Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W10: Insulators and Dielectrics: Electronic Structure and Optical Properties |
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Sponsoring Units: DCMP Chair: Dillon Fong, Argonne National Laboratory Room: 304 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W10.00001: First-principles theory of coloration of WO$_3$ upon charge insertion Yu Xue, Peihong Zhang Tungsten trioxide is one of the most extensively studied electrochromic materials. Here we report density functional theory (DFT) investigations of the coloration mechanism of WO$_3$ upon charge insertion. Our results explains very well the systematic change in color of Na$_x$WO$_3$ from blue to golden-yellow with increasing sodium concentration. We find that proper accounts for the free-carriers contribution to the optical response are critical for a quantitative understanding of the coloration mechanism in this system. We thank Dr. Yong Zhang for his helpful discussion. We thank Dr. M. D. Jones for his assistance in coding. We acknowledge the computational support provided by the Center for Computational Research at the University at Buffalo, SUNY. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W10.00002: First principles studies of Ce and Eu-doped inorganic Andrew Canning, Anurag Chaudhry, Rostyslav Boutchko, Stephen Derenzo This work presents the results of first principles electronic structure calculations for europium and Ce doped inorganic compounds performed using the pseudopotential method based on the local spin density approximation (LSDA) and generalized gradient approximation+U (GGA+U) in density functional theory. The positions of the europium and cerium 4f and 5d states relative to the valence band maximum and conduction band minimum of the host material are determined. Qualitative predictions of the brightness of scintillation in the doped material is made based on the following criteria: (1) The size of the host material bandgap (2) The energy difference between the VBM (Valence Band Maximum) of the host material and the dopant 4f level (3) The energy difference between the occupied Eu or Ce 5d excited state and the host material CBM (Conduction Band Minimum) (4) The level of localization of the 5d excited state on the dopant atom. We have validated this theoretical approach on examples of known bright scintialltors and non-activated scintillators. We have performed calculations on new Eu doped compounds to determine if they are candidates for Eu2+ activated scintillators [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W10.00003: Electronic structure of the quasi-two-dimensional spin-gap system SrCu$_2$(BO$_3$)$_2$ Andres Saul, Guillaume Radtke, H. Dabkowska, B. Gaulin, G. Botton During the last decade, a lot of theoretical and experimental work has been devoted to the study of the magnetic properties of SrCu$_2$(BO$_3$)$_2$. This compound crystallizes in a tetragonal structure where layers of CuBO$_3$ alternate with planes of Sr atoms along the(001) direction. Due to this unusual structure where Cu$\sp{2+}$ atoms are arranged in the layers to form a network of orthogonal dimers, SrCu$_2$(BO$_3$)$_2$ appears as the first realization of a 2D Heisenberg model known as the Shastry-Sutherland model and exhibits a number of unique features such as a spin gap behavior, unusual magnetic excitations or a magnetization {\it plateaux}. In this work, the electronic structure of this system has been investigated using first-principles band structure calculations within the local-density approximation (LDA)+U method as implemented in the {\tt wien2k} code. The comparison of our calculations with available experimental data (exchange integrals estimated from magnetic susceptibility measurements, optical gap from reflectance measurements, and O-K edge recorded in EELS) shows that the (LDA)+U method with a single value of the parameter $U$ can provide an accurate description of both low-energy-scale (magnetic) and high-energy-scale (electronic) properties of this magnetic insulator. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W10.00004: PDMS-BaTiO$_{3}$ Composites with Mechanically Tunable Optical Properties. Nasser Mohamed, Moises Hinojosa, Virgilio Gonzalez Novel composites that show visible light transmittance, mechanically tunable refractive index and good mechanical properties based on PDMS and BaTiO$_{3}$ (BT) nanoparticles (NP), were prepared in 2 steps. First, NP were obtained via mechanical milling; the BT was used as-purchased. Average particle sizes of $\sim $100nm were selected. Second, the NP were embedded into PDMS by in-situ polymerization. PDMS from Dow Corning (Sylgard 184) was supplied as a kit containing 2 components: the Base and the Curing Agent. The BT content was varied up to 1.0wt{\%}. Finally, thick films were prepared by solvent casting and cured in a vacuum furnace, where the trapped air and solvent were extracted. Weight content of the NP was examined. XRD and Raman confirmed the desired tetragonal phase of BT NP. Average particle size was determined by SEM. EDS maps revealed a homogeneous dispersion of the NP. UV-Vis analysis showed transmittances of $\sim $70{\%}. The ellipsometry results revealed that the wt{\%} of BT significantly influences the optical response of the composite when it is stressed; however the response is not linear. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W10.00005: Measuring Quantum Efficiency of Organic Dyes Encapsulated in Dielectric NanoSpheres Timothy Russin, Erhan Altinoglu, James Adair, Peter Eklund We present results of a fluorescent quantum efficiency ($\Phi )$ study on the encapsulation of the near infrared dye indocyanine green (ICG) in calcium phosphate (CP) nanoparticles (dia$\sim $50 nm). The quantum efficiency ($\Phi $, described as the ratio of photons emitted to photons absorbed) provides a quantitative means of describing the fluorescence of an arbitrary molecule. However, standard quantum efficiency measurement techniques provide only $\Phi $ of the smallest fluorescing unit -- in the case of a nanoparticle suspension, the nanoparticle itself. This presents a problem in accurately describing the quantum efficiency of fluorophores embedded in a nanoparticle. We have developed a method to determine the quantum efficiency of the constituent fluorescent molecules embedded in such a nanoparticle, which provides a more meaningful comparison with the unencapsulated fluorophore. While applicable to generic systems, we present results obtained by our method for the ICG/CP nanoparticles in phosphate buffer solution, revealing a dramatic improvement in per-molecule $\Phi $ driven by encapsulation. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W10.00006: Dynamical mean-field analysis of the photo-induced insulator-metal transition in correlated electron systems --- pump-probe spectroscopy Naoto Tsuji, Takashi Oka, Hideo Aoki Recent pump-probe spectroscopy experiments have revealed that photo-excitation can trigger a `phase transition' from an insulator to a metal in various strongly correlated materials. The transition, occurring inherently out of equilibrium, is distinct from conventional phase transitions. In order to identify the nature of the states emerging during the irradiation of an intense laser, we employ the dynamical mean-field theory combined with the Floquet technique for ac fields, which enables us to take account of both the electron correlation and the nonlinear electric-field effect, two essential ingredients in the photo-induced phenomenon. We apply the method to the Falicov-Kimball model, one of the simplest models of correlated electrons, coupled to an ac pump light. The derived optical conductivity spectrum exhibits a Drude-like peak in the low-energy region indicative of metallization. We have also obtained the nonequilibrium distribution of electrons, which turns out to very much deviate from the Fermi distribution, so that the phenomenon is distinct from the heating effect picture. Interestingly, a dip structure is found to emerge in the charge transfer peak, which is shown to come from the vertex correction. We also discuss the dependence of the optical conductivity on the photon energy of the pump light. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W10.00007: Understanding of Nuclear Quadruple Interaction of $^{ 19}$F* and Binding Energies of Solid Fluorine at the First-Principles Level D.R. Mishra, M.M. Aryal, N.P. Adhikari, S.R. Badu, R.H. Pink, R.H. Scheicher, Lee Chow, T.P. Das We have studied the binding energy (BE) and nuclear quadrupole interaction (NQI) parameters for the $^{19}$F* excited nuclear state in solid fluorine as part of our investigation [1] of the properties of solid halogens using the first principles Hartree-Fock Cluster procedure combined with many-body perturbation theory (MBPT), implemented by the Gaussian 03 set of programs. Our results show that Van der Waals interaction obtained from intermolecular electron correlation has dominant effect on the BE but negligible effect on the NQI parameters. For the latter, our e$^{2}$qQ is 117.7MHz forQ($^{19}$F*), 0.072 *10$^{-28 }$m$^{2}$ [2] and $\eta $ is essentially zero.. The influence of vibrational effects on e$^{2}$qQ is being investigated using a first-principles procedure [3] to bridge the small remaining difference with experiment. [1] M.M. Aryal et al., Hyperfine Interact, 176, 51 (2007). [2] K.C.Mishra et al.,Phys. Rev.B25, 3389(1982). [3] N. Sahoo et al. Phys. Rev. Lett. 50, 913(1983) [4] H. Barfuss et al., Phys. Lett. 90A, 33(1982). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W10.00008: Group IIIA doping in \textit{$\alpha $}-Fe$_{2}$O$_{3}$ for PEC hydrogen production Muhammad N. Huda, Aron Walsh, Yanfa Yan, Su-Huai Wei, Yong-Sheng Hu, Alan Kleiman-Shwarsctein, Eric McFarland, Mowafak Al-Jassim Among iron oxides, \textit{$\alpha $}-Fe$_{2}$O$_{3}$ is the most abundant on earth. Because it has a band gap of approximately 2 eV, it is stable and inexpensive to process, \textit{$\alpha $}-Fe$_{2}$O$_{3}$ has been considered as a potential photoelectrocatalyst for solar driven photoelectrochemical (PEC) water splitting to make hydrogen. However, as \textit{$\alpha $}-Fe$_{2}$O$_{3}$ is a charge-transfer type insulator, the poor conduction properties have limited its efficiency as a PEC material. We will present our study on the doping of group IIIA elements in \textit{$\alpha $}-Fe$_{2}$O$_{3}$ to improve its performance. All the calculations were done with DFT+$U$. The main electronic features of \textit{$\alpha $}-Fe$_{2}$O$_{3}$ remained almost unchanged for group IIIA doping. While for Al-doping, the band gap remained almost the same, for Ga and In substitution the band gap marginally increased. However, increased conduction and PEC efficiency has been experimentally reported for Al-doped \textit{$\alpha $}-Fe$_{2}$O$_{3}$. It will be shown that the change in volume plays an important role in this behavior. A dramatic increase in photo-response cannot be expected for this type of doping in \textit{$\alpha $}-Fe$_{2}$O$_{3}$. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W10.00009: The band gap of ultra-thin amorphous and well-ordered Al$_{2}$O$_{3}$ films on CoAl(100) Volker Rose, Rene Franchy Understanding the insulating properties of thin oxide films is key to developing novel devices. In this work, the band gaps of ultra-thin amorphous and well-ordered alumina films on CoAl(100) were investigated by means of scanning tunneling spectroscopy (STS). The ordered intermetallic alloy CoAl(100) exhibits a magnetic surface, although the bulk is nonmagnetic. Such a material is extremely attractive for innovative technical applications. Utilizing selective oxidation, by which the oxidation of CoAl leads to surface segregation of the element with higher oxygen affinity, thin high-quality Al$_{2}$O$_{3}$ films are formed. Oxidation at 300 K leads to the growth of amorphous oxide, while well-ordered films result at elevated temperatures. In both cases, the self-limiting thickness of the oxide film amounts to around 1 nm. The analysis yields band gaps of 2.8 and 3.6 eV for amorphous and well-ordered Al$_{2}$O$_{3}$, respectively. The with respect to the bulk oxide reduced band gap can be explained by the appearance of defect induced states localized in the band gap. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W10.00010: Spectroscopic analysis of ALD-coated 3D structures and origin of the Berreman effect Giovanna Scarel, Jeong-Seok Na, Kevin Hyde, Gregory Parsons The Berreman effect shed light on various phenomena in 2D systems. However, coatings of 3D systems in soft-lithography and photonic devices, or 3D fibers suggest that the Berreman effect in 3D structures could be different. Experimental and computational infrared spectroscopy studies of 3D structures conformally coated with Al$_{2}$O$_{3}$ and ZnO layers using atomic layer deposition support this conclusion. In 2D systems, defining $\theta _{0}$ the macroscopic incidence angle of the IR beam on a sample, the LO mode absorbance increases as [Sin($\theta _{0})$]$^{4}$ when $\theta _{0}$ becomes grazing. On the other hand, in 3D systems a linear combination of [Sin($\theta _{0})$]$^{4}$ with appropriate coefficients must be considered. Accounting for Snell's law in the simulation model is essential to explain these results and the origin of the Berreman effect. We conclude that sample geometry determines infrared absorbance of LO modes versus $\theta $ and vice-versa Our results promise a new tool to investigate topography of insulating ionic oxide layers. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W10.00011: Dielectric properties of solids in the regular and split charge equilibration formalisms Razvan Nistor, Martin M{\" u}ser We investigate the generic dielectric properties of solids in which atomic charges are assigned within the split-charge equilibration (SQE) method, which contains the regular charge equilibration method as a limiting case. It is shown that the latter always mimics ideal conductors, while any positive bond hardness, which is introduced in the SQE method, turns the solid into a dielectric. Crystals with simple cubic and rocksalt structure are considered explicitly. For these symmetries we map the split-charge formalism onto a continuum model, which can be solved analytically, e.g., we provide simple analytical expressions for how the dielectric constant and penetration depth depend on atomic hardness, bond hardness, and lattice constant. This mapping may prove useful when having to solve the dielectric response of a heterogeneous system to external electrical fields not only on the atomic but also on a coarse-grained scale. Successful comparison of numerical data to analytical solutions is made, including those that contain discretization corrections to the continuum solution. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W10.00012: Charge and Bonding States of Ag Atoms in Superionic Conductor ${\alpha}$-AgI Masato Ito, Kazuo Tsumuraya The fast migration mechanism of the cations in the superionic conductors has been little known up to now. In the case of ${\alpha}$-AgI, the charge states of the Ag atoms and the bonding states between Ag and I atoms during the migration remain to be explained. No explanation has also been given for the origin of both the positions and the asymmetric first peaks of the Ag-I and Ag-Ag pair distribution functions. We investigate the electronic states of AgI using the first principles electronic structure calculations. We use the Bader analysis to evaluate the charges that belong to each atom and obtain the ionicity of the atoms. The stability of the cation pairs in the conductor will be discussed using their binding energies. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W10.00013: Multiplets and Crystal Fields: Systematics for X-Ray Spectroscopies Francois Vernay, Bernard Delley Recently Soft X-ray spectroscopies such as XAS and RIXS, became tools of choice to investigate transition metal oxides. The current resolving power is such that it is nowadays possible to investigate multiparticle excitations like, for instance, bi-magnon dispersion throughout the entire Brillouin zone. Yet, these spectroscopies are strongly linked to local physics: the absorption of a photon and creation of a localized core-hole opens up a shell and therefore a multiplet structure becomes apparent in the spectra. From here we see that it becomes crucial, while interpreting the experimental data, to have a systematic, user-friendly and transparent way of computing the multiplet spectra in order to disentangle in the experiment the information arising from single-particle excitations from the information relevant to collective excitations. We present our approach for arbitrary core-valence multiplets arising from a single configuration. The method covers the full range LS-intermediate-jj and allows to introduce splitting by an arbitrary crystal field easily. [Preview Abstract] |
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