Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z35: Insulators and Dielectrics II |
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Sponsoring Units: DCMP Chair: Oleksandr Kutana, Naval Research Laboratory Room: C140 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z35.00001: Opacification of dielectrics oxides investigated by infrared emittance spectroscopy Myriam Eckes, Domingos de Sousa Meneses, Mohammed Malki, Patrick Echegut With increasing temperature, some compounds that are transparent in the near infrared range at room temperature become progressively opaque towards the liquid phase. Such a behavior deeply impacts their thermal radiative properties which knowledge is of main importance in crystal growth processes for example. To understand this phenomena, infrared emittance spectra were acquired from room temperature up to the liquid state on crystalline LaAlO$_{3}$ and LiAlO$_{2}$. The samples are heated with a CO$_{2}$ laser that allows a direct measurement of emittance up to their melting point and in a wide spectral range, i.e. between 50 and 10000 cm$^{-1}$. In the transparency region, we observed an increase of emittance up to 1000 K for LaAlO$_{3}$ (more than 1000 K before the solid to liquid transition) and no significant increase of emittance for LiAlO$_{2}$ before the solid to liquid phase transition. The experimental data have been fitted with a dielectric function model including a Debye relaxation term. The absorption mechanism at the origin of the opacification is thermally activated and has a relaxation time compatible with a motion due to electrons. Electrical conductivity measurements of these compounds were also made up to 800K. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z35.00002: Non-equilibrium ballistic phonon transport in microstructures Jared Hertzberg, Obafemi Otelaja, Richard Robinson We demonstrate a method to locally excite and detect phonon modes in silicon microstructures. Decay of quasiparticles injected into an adjacent superconducting film excites phonons in a non-thermal spectral distribution [1]. Phonons of frequency of order 100 GHz are detected by the excitations they cause in a second superconducting film, after ballistically traversing microstructures of 10 to 50 micron dimension. Measurements are made at temperatures of 0.3 to 1.2 K. Such a device advances the goal of building a nanoscale phonon spectrometer to study acoustic confinement and surface scattering effects. This work is supported by KAUST (KUS-C1-018-02), NSF (DMR 0520404), and DOE (DE-SC0001086). \\[4pt] [1] W. Eisenmenger, A. H. Dayem, Phys. Rev. Lett. 18, 125 (1967). [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z35.00003: Dielectric measurements above 100 GHz using a high-Q open hemispherical resonator Rezwanur Rahman, John Scales High-Q cavities can be used to study materials (or perturbations to materials) whose effects are too small to be seen by other methods. We have developed a millimeter wave cavity operating above 100 GHz in order to measure the dielectric properties of thin films and ultra low loss materials. The cavity is a open hemispherical resonator. Millimeter waves are introduced and measured via 2 closely spaced sub-wavelength holes in the center of a 15cm spherical, copper mirror. Cavity perturbation techniques are applied to extract the complex permittivity of a sample. This is a paraxial system and axissymmetric modes are of primary interest but nonaxisymmetric modes area also generated and need to be dealt with. Applications to thin films and other materials will be shown. \noindent This was was partially supported by US Department of Energy under grant DE-FG02-09ER16018 [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z35.00004: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z35.00005: Comparison between Resonant Inelastic X-Ray Scattering and the Dynamical Structure Factor Chunjing Jia, Cheng-Chien Chen, Brian Moritz, Adam Sorini, Thomas Devereaux Momentum dependent resonant inelastic X-ray scattering (RIXS) is an effective probe of many-body excitations. Theoretical and experimental work has shown that under certain circumstances RIXS can be viewed as an approximate probe of the dynamical structure factor $S(q,\omega)$. We perform cluster diagonalization combined with the bi-conjugate gradient stabilized method to model the RIXS spectra and $S(q,\omega)$ for the single-band and multi-orbital Hubbard models. While these two cross sections share some similar features, there are significant quantitative differences, which highlight the qualitative distinction between these two probes. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z35.00006: Observations of ferroelastic switching by Raman spectroscopy in 18-percent ceria-stabilized zirconia Amy Bolon, Juan Munoz Saldana, Molly Gentleman Ferroelastic switching has been shown to be responsible for significant increases in the toughness of tetragonal zirconia ceramics. Observations of switching and measurements of coercive stress have generally been limited to TEM studies on large single crystals. In this study we show that it is possible to observe ferroelastic switching in 18 mole-percent ceria stabilized zirconia using polarized confocal Raman spectroscopy. Observations were made on bulk polycrystalline samples indented with a standard Vicker's indent and exhibited reorientation of crystal domains along the crack as well as near the crack tip. Coercive stress measurements were made by loading the samples uniaxially while making measurements of domain orientation. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z35.00007: Impact of Bond Coordination and Percolation on Mechanical Properties of a-SiC:H Thin Films Sean King, Jeff Bielefeld, Brian Daly Plasma Enhanced Chemically Vapor Deposited a-SiC:H thin films are compelling materials for both semiconductor nano-electronic and MEMS/NEMS technologies due to the extreme chemical inertness of this material and the ability to tune a variety of material properties across an extreme range of values. As one example of the latter, we demonstrate that using PECVD the Young's modulus of a-SiC:H thin films can be varied from $<$ 10 GPa to $>$ 200 GPa and the Hardness can be varied over an equally impressive range of $<$ 0.5 to $>$ 30 GPa. Utilizing Fourier Infrared-Transform Spectroscopy, we show that this remarkable range in materials properties is achieved primarily via the incorporation of terminal hydrogen groups which lowers the overall connectivity of the Si-C network bonding. We find that once the average network coordination number for Si and C falls below 2.6, the Si-C network becomes under constrained and there is a loss of rigidity percolating through the system. There results are compared and found to be in agreement with constraint theory for amorphous materials. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z35.00008: First Principles Investigation of Structure and Electronic Properties of $a-$ Si$_{3}$N$_{4}$ Ravi Pramod Vedula, Nathan L. Anderson, Alejandro Strachan We use a combination of molecular dynamics with empirical inter-atomic potentials and density functional theory (DFT) calculations to generate an ensemble of statistically independent, well relaxed $a$-Si$_{3}$N$_{4}$ structures. Variations in the annealing conditions used to generate the structures lead to zero-stress structures spanning a wide range of densities (2.6g/cm$^{3}$ to 3.1g/cm$^{3})$ but exhibiting very similar cohesive energies. The bulk modulus was found to be varying between 110-180 GPa depending on the density. The predicted density variation agrees well with the range in experimentally observed density, resulting from different fabrication conditions. The radial distribution functions and angle distributions for different densities are in good agreement with diffraction experiments; further validating our models. The slow annealing procedure used to generate the structures leads to well equilibrated structures with relatively small density of coordination defects and several defect free structures. We also compute the formation energy and charge transition levels for the defects found. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z35.00009: Thermal transport and surface sensitivity in suspended amorphous silicon nitride thin films R. Sultan, A.D. Avery, D. Bassett, B.L. Zink Thermal transport in disordered materials continues to provide surprising new results, which often have direct consequences for applications ranging from quantum computation to cutting-edge cosmology. For example, some of the most sensitive detectors of radiation currently in use are thermal detectors that use highly sensitive micromachined thermometers to register the temperature rise caused by absorption of incident light or particles. To achieve this sensitivity, the thermometer is commonly thermally isolated using free-standing amorphous silicon-nitride membranes. As a result, the heat flow through this material is often a critical design parameter. In this talk we present recent measurements of thermal conductivity of a large number of suspended silicon-nitride structures. The results show not only deviation from previously reported measurements, but also very clear but somewhat puzzling dependence on the surface preparation of the structures. Such surface-sensitivity is expected at very low temperatures, but is seen in our experiments even near $300$ Kelvin, where one normally expects heat flow to be dominated by carriers with very short mean-free-paths. We discuss possible interpretation of our results and compare to other recent surprises in the thermal properties of disordered materials. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z35.00010: Accessing short and intermediate range orders of silicate glasses by infrared spectroscopy Domingos de Sousa Meneses, Cristiane N. Santos, Myriam Eckes, Yann Vaills, Patrick Echegut The characterization of short and intermediate range orders in glasses is a very active field since this knowledge is of main importance for understanding how order impacts their properties. The chemical simplicity of binary silicate glasses makes them model systems that are suitable to show how their dielectric functions include such kind of information. We show that it is possible to extract from infrared reflectivity measurements no solely quantitative information on short range order, i.e. populations of Q$^{n}$ tetrahedral units (n : number of bridging oxygens) but also intermediate range information like the presence and evolution of 3D network silicate clusters and silicate sheet clusters. Examples will be given for alkaline silicates glasses and discussed in the light of predictions obtained from structural glass models and literature results. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z35.00011: In situ high-temperature infrared emissivity spectroscopy of silicate glasses and glass-ceramics Cristiane N. Santos, Domingos de Sousa Meneses, Valerie Montouillout, Patrick Echegut Glasses and glass-ceramics are materials of widespread application in industry, building, photonics, microelectronics and medicine. Glass-ceramics are obtained by controlled glass crystallization, and many efforts have been done in the last years to better understand the structural changes occurring in this process. Here we show that in situ infrared emissivity spectroscopy is also a suitable technique for this purpose and a wide spectral and temperature range could be accessed (25-16000 cm$^{-1}$ and 400-1700 K, respectively). We use a home-made instrument composed of two spectrometers, and a CO$_{2}$ laser for locally heat the glass samples up to the melt. A dielectric function model was applied to fit the experimental data and compute the materials optical properties. We show that using new decomposition procedure quantitative information on the distribution of the $Q^{n}$ tetrahedral units (n being the number of bridging oxygen) can be obtained. The results at room temperature are in good agreement with recent molecular dynamics simulations. The major changes occur during quartz crystallization, with a remarkable increase of $Q^{4}$ units. