Bulletin of the American Physical Society
2006 73rd Annual Meeting of the Southeastern Section of the APS
Thursday–Saturday, November 9–11, 2006; Williamsburg, Virginia
Session CB: Materials I |
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Chair: Mary Ellen Zvanut, University of Alabama at Birmingham Room: Williamsburg Hospitality House Jamestown |
Thursday, November 9, 2006 10:45AM - 10:57AM |
CB.00001: Neutron diffraction study of water intercalation in superconducting sodium cobaltate Cinzia Metallo, Takeshi Egami, Thomas Proffen, David Mandrus, Brian Sales Neutron powder diffraction has been used to investigate the role of heavy water in deuterated sodium cobaltate Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O. In spite of the fact that superconductivity appears exclusively when water is intercalated in the (non superconducting) Na-deficient Na $_{x}$CoO$_{2}$, a clear understanding of the role of water has not been achieved. Neutron diffraction data at two different temperatures (T=15K, 100K) were analyzed using the Pair Density Function (PDF) technique, which gives information about local ordering in real space. The measured and calculated PDFs of Na $_{0.7}$CoO$_{2}$, Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O and D$_{2}$O were compared. At both temperatures the D-D distance and the D-O-D angle in Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O are significantly different from those of ordinary water. Two wide coexisting distributions of possible D-O-D bond angles are observed. We speculate that the altered geometry of the intercalated water molecules is due to a modification of the dynamics of the hydrogen bond. The possible implications are discussed in terms of electron conduction and superconductivity. [Preview Abstract] |
Thursday, November 9, 2006 10:57AM - 11:09AM |
CB.00002: EXAFS investigation of the amorphous Ge$_{2}$Sb$_{2}$Te$_{5}$ optical memory material Joseph Washington, David Baker, Gerald Lucovsky, Michael Paesler, Craig Taylor Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied successfully to programmable memory devices, as well as DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching grants researchers today with an active arena of technological as well as fundamental study. Bond constraint theory and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Application of these theories to switching in a-chalcogenides, combined with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy yields the most detailed model to date of the a-Ge$_{2}$Sb$_{2}$Te$_{5}$ system. [Preview Abstract] |
Thursday, November 9, 2006 11:09AM - 11:21AM |
CB.00003: First Principles Study of FCC-HCP Interface Dynamics Under Uniaxial Tension Sungho Kim, Seong-Gon Kim, Mark F. Horstemeyer We studied the dynamics of FCC-HCP interface of Ni crystal at various tension using first-principles density functional theory(DFT) calculations. Both FCC and HCP structures are closed-packed and different stacking orders. We found that FCC-HCP interfaces can have two different stacking orders. We found that the interface can advance by alternating between two stacking orders under uniaxial tension. Intermediate phases and energy bariers for grain growth of two different phases will be presented. [Preview Abstract] |
Thursday, November 9, 2006 11:21AM - 11:33AM |
CB.00004: A First-principles Study of GaSb(001) Surface Reconstruction Seong-Gon Kim, Jeffery Houze, Sungho Kim, Steven C. Erwin We use total-energy and electronic structure calculations based on density functional theory to study the structure of GaSb(001) growth surfaces. We consider different reconstruction models proposed in the literature on the basis of experimental observations under typical GaSb growth conditions and present the $T=0$ surface stability diagram. We found that all $(n\times 5)$-like reconstructions proposed in various literature have too high surface formation energies to adequately model the structures observed experimentally under typical Sb-rich growth conditions. We found several new reconstruction models for GaSb(001) surface with correct symmetries that have lower surface formation energies than existing models and produce correct STM images. [Preview Abstract] |
Thursday, November 9, 2006 11:33AM - 11:45AM |
CB.00005: Optical Excitation Study of Defects in Strontium Titanate Substrates Shehnaz Jeddy, M.E. Zvanut BiFeO$_{3}$ thin films on SrTiO$_{3}$ (STO) substrates exhibit great potential for novel applications. Understanding substrate structure is critical in designing and controlling ferroelectric and magnetoresistive responses of the thin films. Hence, defect levels in STO are being studied using 9.4 GHz electron paramagnetic resonance (EPR) at room temperature before and during optical excitation using a 500 W Hg arc lamp. Initial EPR of STO reveal three defects: Cr$^{3+}$, cubic Fe$^{3+}$ and axial Fe$^{3+}$. Photo induced EPR intensity as a function of the photon energy is measured to determine defect levels and charge transfer effects with respect to the band gap. Preliminary investigation in the energy range 0.51eV to 5.06 eV reveal varying responses of defect centers. Data show Cr$^{3+}$ is unaffected upto 1.3 eV, increasing by a factor of 2 with a peak at 2.0 eV. Both cubic and axial Fe$^{3+ }$ show no response with our present system. The changes seen in Cr$^{3+}$ may be due to Cr$^{3+}$ being converted to Cr$^{2+}$ or Cr$^{4+}$. Further optical study using time dependant photo EPR will be done to enable deeper understanding of defect levels. [Preview Abstract] |
Thursday, November 9, 2006 11:45AM - 11:57AM |
