Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H20: Electronic and Optical Properties of Insulators |
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Sponsoring Units: DMP Chair: Keivan Esfarjani, University of California, Santa Cruz Room: Colorado Convention Center 105 |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H20.00001: Quasiparticle Energies of Liquid Water Deyu Lu, Giulia Galli Understanding the electronic structure of liquid water is of great interest for a broad range of physical, chemical and biological processes occurring in solution. In particular, the ability to obtain an accurate description of water electronic states from first principle plays an important role in interpreting experimental observations. We present results of water electronic properties obtained using Many-Body Green's function approaches within the GW approximation. In particular we compare results obtained using the full dielectric matrix and various model dielectric functions. We analyzed the band structure and the absorption spectrum, with focus on the blue shift with respect to density functional theory results, which has been reported in a recent theoretical study [Garbuio {\it et al.}, PRL: 97, 137402, 2006]. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H20.00002: Temperature- and pressure-induced changes in optical properties of silica Andrea Trave, Babak Sadigh, Eric Schwegler, Jeff Bude Silica is among the most commonly studied materials in its crystalline and amorphous forms, yet it still presents several obscure aspects in its behavior under heating and densification. First-principles simulations of silica glass and quartz are conducted to explain the experimentally observed rapid increase in optical absorption and consequent defect formation when silica glass is heated. Prior to full gap closure, we observe the appearance of optically active localized states, which will be studied in terms of their origin and characterization. Additional analysis of the effect of localized structural defects and of the optical properties of the material under compression will also be undertaken and discussed. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H20.00003: Sum-frequency spectroscopic study of optical phonons in alpha-Quartz Wei-Tao Liu, Y. R. Shen Sum-frequency vibrational spectroscopy (SFVS) was used to probe optical phonons of alpha-quartz. As a second-order nonlinear optical process, SF generation is allowed only with phonon modes that are both infrared and Raman active. This, together with dependences on input/output polarization combination and crystal orientation, enabled us to examine more closely the symmetry properties of such phonons. In the experiment, a tunable infrared ($\sim $7-14$\mu $m) and a visible (532nm) input beam were overlapped in a crystalline alpha-Quartz, and the SF output in the reflected direction was detected. Resonant enhancement of the output when the infrared frequency scanned over phonon modes yielded the phonon spectra. Analysis of the spectra with the help of existing infrared and Raman spectra of alpha-quartz provides concrete information about the observed phonon modes. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H20.00004: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H20.00005: Optical properties of the 1D antiferromagnet KCuF$_3$ Joachim Deisenhofer, Paolo Ghigna, Franz Mayr, Alois Loidl, Dirk van der Marel We present measurements of the optical properties of KCuF3, a paradigm for orbital ordering [1] and one of the best realizations of a quasi-one dimensional spin chain [2]. We can identify the d-d excitations on the Cu sites and the charge- transfer gap of the system. The observed crystal-field level splitting and the gap value will be compared to recent results obtained by LDA+U calculations [3]. Moreover, we find anomalies in the optical properties already above the magnetic phase transition. These features appear concomitantly with a change in the orbital order parameter as reported by resonant x-ray scattering [4] and indicate a symmetry change already above the Neel temperature [5]. [1] K. I. Kugel and D. I. Khomskii, Sov. Phys. Usp. 25, 231 (1982). [2] B. Lake et al., Nature Materials 4, 329 (2005). [3] S.V. Streltsov et al., Phys.~Rev.~B {\bf 71}, 245114 (2005); I. Leonov, unpublished. [4] L. Paolasini et al., PRL 88, 106403 (2002). [5] N. Binggeli and M. Altarelli, PRB 70, 085117 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H20.00006: LDA+U Models of Polarons in LaBr3 and CsI John Jaffe, Sebastien Kerisit, Kevin Rosso We describe calculations of the formation and hopping energies of hole polarons (holes self-consistently localized in lattice distortions) in the wide-bandgap ionic materials LaBr3 and CsI. Both one-center (breathing mode) and two center (anion dimer, also known as Vk center) polarons were treated. The LDA+U method based on the VASP code was employed, since standard DFT methods often fail to represent localized electronic states in solids. We used a 72-atom supercell of the UCl3 structure for LaBr3, and a 54-atom CsCl-structure unit cell for CsI. We attempt to correlate differences in electronic transport between these two compounds with different energy nonproportionality behavior that they exhibit as Ce-activated scintillators in gamma-ray spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H20.00007: Electrostatic resonances and optical responses of cylindrical clusters Chun Wing Choy We develop a Green function formalism (GFF) for computing the electrostatic resonance in clusters of cylindrical particles. In the GFF, we take advantage of a surface integral equation to avoid matching the complicated boundary conditions on the surfaces of the particles. Numerical solutions of the eigenvalue equation yield a pole spectrum in the spectral representation. The pole spectrum can in turn be used to compute the optical response of these particles. For two cylindrical particles, the results are in excellent agreement with the exact results from the multiple image method and normal mode expansion method. The results of this work can be extended to investigate the enhanced nonlinear optical responses of metal-dielectric composites, as well as optical switching in plasmonic waveguides. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H20.00008: Crystal and nonlinear optical properties of triphenylguanidine: theory and experiments Cl\'{a}udia Cardoso, Pedro Silva, Fernando Nogueira, Jos\'{e} Ant\'{o}nio Paix\~{a}o Guanidine compounds have attracted much interest due to the donor and acceptor abilities of the nitrogen and their potencial nonlinear optical properties. Octupolar molecules are particularly interesting from the point of view of nonlinear optics. Their null dipole moment does not stand as a drawback for its crystallization and still allows the crystal to present large third order susceptibilities if some symmetry requirements are fulfilled. In the present work we focused on the triphenylguanidine (TPG) octupolar molecule and its crystalline forms. We present computational and experimental results both for the isolated molecule and TPG crystals. The structural properties as well as optical spectra and response properties will be presented from the point of view of the requirements to obtain octupolar molecules-based materials with enhanced nonlinear optical properties. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H20.00009: Enhanced Luminescence in the Layered Single Crystal Ce[Ag(CN)$_2$]$_3$ Christie Larochelle Single crystals of the form Ln[M(CN)$_2$]$_3$ (Ln=trivalent rare earth; M=Ag, Au, or both) have a layered structure consisting of alternating layers of M(CN)$_2^-$ ions and Ln$^{3+}$ ions. Recent work on this type of crystal has focused on energy transfer from the metal dicyanide donor to the rare earth acceptors, specifically Tb$^{3+}$, Eu$^{3+}$, and Sm$^{3+}$. Crystals of Ln[Ag(CN)$_2$]$_3$ are particularly interesting because they exhibit site-selective excitation. However, the luminescence intensity in these crystals is vanishingly weak at ambient temperatures. We present preliminary luminescence results from a new sample, Ce[Ag(CN)$_2$]$_3$. This crystal displays strong luminesence at all temperatures between 78 K and 295 K, in contrast with all other rare earth dicyanoargentates we have studied. We present steady-state excitation and emission results along with time-resolved measurements of both the cerium doped crystal and the single crystal La[Ag(CN)$_2$]$_3$ for comparison. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H20.00010: Polarizability of Optically Trapped Nanorods Douglas Bonessi, Keith Bonin, Thad Walker We optically trapped C60 polymer nanorods with diameters of 300-500 nm and lengths of 1-3 microns in water in a single beam trap. While in the trap, the nanorods were optically torqued by rotating the plane of polarization of the trapping light. The polarizability of the rod can be found by measuring the rod rotation rate as a function of the polarization rotation rate, and then finding a theoretical fit to this curve that uses a computation of the applied torque as a function of polarizability. We used a discrete dipole approximation (DDA) routine to calculate torques on these trapped C60 rods. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H20.00011: Dynamical structure factor of CaF$_{2}$: Striking coherent dynamical screening of ``atomic'' Ca-derived excitations O.D. Restrepo, M.C. Troparevsky, A.G. Eguiluz, B.C. Larson, J.Z. Tischler, P. Zschack, Y.Q. Cai, H. Ishii, P. Chow, E.L. Shirley, C.C. Kao We report \textit{ab initio} calculations of the dynamical structure factor of CaF$_{2, }$performed within time-dependent density functional theory, together with non-resonant inelastic x-ray scattering measurements. The ``effective'' dielectric function has also been determined. The excitations derived from the ``atomic'' Ca 3p--$>$ 3d process display a striking wave vector dependence. Such dipole-allowed excitation would be expected to lie at about 27 eV. \textit{However, for small q's the leading Ca 3p--$>$ 3d feature lies at about 35 eV.} We demonstrate that this feature corresponds to a collective mode, whose physics embodies a remarkable manifestation of crystal local-field effects induced by charge localization and their interplay with the dynamical screening at the ``natural'' 3p--$>$ 3d energy. For intermediate $q's$, the 27 eV excitation emerges and coexists with the collective mode ---thus highlighting the physics of the ``atomic'' 3p--$>$3d excitation in the condensed matter environment, which is controlled by dynamical coherent screening. For large $q's$ the Ca-derived spectrum consists of the ``single-particle'' Ca 3p--$>$3d excitation, together with the dipole-forbidden Ca 3s--$>$3d excitation. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H20.00012: Vibrational effects on SrTiO$_3$ by K edge X-ray absorption using first-principle methods Silvia Tinte, Eric L. Shirley Using the Bethe-Salpeter-equation methodology has recently become possible to calculate core and valence excited electronic states and spectra, which are usually computed in the ground-state atomic positions. However, vibrational effects can shift and broaden the spectrum through coupling atomic displacements to electron states and electronic excitations. In this work, we analize the phonon effects on the Ti 3d states in cubic SrTiO$_3$ by Ti K edge X-ray absorption fine structure using first-principle methods. LDA total energies and coupled electron-hole Bethe-Salpeter equation calculations are performed for different ionic configurations following relevant normal modes of SrTiO$_3$. As result, we obtain gradients of the excited-state energy and electron-phonon coupling coefficients. Our final goal is to include the Franck-Condon effect on the broadening of the calculated spectra. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H20.00013: Cu $K$-edge Resonant Inelastic X-Ray Scattering in Edge-Sharing Cuprates Thomas Devereaux, Francois Vernay, Brian Moritz, George Sawatzky We present calculations for resonant inelastic x-ray scattering (RIXS) in edge-shared copper oxide systems, such as CuGeO$_{3}$ and Li$_{2}$CuO$_{2}$, appropriate for hard x-ray scattering where the photoexcited electron lies above oxygen 2p and copper 3d orbital energies. We perform exact diagonalizations of the multi-band Hubbard model with and without the presence of a photoexcited 1s core hole, and determine the energies, orbital character and resonance profiles of excitations which can be probed via RIXS. We find excellent agreement with recent results on Li$_{2}$CuO$_{2}$ and CuGeO$_{3}$ in the 2-7 eV photon energy loss range. [Preview Abstract] |
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