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z35.00012: CrN electronic structure and vibrational modes: an optical analysis Xunyuan Zhang, Daniel Gall The electronic structure of the paramagnetic insulating phase of CrN is investigated using optical spectra from epitaxial CrN(001) layers. The imaginary part of the dielectric function indicates direct interband transitions at \textit{h$\omega $} = 0.64, 1.5 and 2.9 eV, and suggests a depletion in the density of states at the Fermi level. This is attributed to local magnetic moments that cause splitting of the $t_{2g}$ bands and the formation of an indirect band gap of 0.19$\pm $0.46 eV, as estimated by comparing the optical transition energies with reported direct gap energies from calculations with different magnetic ordering and Coulomb interaction terms. The dielectric function shows a strong resonance at \textit{h$\omega $}$_{0}$ = 48.7$\pm $0.2 meV, and values of dielectric constants \textit{$\varepsilon $}$_{dc}$ = 53$\pm $5 and \textit{$\varepsilon $}$_{\infty }$ = 22$\pm $2, respectively, providing values for transverse and longitudinal optical phonon frequencies at the zone center of 11.7 and 18.2 THz, respectively. The vibrational frequencies are confirmed by Raman spectroscopy peaks at 800, 1170, and 1330 cm$^{-1}$ which are attributed to 2TO(X), 2LO(X), and 2LO(L) modes.\\[4pt] X.Y. Zhang and D. Gall, \textit{Phys. Rev. B} \textbf{82}, 045116 (2010). [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z35.00013: Surface states control and thermoelectric properties of CrN films Antia S. Botana, Alberto Pi\~neiro, Victor Pardo, Daniel Baldomir The manipulation of the electronic structure of a material by quantum confinement has attracted much attention recently, e.g. the appearance of conducting surface states or the novel phenomena at the interface between oxides being the most notorious. CrN is a degenerate semiconductor with large thermoelectric power in the bulk[1]. We have performed electronic structure calculations in thin CrN films and studied the evolution of the electronic structure and conduction properties (calculated using Boltzmann formalism) with thickness, focussing on the evolution of the Seebeck coefficient and electrical conductivity. We have utilized a density functional theory (LDA+U) formalism for our calculations with the U value (4 eV) that reproduces the experimental gap for the material as a bulk. When nanostructured, (almost conducting) surface states arise due to the dangling bonds at the surface that reduce the symmetry of the octahedral crystal field around Cr. When the film is relaxed, a gap opens and the material remains semiconducting. In addition, we have analyzed the role of these surface states in a possible improvement of the CrN thermoelectric properties by studying the figure of merit dependence with thickness. \newline [1] C. X. Quintela et al., Appl. Phys. Lett. 94, 152103 (2009). [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z35.00014: Temperature Dependence of Internal Deformation Field in Zeolites Wonsuk Cha, Sanghoon Song, Nak Cheon Jeong, Tung Pham, Ross Harder, Gang Xiong, Kyung Byung Yoon, Ian K. Robinson, Hyunjung Kim We studied temperature dependent internal deformation field distributions in zeolite microcrystals using coherent x-ray diffraction. We measured the coherent x-ray diffraction patterns around (200) and (020) Bragg peaks of the crystals. The three-dimensional real space images were obtained by phasing and inverting the oversampled diffraction patterns using the phase retrieval algorithm combined with error reduction and hybrid input-output method. The internal deformation fields show unusual temperature dependent behaviors which might be originated from the synthesis and calcination process. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z35.00015: Tension in the Initial Growth Stages of Sputter Deposited WSi$_{2}$ on Si in Multilayers Albert Macrander, Kimberley MacArthur, Bing Shi, Ray Conley We concentrate on the initial stages of growth of sputtered amorphous multilayers with equal WSi$_{2}$ and Si layer thicknesses of 5.5 nm, and we report observations of tension in the first 1.1 nm of WSi$_{2}$ grown on Si layers. Measurements of wafer curvature were made \textit{in-situ} in the sputtering chamber. Stresses in the conjugate interface of Si on WSi$_{2}$ were observed to be significantly smaller. A clear asymmetry in the stress of these conjugate interfaces rules out an explanation based solely on lattice misfit. We find a value of 1.3 x 10$^{10}$ dynes/cm$^{2}$ for the biaxial film stress at the WSi$_{2}$ on Si interface grown at 2.3 mTorr of Ar pressure, a value comparable to values calculated for hybridization of Si(111) facets by various adatoms [1]. Our observations thereby support a model for chemically induced changes in packing density during sputtering of the interfaces. As an example of a particular application, WSi$_{2}$/Si multilayers consisting of many hundreds periods have been used to make lenses for nanofocusing of hard x-rays [2-3].\\[0pt] [1] D. Vanderbilt, PRL 59, 1456 (1987). [2] H.C. Kang et al. PRL 96, 127401 (2006); APL 92, 221114 (2008). [4] L. Zhou et al. PRB 82, 075408 (2010). [Preview Abstract] |
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