CB.00006: Effects of heat treatments on defects in strontium titanate substrates for magnetoelectric multiferroics Brian Lassiter, Mary Ellen Zvanut SrTiO$_{3}$ (STO) shows potential as a substrate for deposition of thin film multiferroic devices; however, there is little research on the effects of heat treatments under ambient conditions and temperature ranges where deposition typically occurs. To this effect, defects in STO are analyzed using electron paramagnetic resonance (EPR) at room temperature after anneals in the temperature range 200-500\r{ } C in argon, oxygen, and vacuum. These conditions are similar to those used in the deposition of bismuth ferrite (BFO), which are thought to have enhanced ferroelectric properties. Three different defects, common in bulk STO, are observed in the as-received substrates: Fe$^{+3}$, Cr$^{+3}$, and a proposed Fe$^{+3}$V$_{O}$ complex. The data trends show Cr$^{3+}$ decreasing during oxygen anneals, Fe$^{3+}$ remaining negligibly affected for most cases, and Cr$^{3+}$ and Fe$^{3+}$V$_{O}$ increasing between 2 to 5 times during vacuum anneals. The increase in Fe$^{3+}$V$_{O}$ supports the conclusion that there is considerable in-diffusion of oxygen occurring during the vacuum anneals, but higher temperature may be required to replace the oxygen and fill the vacancy. [Preview Abstract] |
Thursday, November 9, 2006 11:57AM - 12:09PM |
CB.00007: Mid-infrared spectroscopy of Praseodymium doped Ternary Lead Halides Peter Amedzake, Ei Brown, Uwe Hommerich, Moronkeji Bandele, Sudhir Trivedi, John Zavada Solid-state lasers operating in the mid-infrared (MIR) wavelength (3-5$\mu $m) region are important for applications such as remote sensing of biochemical agents and military countermeasures. Solid-state laser hosts based on oxide and fluoride crystals are limited by non-radiative decay through multi-phonon relaxation. Ternary lead halides have recently become of interest as new hosts for mid-infrared gain media because of their low maximum phonon energies ranging from 140 to 205 cm$^{-1}$. The low-phonon energies allow for efficient rare earth emission at mid-infrared wavelengths. In this paper we report on the optical properties of several Pr$^{3+}$ doped ternary lead halides including KPb$_{2}$Cl$_{5}$, RbPb$_{2}$Cl$_{5, }$KPb$_{2}$Br$_{5}$, and RbPb$_{2}$Br$_{5}$. The crystals were grown using a self-seeded Brigdman technique. Broad MIR emission spectra centered on $\sim $ 4.6 $\mu $m with a band width of $\sim $0.9 $\mu $m were observed upon optical excitation at 1.907 $\mu $m. Room temperature lifetimes varied between 1.5 ms and 5.0 ms depending on the host composition. More details of MIR emission properties and energy level assignments will be presented at the conference. [Preview Abstract] |
Thursday, November 9, 2006 12:09PM - 12:21PM |
CB.00008: Modeling defect level occupation for recombination statistics Adam Topaz, Tim Gfroerer, Mark Wanlass Measurements of luminescence intensity as a function of temperature and laser illumination can provide considerable insight into the energy level distribution and recombination statistics of a semiconductor. Since the radiative recombination rate is proportional to the product of carrier densities in the valence and conduction bands, measurements of radiative efficiency vs. photoexcitation reveal how non-radiative, defect-related recombination mechanisms vary with band occupation. In this context, recent experimental results for a high-quality GaAs/GaInP double heterostructure contain two informative features. First, the radiative efficiency increases with temperature, indicating that shallow nonradiative recombination centers are being thermally depleted. Second, the defect-related recombination rate increases unusually slowly against the band carrier density product. Using a sophisticated model for non-equilibrium band and defect level occupation, we show that this latter result requires an asymmetric distribution of defect levels within the gap. Trap filling produces a rapid increase in that carrier’s band occupation, while the density of carriers in the band responsible for defect-related recombination remains nearly constant, such that the product increases without augmenting the recombination rate appreciably. [Preview Abstract] |
Thursday, November 9, 2006 12:21PM - 12:33PM |
CB.00009: High Resolution Acoustoelastic Measurements of Materials Mark McKenna, Samuel Guy, Joseph Heyman As materials become more complex, there is an increasing need for high resolution measurements to characterize strength and damage in the materials. Typically, the criterion for rejecting a part is based on the detection of a flaw of a specific size in a critical location. Interestingly, if a low stress field exists at the flaw site, the flaw may not grow over time. Similarly, in a part that shows no unacceptable indications, a high stress state may cause the flaw to quickly grow through the part leading to failure. In other cases, a controlled amount of stress (in a specific direction or type) is purposely added to the material to prevent flaw growth. Inspection time intervals are based knowing and controlling the stress environment to predict the flaw growth. Luna Innovations Incorporated has developed a high resolution ultrasonic instrument that can enhance the integrity of critical hardware by measuring changes in the stress state in a material. Knowledge of the stress state plus knowledge of crack sizes greatly improves structural engineers' capability of life prediction. System data will be shown for tests to stresses near holes in laboratory fabricated aircraft metal samples. Scans of the spatial distribution of stresses will be compared with finite element models of the structure. [Preview Abstract] |
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