Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A9: Electronic Structure on Surfaces and in Reduced Dimensions |
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Sponsoring Units: DCMP Chair: Hanno Weitering, University of Tennessee Room: Baltimore Convention Center 301 |
Monday, March 13, 2006 8:00AM - 8:12AM |
A9.00001: Single-electron tunneling force spectroscopy of electronic states in nonconducting surfaces Ezra Bussmann, Ning Zheng, Clayton C. Williams Typically, the scanning tunneling microscope (STM) cannot directly perform current-voltage spectroscopy on any electronic state with a lifetime greater than $\sim $10$^{-6}$ seconds---the state cannot empty fast enough to supply the necessary $\sim $0.1 pA ($\sim $10$^{6}$ e/s) imaging current. Recently, we reported a scanning probe technique that detects, by electrostatic force, single-electron tunneling events between a probe and states in a nonconducting surface. Here we determine the energy level of such a state by a single-electron tunneling spectroscopy, implemented by tuning the probe Fermi level with respect to the state by a dc voltage. A random telegraph signal (RTS), due to an electron tunneling back-and-forth between the probe and state, is observed when the Fermi level is near the state energy. We present spectroscopic data and extract the energy of a state in a thermal silicon dioxide film. The origin of the RTS is discussed. Additionally, we find evidence for energy relaxation and charge movement in these states. This new nanometer-scale approach provides the means to characterize electronic states in nonconducting surfaces, opening for exploration materials not accessible to the STM. \newline [1] E. Bussmann, D. J. Kim {\&} C.C. Williams, \textit{Appl. Phys. Lett.} \textbf{85}, 2538 (2004) [Preview Abstract] |
Monday, March 13, 2006 8:12AM - 8:24AM |
A9.00002: High Resolution Studies of the Electronic Properties of Graphite and Graphene Shuyun Zhou, Gey-Hong Gweon, Daniel Garcia, Elizabeth Rollings, Catalin Spataru, Steven Louie, Dung-Hai Lee, Alessandra Lanzara We report a high-resolution angle-resolved photoemission spectroscopy (ARPES) study on the electronic properties of bulk graphite and atomically-thin graphene layers. Data as a function of both in-plane and out-of-plane momenta, binding energy and photon energy will be presented. We have observed for the first time using ARPES some very interesting electronic properties near the Fermi energy E$_{F, }$which$_{ }$shows that interlayer coupling is an important ingredient in understanding the electronic properties of graphite. In addition, the unique electronic properties in atomically-thin graphene layers will be discussed and compared with those from the bulk graphite sample. [Preview Abstract] |
Monday, March 13, 2006 8:24AM - 8:36AM |
A9.00003: Positron Trapping and Annihilation at Reconstructed Ge(100)-(2x1) and Ge(111)-(2x8) Surfaces Arnab K. Pal, Nail G. Fazleev The results of experimental studies of Ge(100) and Ge(111) surfaces using high-resolution positron-annihilation-induced Auger electron spectroscopy are analyzed by performing 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 positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction of the topmost layers of clean Ge. These results are compared to the ones obtained for the reconstructed Si(100)-(2x1), Si(100)-p(2x2), and Si(111)-(7x7) surfaces. Comparison of theoretical positron annihilation probabilities computed for different reconstructed surfaces of Ge 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] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A9.00004: Probing electron correlation effects of Ni(111) with STM Kees Flipse, K. Braun, A. Grechnev, M. Katsnelson, K. Rieder The role of electron correlation effects in the electronic structure of Ni-metal has been attracting interest for a long time. Here we present an extensive scanning tunneling microscopy and spectroscopy investigation on Ni(111) at low temperature which shows a parabolic surface state with a surprisingly low effective mass compared to the noble metals like Au, Ag and Cu and a d-surface resonance showing an electron-electron interaction signature which can be understood by a many-body calculation of the electronic structure (DMFT). For the first time, a small energy shift and a significant broadening of the electron state due to electron-electron correlation effects are obtained in a STM experiment. This opens the possibility to study electron many-body effects of surface states in detail with a very high energy resolution on a clean and defect free part of the surface. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A9.00005: Low temperature scanning tunneling microscopy study on electronic standing waves and step motion on Au(111) surface Hui Wang, Jonghee Lee, Dan Sullivan, Michael Dreyer, Barry Barker We present a Low Temperature Scanning Tunneling Microscope investigation of step motion and electronic standing waves of Au(111) surface epitaxially grown on Mica. By performing Fourier Transform Scanning Tunneling Spectroscopy, we measured the parabolic surface state dispersion with an effective mass of 0.25m$_{e}$. This agrees well with the theory and the photo-emission data. We also report the step motion of Au(111) surface at 4K. The speed of the motion is around 5 to 20nm/hr. Potential causes for this motion include tip-sample interaction and surface contamination. These and other possible causes will be discussed. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A9.00006: On the inhomogeneous structure and charge transfer of C$_{60}$ monolayer on Ag(001) Xieqiu Zhang, Aidi Zhao, Xudong Xiao, Wei He, Jinlong Yang We have studied the morphological and electronic structure of C$_{60}$ ML/Ag(001) by STM/STS and first principles calculation. The bright-dim contrast of adsorbed C$_{60}$ molecules is identified as originating from geometric effects. Among the dim C$_{60}$ molecules, there consist of dim monomers (DM) and dim dimers (DD), which display distinguished STS. The dI/dV spectra give the energy locations of HOMO, LUMO and LUMO+1-derivated energy bands and the different STS spectra for B, DM, and DD C$_{60}$ molecules indicate that the charge transfer from the substrate is strongly inhomogeneous. The charge transfer has been estimated as $\sim $0, $\sim $1, $\sim $2 electrons/molecule for B, DM and DD from the shift of their corresponding LUMO+1 energy bands, respectively. The DFT calculations give the consistent spectra for B, DM and DD C$_{60}$ molecules with experimental STS data, but with smaller charge transfer. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A9.00007: Electronic Effects in the Length Distribution of Si(553)-Au Chains Jason Crain, Mark Stiles, Joseph Stroscio, Daniel Pierce The electronic structure of nanostructures directly impacts their energetics. For example, the confinement of electronic states determines preferred sizes for clusters and thin films. In the present study we investigate the effects of the electronic structure on the self-assembly of Si(553)-Au atomic chains, which are broken into finite-size segments by defects [1,2]. Chains are first fabricated by depositing gold on stepped silicon surfaces [3]. Scanning tunneling microscopy measures the distribution of chain lengths and the correlation between defects separating chains. The distribution of chain lengths reveals incommensurate oscillations that are linked to the electronic scattering vectors at the Fermi surface of the surface states. The pairwise correlation function between defects shows long-range correlations that extend beyond nearest-neighbor defects, indicating coupling between chains. [1] J. N. Crain, A. Kirakosian, K. N. Altmann, C. Bromberger, S. C. Erwin, J. L. McChesney, J. L. Lin, and F. J. Himpsel, Phys. Rev. Lett., 90, 176805 (2003). [2] J. N. Crain and D. T. Pierce, Science, 307, 703 (2005). [3] J. N. Crain, J. L. McChesney, F. Zheng, M. C. Gallagher, P. C. Snijders, M. Bissen, C. Gundelach, S. C. Erwin, and F. J. Himpsel, Phys. Rev. B, 69, 125401 (2004). [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A9.00008: An Electronic View of the Step-edge Schwoebel Barrier Problems Yina Mo, Zhenyu Zhang, Efthimios Kaxiras Using First-principle calculations, we studied the energetics of adatoms Co, Fe, Cu, and Zn on the stepped Cu(111) surfaces and that of adatoms Rh, Pd, and Ag on the stepped Pd(111) surfaces. We found that the behavior of the adatoms at the step edge of these substrates is governed by the electronic interactions instead of stain strain effects. The different energetics and kinetics of the different adatoms on the same substrates result from the difference in which these adatoms {\it see} the charge distributions from the identical substrates. We further clarified that atomic level studies of these cases are required because the bond counting rules are not univeral. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A9.00009: Electronic Structure of NiO (100) and CoO (100) Ultrathin Films on Fe$_{3}$O$_{4 }$(100) Hui-Qiong Wang, Victor Henrich The electronic structure of ultrathin epitaxial films of two insulating antiferromagnets, NiO and CoO, grown on the metallic ferrimagnet Fe$_{3}$O$_{4}$ is being investigated. NiO (100) and CoO (100) are grown monolayer by monolayer on Fe$_{3}$O$_{4}$ (100) using molecular beam expitaxy; the thin-film electronic properties are characterized by measuring ultraviolet photoelectron spectra (UPS) as a function of overlayer thickness. The evolution of the density-of-states in the O 2p/Fe 3d and O 2p/Ni 3d bands exhibits a shift in the position of the NiO valence band for ultrathin films relative to bulk-like thick films; that band shift is also observed for ultrathin CoO (100) films grown on Fe$_{3}$O$_{4}$ (100). Several possibilities for the origin of that shift will be discussed: one is modification of metal 3d - O 2p hybridization in the thin film; another is change of the on-site Coulomb interaction, U, due to polarization energy and substrate image potentials. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A9.00010: Modification of Surface States in Ultrathin Films via Hybridization with the Substrate -- a Study of Ag on Ge Shu-Jung Tang, Tom Miller, Tai-Chang Chiang The Shockley surface state of Ag(111) develops unusual band dispersion relations for Ag films of decreasing thicknesses on Ge(111), as observed by angle-resolved photoemission. Its parabolic dispersion in the thick-film limit shifts toward higher binding energies and splits into multiple bands with dispersions that reflect the valence band structure of Ge including the heavy-hole, light-hole, and split-off bands. The results are explained in terms of a hybridization interaction between the Ag surface state and the Ge substrate states. