Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y12: Electronic and Lattice Properties, Including Quantum Size Effects |
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Sponsoring Units: DCMP DMP Chair: Tai Chiang, University of Illlinois, Champaign-Urbana Room: 308 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y12.00001: Bi(114): A quasi one-dimensional metal with strong spin-orbit splitting Ph. Hofmann, J.W. Wells, H. Dil, F. Meier, J. Lobo-Checa, V.N. Petrov, J. Osterwalder, M.M. Ugeda, I. Fernandez-Torrente, J.I. Pascual, E. Rienks, M.F. Jensen The (114) vicinal surface of the semimetal Bi is found to support a quasi one-dimensional, metallic surface state. As required by symmetry, the state is degenerate along the $\bar{\Gamma}-\bar{Y}$ line of the surface Brillouin zone with a binding energy of $\approx$100 meV. In the $\bar{\Gamma}-\bar{X}$ direction the degeneracy is lifted by the strong spin-orbit interaction, as directly shown by spin-resolved photoemission. This results in a Fermi surface consisting of two closely separated, paralell lines of opposite spin direction. We discuss these findings in the light of the recently discovered topological stability of surface states on BiSb topological insulators. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y12.00002: Plasmon Resonances in Ultrathin Magnesium Films Mustafa M. Ozer, Eun Ju Moon, Adolfo G. Eguiluz, Hanno H. Weitering Low temperature growth of Mg on Si(111) results in the formation of atomically smooth thin films with precisely controlled film thickness. We employed x-ray photoelectron spectroscopy to monitor the evolution of a sharp shake-up satellite in the Mg 1s core level as a function of the film thickness. For films with thicknesses between five and twenty five atomic layers the energy position of this peak is inversely proportional to the square of the film thickness. These results are consistent with the existence of quantized plasmons, which we interpret on the basis of theoretical (hydrodynamics and RPA) descriptions of the density-response function. We demonstrate that the observed loss feature corresponds to the n = 1 antisymmetric normal mode of the thin film, consistent with the fact that in the ultrathin film limit the symmetric plasmons have vanishing spectral weight - a striking manifestation of the role of size quantization on plasmon resonances in precisely controlled nanostructures. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y12.00003: Structural analysis of the surface of bilayered ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ Biao Hu, D. Mandrus, A.P. Li, M.H. Pan, Shuheng Pan, V.B. Nascimento, E.W. Plummer, R. Jin The bilayered ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits interesting structural properties. For example, in the bulk the in-plane lattice parameter $a$ expands while the out of plane lattice parameter $c$ contracts as temperature increases, accompanied by a rotation of RuO$_{6}$ octahedra. Remarkably, the broken translational symmetry at the surface causes a tilt of RuO$_{6}$ octahedra. The Low Energy Electron Diffraction (LEED) $I-V$ analysis shows that the tilt angle of RuO$_{6}$ is temperature dependent and hysteretic. The correlation between RuO$_{6}$ tilt at the surface, the surface thermal expansion, and the electrical properties were investigated using Scanning Tunneling Spectroscopy, (LEED)-$I-V, $and Scanning Electron Microscopy. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y12.00004: Interface-Induced Complex Electronic Interference Structures in Ag Films on Ge(111) Tai Chiang, Yang Liu, Nathan Speer, Shu Tang, Tom Miller We have mapped the electronic structure of atomically-uniform films of Ag grown on Ge(111) by angle-resolved photoemission. Cuts in momentum space at constant energies near the Fermi surface reveal intricate patterns resembling interfering waves emanating from multiple centers. The measured dispersion relations exhibit zigzag patterns with multiple energy gaps. These features are attributed to the mixing of electronic standing waves by the Ag-Ge interface potential, as confirmed by the observed pattern symmetry and by an experimentally deduced interaction strength that scales as the inverse film thickness. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y12.00005: Influence of quantum well states on the formation of surface Au/Pb alloy in Pb/Si(111) quantum thin films. Jungdae Kim, Shengyong Qin, Alexander Khajetoorians, Wenguang Zhu, Zhenyu Zhang, Chih-Kang Shih The thickness dependence of Au/Pb alloy formation on thin Pb quantum films is studied using \textit{in-situ} low temperature STM/S. Sub-monolayer Au was deposited on to thin Pb films on Si(111) substrates. When deposition is carried out at a substrate temperature close to room temperature, it is found that local Au/Pb surface alloys are formed in the form of nano-islands, with preferential formation probability at certain thicknesses. STS data shows this is directly related to quantum well states (QWS) of underlying Pb mesas. When the growth is carried out at low temperature ($\sim $80K), the alloy formation probability doesn't show strong thickness preference, but QWS has still strong influence on the nature of the Au/Pb alloy. Two types of Au/Pb alloy nano-islands are formed with quite different electronic properties. