Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U41: Dielectrics: Response Properties |
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Sponsoring Units: DCMP Chair: Serge M. Nakhmanson, Rutgers University Room: Baltimore Convention Center 344 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U41.00001: Enhanced performance in nanotemplated dielectric structures Yun Peng, K. Kempa, M.J. Naughton The dielectric properties of nanoscale metallic inclusions in insulating media are anticipated to be significantly enhanced (1,2). We have prepared such nanocomposites via electrochemical deposition of metal (gold) in polycarbonate template membranes. We have characterized the properties of these via frequency-dependent capacitance measurements, and compare our results with a theory of enhanced $\varepsilon $ ($\omega )$ at the nanoscale (2). \newline \newline - 1. J.Xu and C.P.Wong, Proceedings of the Ninth International Symposium on Advanced Packaging Materials: Process, Properties and Interfaces, Atlanta, GA, 24 March 2004 (IEEE, New York, 2004,) p.158 \newline - 2. T. Kempa, D. Carnahan etc., Dielectric media based on isolated metallic nanostructures, J.Appl.Phys, 98, 34310 (2005) [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U41.00002: Low temperature specific heat and Brillioun scattering in nano-oscillator arrays Douglas Photiadis, Joseph Bucaro, Xiao Liu We consider a large, free-standing array of coupled, planar oscillators each several hundred nanometers on a side fabricated from a single layer of dielectric. In particular, we predict the low temperature heat capacity and Brillouin scattered cross section based upon a numerical calculation of the density of states (DOS) for this nano-structured array. The DOS, which is interesting in its own right, is found to have an average value nearly independent of frequency and a number of gaps of varying depths. The predictions suggest that it should be possible to use low temperature measurements of Brillouin cross section and/or the specific heat to observe the quantum statistics obeyed by various rigid body modes of the array, some of which involve the center of mass motion of a large number of atoms. As such, these measurements would result in a considerable extension of the domain in which quantum mechanics has been tested. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U41.00003: Determining the quantum phase coherence time of a NEMS resonator Emily Pritchett Recently steps have been made toward characterizing macroscopic quantum behavior in nanoelectromechanical devices (NEMS), particularly resonators with large frequencies and high Q factors. While the quantum phase coherence time as well as energy relaxation time of NEMS resonators are believed to be long, this cannot be tested directly using standard techniques. Using formalism typically found in quantum computation, we propose a procedure for directly measuring both phase coherence and energy relaxation times of NEMs resonators by coupling them to anharmonic Josephson junction devices. We hope that using this proposed method experiments will verify current models of decoherence in harmonic oscillator systems. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U41.00004: Tunable Electronic Energy Transfer in Layered Inorganic Solids Codoped with Tb$^{3+}$ and Eu$^{3+}$ Christie Larochelle Ln[M(CN)$_2$]$_3$ systems (Ln=trivalent rare earth, M=Au,Ag) have a layered structure consisting of alternating layers of the M(CN)$_2^-$ ions and Ln$^{3+}$ ions. Our past studies of tunable energy transfer have focused on transfer from Au(CN) $_2^-$ or Ag(CN)$_2^-$ donor ions to a variety of rare earth ions, including Tb$^{3+}$ and Eu$^{3+}$. Most recently, we have characterized systems with mixed metal donors, such as La[Ag$_ {.5}$Au$_{.5}$(CN)$_2$]$_3$, La[Ag$_{.75}$Au$_{.25}$(CN)$_2$] $_3$, and La[Ag$_{.9}$Au$_{.1}$(CN)$_2$] $_3$. We have found that these systems exhibit ``tunability'' of emission energy due to the variation of the donor emission associated with varying the Ag/Au ratio. Tunability also occurs with changing temperature and excitation wavelength. Also, the steady-state luminescence spectra of these compounds reveals that they are strongly luminescent at room temperature, in contrast to the corresponding La[Ag(CN)$_2$]$_3$ and La[Au(CN) $_2$]$_3$ pure metal systems. Results will be presented from a new series of samples incorporating both Tb$^{3+}$ and Eu$^{3+}$ as acceptors. The tunability of the donor's emission wavelength allows for changes in the spectral overlap with each of the two donors. Preliminary results show variation in the rare earth (acceptor) emission with changing temperature and excitation wavelength, indicating the possibility of tuning the energy transfer off of one acceptor and onto the other. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U41.00005: EPR and ENDOR of two axial Fe$^{3+}$ centers in stoichiometric lithium tantalate Valentin Grachev, Galina Malovichko, Robert Petersen, Christoff Baeuman The determination of structures of centers in oxide crystals created by impurity ions, including those of transition metals and rare- earth elements, is one of the most important tasks in defect study. The iron ions play a key-role in the photovoltaic and photorefractive effects, holographic records and many other physical properties of lithium tantalate. The axial Fe$^{3+}$ center, Fe1 with the crystal field parameter b$_{2}^ {0}$ $\approx $ 0.313 1/cm is well studied in congruent lithium tantalate crystals. Using the EPR we have discovered and investigated a new axial Fe$^{3+}$ center, Fe2 in stoichiometric samples prepared by vapor transport equilibrium treatment. The crystal field parameter of the Fe2 center b$_{2}^{0} \quad \approx $ 0.205 1/cm is significantly smaller than for Fe1. The ENDOR measurements have shown that hyperfine interactions of the Fe$^{3+}$ electrons with the surrounding Li nuclei for Fe2 are stronger than for Fe1. Therefore, the conclusion was made that in the case of Fe2 center the iron ion substitutes for Ta and has Li nuclei in the nearest neighborhood, whereas in the case of Fe1 it substitutes for Li, has Ta nuclei as nearest neighbors and Li nuclei in the second shell only. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U41.00006: Near infrared emission properties of Nd doped Potassium Lead Halides Jessica Freeman, Ei Nyein, Uwe Hommerich, Sudhir Trivedi, John Zavada The incorporation of rare earth (RE) ions into host materials with low maximum phonon energies provides opportunities for improved RE infrared emission properties. In this work, we evaluated the IR emission from Nd doped potassium lead halides, namely Nd: KPb$_{2}$Cl$_{5}$ and Nd: KPb$_{2}$Br$_{5}$, for possible applications in IR lasers and optical communications. Both halides are nearly non-hygroscopic and have low maximum phonon energies, which reduces non-radiative decay rates through multi-phonon relaxations. Following optical excitation at 800nm, near IR emission bands were observed from the $^{4}$F$_{5/2}$ and $^{4}$F$_{3/2}$ excited states of Nd$^{3+}$. The $^{4}$F$_{5/2}$ level was strongly quenched through non-radiative processes in Nd: KPb$_{2}$Cl$_{5}$. On the contrary, for Nd: KPb$_{2}$Br$_{5}$ the $^{4}$F$_{5/2}$ was highly radiative with an emission efficiency of $\sim $50{\%}. More detailed results of the material synthesis, purification, steady-state and time-resolved emission spectroscopy will be presented at the conference. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U41.00007: Non-resonant Inelastic X-Ray Scattering and Energy-Resolved Wannier Function Investigation of Local Excitations in Transition Metal Monoxides NiO and CoO B.C. Larson, J.Z. Tischler, Wei Ku, Chii-Cheng Lee, Oscar Restrepo, A.G. Eguiluz, P. Zschack, K.D. Finkelstein Non-resonant inelastic x-ray scattering (NIXS) and energy- resolved Wannier function analysis have been used to probe the strongly correlated electronic structure of NiO and CoO. NIXS measurements of the dynamical structure factor s(q,w) as a function of momentum transfer q and frequency w have shown that dipole-forbidden, d-d excitations appear within the optical gap for large wavevectors (q $>$ 2/A), become the dominant structure in the loss spectra for q $>$ 3/A, and reach a maximum at q $\sim$ 7/A. In contrast to the loss-spectra observed in resonant-probe studies of NiO and CoO, non-resonant spectra show only two excitations that are highly anisotropic - strongest in the [111] direction and weakest (or missing) in the [001] direction. Energy-resolved Wannier function analyses of vertex matrix elements within LDA+U demonstrate that the anisotropy provides a sensitive measure of electronic symmetry-breaking in these atomic-like d-d excitations as a result of point-group symmetry selection rules. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U41.00008: Self-consistent linear density response within the LDA+U method: Application to transition-metal oxides(*) A. G. Eguiluz(1), O. D. Restrepo(1), J. Kunes, W. E. Pickett (3) We formulate a scheme to calculate self-consistently the dynamical linear density-response function based on correlated LDA+U theory. The orbital dependent V$_{U}$ term in the Kohn- Sham potential, leads to an additional self-consistent condition in the density fluctuations. The end result is a density response function which includes electron-hole interactions (that is, it goes beyond the random-phase approximation). We assess the performance of our scheme by calculating the electron-hole excitation spectrum of prototype transition metal oxides for arbitrary momentum transfers. (*) DOE-CMSN PCSCS collaboration. (1) Supported by NSF ITR-DMR 0219332. (2) Managed by UT-Battelle for the U.S. DOE under contract DE- AC05-00OR22725. (3) Supported by DOE Grant DE-FG03-01ER45876 [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U41.00009: Determining the Critical Dose Threshold of Electron-Induced Electron Yield for Minimally Charged Highly Insulating Materials Ryan Hoffmann, J.R. Dennison, Jonathan Abbott When incident energetic electrons interact with a material, they excite electrons within the material to escape energies. The electron emission is quantified as the ratio of emitted electrons to incident particle flux, termed electron yield. Measuring the electron yield of insulators is difficult due to dynamic surface charge accumulation which directly affects landing energies and the potential barrier that emitted electrons must overcome. Our recent measurements of highly insulating materials have demonstrated significant changes in total yield curves and yield decay curves for very small electron doses equivalent to a trapped charge density of $<$10$^{10}$ electrons /cm$^{3}$. The Chung-Everhart theory provides a basic model for the behavior of the electron emission spectra which we relate to yield decay curves as charge is allowed to accumulate. Yield measurements as a function of dose for polyimide (Kapton$^{TM})$ and microcrystalline SiO$_{2}$ will be presented. We use our data and model to address the question of whether there is a minimal dose threshold at which the accumulated charge no longer affects the yield. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U41.00010: Comparison of Methods for Determining Crossover Energies in Insulators Jonathan Abbott, Ryan Hoffmann, J.R. Dennison, Sarah Barton When a material is irradiated with energetic particles electrons can be emitted from the material. For electron-induced emission, the number of electrons that leave a particular material depends on the incident energy of the electrons, among other things. There are two critical energies where the ratio of emitted electrons to incident electrons crosses unity, called crossover energies. Measurements of the absolute total yield, secondary electron emission spectra, and sample and detector currents are used for a variety of methods to determine first and second crossover energies of both conductors and insulators. Precision is discussed for the following methods: i) Total Yield Curve, ii) Backscattered-to-Secondary Yield Ratio, iii) Mirror Potential, iv) Emission Spectral Shift, and v) Sample Null Current. Also, theoretical models for the Emission Spectral shift and Sample Null Current methods will be discussed. This work was funded by the NASA Space Environments and Effects Program, a Willard L. Eccles Undergraduate Research Fellowship, and a USU Undergraduate Research and Creative Opportunities award. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U41.00011: Piezoelectric Coupling in Non-piezoelectric Materials due to Nonlocal Size Effects at the Nanoscale: Fundamental Solutions, Embedded Inclusions and Piezoelectric Composites without Electromechanical Constituents Pradeep Sharma In a piezoelectric material an applied \textit{uniform} strain can induce an electric polarization (or vice-versa). Crystallographic considerations restrict this technologically important property to non-centrosymmetric systems. It has been shown both mathematically and physically, that a \textit{non-uniform strain} can potentially break the inversion symmetry and induce polarization in non-piezoelectric materials. The coupling between strain gradients and polarization; and strain and polarization gradients, is investigated in this work. Based on a field theoretic framework accounting for this phenomena, we (i) develop the fundamental solutions (Green's functions) for the governing equations (ii) solve the general embedded inclusion problem with explicit results for the spherical and cylindrical inclusion shape and, (iii) Illustrate using the simple example of a bilaminate how an apparently piezoelectric composite may be created without using constituent piezoelectric materials. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U41.00012: Ionic-Mode Contributions to the Refractive Index of Glasses Carrie E. Black, E. Shiles, D. Y. Smith The refractive index of materials transparent in the visible is commonly much smaller in the infrared than at shorter wavelengths because low-frequency ionic polarization lags the \textbf{E} field of higher-frequency light. This is often described by an IR Sellmeier term in empirical index fits. However, this separation of ionic and electronic contributions is not unique. A unique separation is given by Taylor expansion of the K-K relations in the region of transparency that yields a Laurent series in photon energy squared as the IR contribution. The coefficients are odd moments of the IR extinction coefficient. We studied this for vitreous silica and Corning ULE glass (92.5 {\%} SiO$_ {2}$ + 7.5{\%} TiO$_{2})$. While the oscillator strength of the IR modes is four orders of magnitude less than that of the electronic transitions, the IR contribution to the index is comparable to the electronic contribution in the IR. In our examples, IR terms are sufficiently negative to bring the total index well below unity (but greater than zero) between 7 to 9 $\mu $m. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U41.00013: Enhanced fluorescence in rare earth doped sol-gel glasses containing Al$^{3+}$ Gregory Armstrong, Ann Silversmith, Daniel Boye Sol-gel synthesis is a low temperature method for preparing rare earth (RE) doped glass. Aluminum is often used as a co-dopant because it increases fluorescence yield from RE's. There is a tendency for RE ions to form clusters in the sol-gel preparation, facilitating inter-ion interactions and fluorescence quenching from cross relaxation. It is generally believed that Al prevents clustering of RE ions, but recent work questions this long-established role of Al. We report on spectroscopic investigations of energy transfer in Tb- and Eu- doped glasses that probe the effect of Al co-doping. Pulsed laser excitation of $^{5}$D$_{3}$ fluorescence is used to measure Tb-Tb cross-relaxation rates. In materials containing Al, cross-relaxation occurs in concentrations much lower than 0.1{\%}Tb, indicating that RE clustering persists in glasses with Al. Line narrowing experiments in dilute Eu glasses confirm that ions remain clustered in co-doped samples. Our results point toward a model of aluminum rich regions in the glass that attract and confine RE ions. Reduced association with OH$^{-}$ in the confined regions causes RE fluorescence enhancement. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U41.00014: Site-specific modification of oxide nanoclusters: Towards atomic-scale surface structuring Kenneth M. Beck, Matthias Henyk, Chongmin Wang, Wayne P. Hess, Paolo E. Trevisanutto, Peter V. Sushko, Alexander L. Shluger Atomic emission from MgO and CaO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. Using selective 4.66 eV laser excitation of nanocrystalline thin films and nanocube metal oxide samples we have recorded a unique pattern of hyperthermal atomic desorption. Not only neutral O-atoms, but neutral Mg-atoms, with hyper-thermal kinetic energies in the range of 0.1--0.4 eV are readily observed. Our \textit{ab initio} calculations suggest that metal atom emission is induced predominantly by electron trapping at surface 3-coordinated metal sites followed by electronic excitation at these sites- an `electron plus an exciton' mechanism. The proposed elementary mechanism involves both sequential excitation and localization of excitons as well as electrons and holes at 3-coodinated surface sites. This mechanism differs from all previously suggested mechanisms for desorption induced by electronic transitions. This desorption process serves as an example of atomic scale modification of a nanostructured metal oxide using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U41.00015: Optical Spectroscopy of Low-k Dielectric Films Joanna Atkin, Daohua Song, Srikanth Iyer, Nils Hoivik, Robert R. Rosenberg, Thomas M. Shaw, Robert B. Laibowitz, Tony F. Heinz Low-k dielectric materials based on porous carbon-doped oxides are widely used in the microelectronics industry. Despite their importance, relatively little is known about their spectroscopic properties. In this paper we report results of two classes of optical spectroscopy measurements, absorption spectroscopy and photocurrent spectroscopy. Optical absorption spectroscopy has been performed on various thin-film low-k materials. These measurements show the presence of strong optical absorption in the ultraviolet and yield the effective band gap of the medium. Photocurrent spectroscopy has been performed on films of low-k material deposited on both Si and metallic substrates using a transparent counter-electrode. A well-defined spectral dependence of the photocurrent efficiency is observed. The data provide information on the band offsets of the low-k materials, parameters that play a crucial role in models of electrical conduction. [Preview Abstract] |
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