, in samples where the average grain diameter d $>$ 38 nm and the resistivity is determined by electron-surface plus electron-phonon scattering, to a regime where it is determined by electron-grain boundary plus electron phonon scattering , in samples where d $<$ 38nm. l(300)=38 nm is the electron mean free path in the bulk at 300 K. The resistivity can described by Drude's model. It can be described as well by Mayadas's theory using the grain boundary reflectivity R \textit{as the only adjustable parameter.} Funded by FONDECYT 1085026. \textbf{References.} R. Henriquez et al., Phys. Rev. \textbf{B82} (2010) 113409. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y10.00006: Atomic and Electronic Structures of the Cu2O/TiO2 Heterostructure Interface Shuzhi Wang, Balasubramaniam Kavaipatti, Joel Ager, Ramamoorthy Ramesh, Lin-Wang Wang Earth-abundant metal oxides have great potentials in replacing Si in semiconductor solar cells and meeting the terawatt scale global energy demand. The structural and electronic properties of the heterojunction interface in oxide-based thin film solar cells, which is of great importance to the energy conversion efficiency, however, is not well understood yet. In this talk, we will present our experimental and theoretical work on the atomic and electronic structures of the interface of Cu$_2$O and anatase TiO$_2$. Despite the large lattice mismatch of 13\%, Cu$_2$O can be grown epitaxially on TiO$_2$(001) in the cube-on-cube orientation by pulsed laser deposition. The interface is found to form a regular coincidence lattice of 8 Cu$_2$O and 9 TiO$_2$ unit cells in each in-plane direction. The relaxed structure of this coincidence lattice is simulated using density functional theory calculations. The local density of states along the interface is found to shift as much as 0.4 eV, depending on the local alignment of the two lattices. As a result, the valence band and conduction band edge wave functions are well separated. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y10.00007: Response of the Shockley surface state on Cu(111) to an external electrical field: A density-functional theory study Kristian Berland, Per Hyldgaard, T.L. Einstein We study the response of the Cu(111) Shockley surface state to an external electrical field E by combining a density-functional theory calculation for a finite slab geometry with an analysis of the Kohn-Sham wavefunctions to obtain a well-converged characterization. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We find that the shift in band minimum and effective mass depend linearly on E. Most change in electrostatic potential profile, and charge transfer occurs outside the outermost copper atoms, and most of the screening is due to bulk electrons. Our analysis is facilitated by a method used to decouple the Kohn-Sham states due to the finite slab geometry, using a rotation in Hilbert space. We discuss applications to tuning the Fermi wavelength and so the many patterns attributed to metallic surface states. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y10.00008: Measuring Transport Properties of Thin Films Under Isotropic and Anisotropic Strain Using Piezoelectric Substrates S. Wolgast, C. Kurdak, A. Gaitas, W. Zhu Thin film systems have been of great technological interest in the last few decades due to their unique properties. It is crucially important to understand transport properties of such films under strain for some applications such as in strain gauges. Piezoelectric materials have been used in the past to study the isotropic strain-dependent properties of magnetotransport devices. We have extended this technique using one of the shearing modes of a Lead Magnesium Niobate-Lead Titanate (PMN-PT) crystal poled in the $<$011$>$ direction to study anisotropic strain in thin films. A double Hall Bar pattern oriented along the eigenaxes of the piezoelectric shearing mode permits the characterization of the film in both directions simultaneously. A uniform field in the piezoelectric substrate may be achieved for patterned devices by growing a metal surface surrounding the entire pattern. We will discuss how the changes in the carrier density and electron mobility associated with strain can be characterized in thin metal films deposited directly on the PMN-PT substrate. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y10.00009: Symmetry-induced confinement in reconstructed Au(100) S. Bengio, V. Navarro, M.A. Gonzalez-Barrio, R. Cortes, I. Vobornik, E.G. Michel, A. Mascaraque The clean reconstructed Au(100) surface was investigated by angle-resolved photoemission spectroscopy, Low Energy Electron Diffraction and Scanning Tunneling Microscopy. The reconstruction can be described as a floating, corrugated hexagonal layer on top of the bulk-terminated substrate, as in the case of Ir(100) and Pt(100) surfaces. We determine a superperiodicity of (5x26). The substrate Shockley surface state survives the reconstruction and becomes an interfacial surface state. Compelling evidence supports that the overlayer behaves as a quasi-1D system. The presence of quasi-1D states and Shockley surface states are both a consequence of a certain degree of vertical electronic confinement induced by the different symmetry of the hexagonal overlayer and the square bulk-terminated Au(100). The existence of quasi-1D states reveals a significant lateral confinement perpendicular to the atomic chains of the reconstructed Au layer. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y10.00010: Surface states localization induced by single adatoms at metal surfaces Simona Achilli, Mario Italo Trioni The perturbation introduced at metal surfaces by the adsorption of a single adatom affects the surface states. Due to their two dimensional character, localized bound states could result at energy lower then the pristine surface states. The extent of such a localization depends on a variety of aspects such as the attractive strength of the adatom induced potential, the adsorption distance, the nature of the surface state. We investigate this effect through a density functional theory approach that accounts for the semi-infinite character of the substrate and which reproduces the experimental surface states and the surface projected energy gap. The results obtained for different kind of adatoms (both magnetic and paramagnetic) on metal surfaces are discussed, focusing on the localization of the Shockley state and of the image states for different adsorption configurations. The spin splitting of the localized bound state will be also analyzed for magnetic adatoms. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y10.00011: Structure, Morphology and SRF Characteristics of Superconducting Niobium Thin Films on Ceramic Substrates Douglas Beringer, William Roach, Cesar Clavero, Rosa Alejandra Lukaszew, Charles Reece The need to improve superconducting thin film coatings for radio frequency (SRF) cavities used in linear accelerators has inspired recent niobium thin film research. To better understand the SRF properties in thin film niobium, correlated studies of structure, surface morphology and SRF performance are examined. Recent work on epitaxial growth of niobium on insulating ceramic substrates --- a-plane sapphire and MgO (001) --- anticipates Superconducting / Insulating / Superconducting (SIS) multilayer structures, which have been proposed as a means to achieve higher field gradients in SRF cavities, overcoming the intrinsic SRF limitations of bulk niobium. A fundamental study correlating structure, morphology and SRF superconducting properties of niobium thin films is an imperative first step towards realizing next generation SRF materials. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y10.00012: The Electronic Structure and Properties of Different Surface Terminations of Li2B4O6 Single Crystal Ihor Ketsman, Yaroslav Losovyj, Volodymyr Adamiv, Yaroslav Burak, David Wooten, James Petrosky, John McClory, Peter Dowben The electronic structure of the(100) and (110) surfaces of Li2B4O6 single crystal was investigated by combined angle- resolved photoemission and inverse photoemission spectroscopies. The obtained results are in a qualitative agreement with the available model bulk band structure calculations.Together with some common features, they reveal clear differences between the two surfaces. For both of them the observed dispersion of the conduction band is much greater than that of valence band and both surfaces are of n-type, though the feature is more pronounced for (100) surface, which, on the whole, is more polar. However, the (110) surface demonstrates much more sofisticated properties exhibiting, in particular, the true surface states and complicated temperature and time dependent photovoltaic charging behaviour. For this surface, in the temperature range of(80-280)K, the off-axis pyroelectric effect was observed with strongly temperature dependent currents in the $<$110$>$ direction and much smaller pyroelectric coefficient than that measured in the $<$001$>$ direction. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y10.00013: Correlated electron effects in low energy alkaline earth ion scattering Xiaoxiao He, Jory Yarmoff The spin correlations of many electrons can lead to emergent phenomena that cannot be extrapolated from the behavior of independent electrons. The role of such multi-electron processes in charge exchange during atom-surface collisions remains a challenging and unsolved problem. Two prior independent theoretical investigations predicted that when a projectile has a single unpaired electron or hole, this localized spin impurity would induce a Kondo resonance at the Fermi energy leading to a mixed-valent state in the metal conduction band. The occupancy of this sharp state would be a strong function of the surface temperature, which would cause an anomalous temperature dependence of the neutralization probability in a scattering experiment. We demonstrate such dependence for low energy Sr$^{+}$ scattered from clean polycrystalline gold. This unusual temperature dependence is amplified when the metal work function is reduced by embedding Sr atoms into the material. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y10.00014: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y10.00015: ABSTRACT WITHDRAWN |
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