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A9.00011: Surface electronic structure of H/Li(110) -- an application of the semi-infinite method. Yonas Abraham, N.A.W. Holzwarth We report the application of our new semi-infinite method to study the Li (110) surface. This method calculates continuum and bound electronic states in the vicinity of a surface of a semi-infinite crystal. It is designed for solving the Kohn-Sham equations in a pseudopotential formulation, including both local and separable non-local contributions. It is based on the Numerov integration algorithm and uses singular value decomposition to control the exponentially growing contributions. For Li (110), we see that H changes the interference patterns of the continuum states. In addition, we are able to locate the energies of H-induced surface states relative to the bulk band edges. Comparison with supercell calculations of the same system, reveal interesting differences. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A9.00012: Possible superconductivity in hetero-polar interfaces of CuCl/Si superlattices: (001) and (111) S. H. Rhim, R. Saniz, A.J. Freeman, J.J. Yu To investigate possible interfacial superconductivity\footnote{ \%Mattes, \%Physica C {\bf 162},554 (1989); Mattes and Foiles, Physica 135B, 139 (1985)} in CuCl/Si superlattices, we carried out first-principles calculations using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. Two possible growth directions, (001) and (111), are compared through their band structures, density of states (DOS), charge densities and Fermi surfaces. While the (111) superlattice is always metallic, the (001) superlattice is metallic or insulating depending on the number of Si and CuCl layers. Both directions exhibit two dimensional (2D) character at the interfaces which is a result of charge transfer between CuCl and Si layers. For metallic superlattices, the 2D conduction bands at the interfaces, resemble the 2D Cu-O $dp\sigma$ bands of cuprate superconductors. To obtain $T_C$ based on conventional electron-phonon (e-p) interactions, we calculated the e-p coupling constant, $\lambda$, within the rigid muffin-tin approximation.\footnote{Gaspari and Gyoffry, Phys. Rev. Lett. {\bf 28}, 801, (1972)} The results indicate that while e-p coupling is present in both directions, it gives $T_C=0.41\sim1.69$ K, i.e. one order of magnitude lower than the previously reported high transition temperature$^{2}$- which, if confirmed, would indicate a possible role for excitonic effects. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A9.00013: Measurement of Valley Splitting in a Si/SiGe 2DEG Point Contact L.M. McGuire, K.A. Slinker, S. Goswami, J.O. Chu, M.A. Eriksson We measure the valley splitting as a function of magnetic field in a Si/SiGe two-dimensional electron gas (2DEG) point contact defined by metal top-gates. Using a pair of point contacts on a quantum dot, and a two-point measurement technique, we apply a small ac bias to the source-drain and measure the differential current as we pinch off the channel by applying a negative voltage to the top-gate. As the voltage on the top-gate is varied in zero magnetic field, we observe the conventional steps at conductance values of multiples of $\frac{4e^2}{h}$. By applying a perpendicular magnetic field, we lift both the spin and valley degeneracies, and we see corresponding steps in conductance at every $\frac{e^2}{h}$. By fitting the conductance as a function of magnetic field, we can extract both the subband spacing and the valley splitting energy. Temperature dependence and source-drain spectroscopy plots are shown. At high magnetic fields, step-like features appear at non-integer conductance values in addition to the integer steps. Research made possible by ARDA, the NSA, and NSF. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A9.00014: Quantum effects in the conductivity of high-mobility Si MOSFETs at ultra-low temperatures. Nikolai N. Klimov, Alexander Kuntsevich, Vladimir M. Pudalov, Harry Kojima, Michael E. Gershenson By thorough suppression of electromagnetic noise in our experimental set-up, we were able to cool the electrons in high-mobility Si MOSFETs down to 17mK. We have studied how the conductivity depends on the temperature and the in-plane magnetic field over the density range $n=(2-10)\cdot10^{11} cm^{-2}$, with the focus on the crossover from ballistic transport ($T\tau\gg 1$, where $\tau$ is the momentum relaxation time) to diffusive transport ($T\tau\ll 1$). For our samples, this crossover was observed $T \sim 0.3K$. The quasi-linear dependence $\sigma(T)$ observed in the ballistic regime [1] is in a quantitative agreement with the theory of interaction corrections to the conductivity [2]. At lower temperatures, the interaction corrections are strongly affected by the inter-valley scattering. We have determined the inter-valley scattering rate by analyzing the weak-localization corrections and the dephasing time in the studied Si inversion layers. We will discuss how the interaction corrections in the diffusive regime are modified by the inter-valley scattering. [1] V.M.Pudalov, M.E.Gershenson, H.Kojima, G.Brunthaler, A.Prinz, G.Bauer, Phys.Rev.Lett. 91,126403 (2003) [2] G.Zala, B.N.Narozhny, and I.L.Aleiner, Phys. Rev.B 64, 214204 (2001); 65, 020201 (2002). [Preview Abstract] |
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