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y12.00006: Coherent quantum-well electronic structure in bimetallic Pb/Ag films prepared on Si(111). Matthew Brinkley, Yang Liu, Nathan Speer, Thomas Miller, Tai-Chang Chiang Angle-resolved photoemission is employed to investigate the electronic structure of Pb films of various thicknesses grown on atomically uniform Ag(111) films. The Ag films, which were deposited on Si(111) substrates, host fully confined electrons at energies within the absolute gap of Si and partially confined electrons outside. The question is: What is the electronic structure of the Pb films prepared over the Ag films? Our results reveal that the quantized electronic structure of the Ag films can be detected for Pb overlayers with thicknesses much larger than the photoemission escape depth. Comprehensive simulations have been performed and are in agreement with the experimental results. This study reveals a strong coherent coupling of the Ag and Pb electronic structures despite the incommensurate Ag/Pb interface. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y12.00007: Pseudogap Mediated by Quantum-Size Effects in Lead Islands Kedong Wang, Xieqiu Zhang, M.M.T. Loy, T.-C. Chiang, Xudong Xiao Quantum confinement effects in both metallic and semiconducting materials are subjects of intense prevailing interest. For thin films and islands of Pb grown on semiconductor surface, quantum well states have been clearly identified and their consequences in growth, thermal stability, and superconductivity have been well studied. In this talk, we will present scanning tunneling spectroscopy measurement results of Pb islands on Si(111) at high energy resolution that reveal a novel pseudogap, or a pseudopeak in special cases, around the Fermi level in addition to the usual quantum well states. These gap or peak features persist to temperatures as high as $\sim $80 K and are uniquely related to the quantum well nanostructure of the Pb islands. A systematic analysis indicates that electron-phonon scattering is responsible for the observed electronic structure. The behavior of the pseudogap has a strong resemblance to that of the pseudogap in high temperature superconductors and certain connections may be speculated. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y12.00008: Quantized Electronic Structure and Growth of Pb Films on Highly Oriented Pyrolytic Graphite Yang Liu, Jens Paggel, Mary Upton, Tom Miller, Tai Chiang We have measured the electronic structure of thin Pb films grown on highly oriented pyrolytic graphite (HOPG) by angle-resolved photoemission spectroscopy. Quantum well states (QWS) corresponding to confined Pb valence electrons are observed. Although the films are rough, the thickness distribution is sufficiently narrow to allow a unique assignment for each QWS peak in terms of a quantum number and the exact film thickness in atomic layers. The even film thicknesses are found to be much more prevalent than the odd film thicknesses. These results are consistent with an available first-principles calculation of the surface energies of freestanding films; an implication is that the interaction between the Pb film and the HOPG substrate is weak. The effective masses of QWS at the surface zone center agree well with the results calculated from the bulk Pb band structure, in sharp contrast to the strongly enhanced or anomalous effective masses in Pb films grown on Si(111) as reported previously. This finding indicates that the anomalous effective masses in Pb/Si(111) are not caused by increased electron correlation effects in a confined geometry, but rather attributable to a strong interfacial interaction between the QWS and the substrate electronic structure. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y12.00009: Quantum stability and superconductive properties of atomically smooth ultrathin alloy films of thermodynamically immiscible metal elements Eun Ju Moon, Mustafa M. Ozer, James R. Thompson, Hanno H. Weitering Pb and Ga are immiscible in bulk form. However, atomically smooth ultrathin films of Pb $_{1-x}$Ga $_{x}$ alloy (x=0.06) can be stabilized on a Si(111)7x7 substrate through the quantum size effect. The quantum stability and superconductive properties of these films were investigated using STM, XPS, and SQUID magnetometry measurements. Quantum stabilized growth defects, consisting of deep holes extending to the film-substrate interface, act as pinning centers for vortices in the superconducting state. The pinning centers support an extraordinarily robust critical state with critical current densities in excess of 3 MA/cm2 in 10 monolayer thick films. Anomalies in the dc magnetization and ac magnetic response below 2.5-3.5 K indicate a reduction of the flux pinning below these temperatures, which we attribute to the nature of the holes (deep holes as opposed to blind holes in pure Pb films). The present study highlights the possibility of growing new alloys beyond the solid solubility limit and controlling critical state properties in the quantum regime. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y12.00010: First-principles study of SiON/SiC(0001) surface insulating layer Yasunobu Ando, Kazuto Akagi, Shinji Tsuneyuki, Tetsuroh Shirasawa, Hiroshi Tochihara Silicon oxynitride (SiON) formed on a 6H-SiC (0001) surface could be an ideal insulating layer in the application of SiC for future semiconductor devices: it is not amorphous but crystalline with only three-atomic-layer (5A) thickness, while STS-IV measurements show that the surface has a band gap as large as crystalline SiO2 (Shirasawa et al., Phys. Rev. Lett. 98, 136105(2007)). In this study, we precisely calculated band gap profile of the system in the surface normal direction with a first-principles method based on density functional theory to find reasonable agreement with that obtained by the element-specific XAS and XES measurements. In view of its application to a metal-oxide-semiconductor (MOS) structure, we also investigated the effect of aluminum atoms deposited on the insulating layer at low coverage. Starting from several candidate structures, we optimized the structure and investigated the band gap profile. We found that adsorbate-originated states appeared within the large band gap at the surface, though the states are confined within a very thin layer apparently preserving the surface insulating layer. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y12.00011: Electronic structures of intrinsic \textit{n}-type SrTiO$_{3}$-LaAlO$_{3}$ interface: density and spatial distribution of free carriers Won-joon Son, Eunae Cho, Seungwu Han The seminal paper by Ohtomo and Hwang reporting the unexpected conductivity when two perovskite insulators, SrTiO$_{3}$ and LaAlO$_{3}$, formed an atomically abrupt interface along [001] direction, has aroused an immense interest on the origin of the conductivity. While it is widely accepted that the intrinsic $n$-type interface is conducting, the carrier density and its spatial distribution are not fully understood. In presentation, first-principles study for establishing the fundamental property of the charge carrier in the intrinsic $n$-type LaAlO$_{3}$/SrTiO$_{3}$ interface is introduced. To this end, large model systems including up to 20 layers of SrTiO$_{3}$ and 1--10 layers of LaAlO$_{3}$ are employed. The charge carrier is observed when LaAlO$_{3}$ is larger than 3 unit cells and it converges to 0.5 electrons per unit cell. It is also found that the charge carriers transferred from LaAlO$_{3}$ surface are mostly localized within a few layers from the interface. Furthermore, the electronic states are quantized at the interface with different localization widths, which is similar to electrons in a wedge potential. The carrier density contributed by the interface-localized state shows a good agreement with the experiment. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y12.00012: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y12.00013: Positron states and annihilation characteristics at the reconstructed (100) and (111) surfaces of Si with adsorbed hydrogen and oxygen. N.G. Fazleev, W.B. Maddox, A.H. Weiss We present results of theoretical studies of positron surface states, work functions and annihilation characteristics at the reconstructed (100) and (111) surfaces of Si with adsorbed hydrogen and oxygen. Calculations are performed taking into account geometrically optimized surface structures using a modified superimposed-atom method, and employing the corrugated-mirror model in a full three-dimensional geometry. The effects of adsorption of hydrogen and oxygen on the positron binding energy at the reconstructed (100) and (111) surfaces of Si and the positron work function are explored. The positron surface-state annihilation characteristics are computed for different hydrogen and oxygen coverages of both reconstructed surfaces of Si. The obtained results predict attenuation of the Si positron-annihilation-induced Auger electron signal intensity with the increase of the hydrogen and oxygen coverage consistent with experimental data. These studies confirm that positron-annihilation-induced Auger electron spectroscopy can be used to study changes in the properties of semiconductor surfaces due to the presence of adsorbed hydrogen and oxygen. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y12.00014: Fabrication and transport measurement of periodic wavy structures of Si/SiGe nanomembranes Minrui Yu, Robert Blick, Arnold Kiefer, Don Savage, Max Lagally We demonstrate fabrication of completely under-etched Hall-bars made from Si/SiGe nanomembranes with a highly doped SiGe layer. The sample material is epitaxially grown by chemical vapor deposition (CVD) on silicon-on-insulator (SOI) wafers. Hall-bars are defined by optical photolithography and etched by reactive ion etching (RIE). They are then completely released from the substrate through hydrogen fluoride (HF) vapor etching. The lattice mismatch between silicon (Si) and germanium (Ge) generates an initial strain inside the material, which tends to recover once the sacrificial oxide layer is removed. This combined with carefully designed geometric constraints causes the structures to buckle and generate periodic wavy patterns after releasing and rebonding to the wafer surface. We study the magneto-transport at both room and low temperatures, with and without light illumination. Our results show the effect of strain on band structure and electron mobility. This will further the understanding of mechanically modulated electron transport. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y12.00015: Electron accumulation on bare and hydrogenated indium nitride surfaces Brian Thoms, Rudra Bhatta, Ananta Acharya, Mustafa Alevli, Nikolaus Dietz Electron accumulation layers which affect device and contact properties have been reported on several semiconductor surfaces such as InAs, InN, and CdO. Adsorbates have been shown to affect the electron density on InAs surfaces, however, surface termination effects for InN have not been determined. In this work surface-sensitive electron spectroscopic techniques are used to study both the electron accumulation layer and the surface structure on N-polar InN. High resolution electron energy loss spectroscopy (HREELS) has been used to characterize the surface electron accumulation by observing changes in the energy of the conduction band plasmon loss with variations in incident electron energy. In addition, HREELS along with low energy electron diffraction and Auger electron spectroscopy allow characterization of the surface structure and bonding. By this method it is shown that both hydrogen-terminated and bare N-polar InN surfaces exhibit electron accumulation. These results indicate that surface electron accumulation on InN is not due to indium-indium bonding and is not substantially affected by the presence or absence of surface hydrogen, but may instead be intrinsic to the N-polar InN surface. The effects of other adsorbates will also be discussed. [Preview Abstract] |
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