Bulletin of the American Physical Society
9th Annual Meeting of the Northwest Section of the APS
Volume 52, Number 6
Thursday–Saturday, May 17–19, 2007; Pocatello, Idaho
Session B1: Condensed Matter Physics I |
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Chair: You Qiang, University of Idaho Room: PSUB South Fork Room |
Friday, May 18, 2007 2:00PM - 2:36PM |
B1.00001: Zinc oxide - a material for energy applications Invited Speaker: Zinc oxide (ZnO) is a wide-band-gap semiconductor that has attracted resurgent interest as an electronic material for a range of applications. The efficiency of the emission is higher than more conventional materials such as GaN, making ZnO a strong candidate for energy-efficient white lighting. Another major advantage of ZnO is the fact that, in contrast to GaN, large single crystals can be grown. ZnO has been used as a transparent conductor in solar cells, and is a preferred material in transparent transistors, ``invisible'' devices which could be very useful in products such as liquid-crystal displays. In addition to optoelectronic and electronic devices, ZnO has emerged as a potentially important material for spintronic applications. Despite its numerous advantages and potential applications, ZnO suffers from a relatively high level of donor defects. These compensating impurities prevent p- type doping, which is essential for practical applications. In our work, we have focused on hydrogen donors in bulk ZnO, combining IR spectroscopy with electrical measurements. As dimensions approach the nano-scale, the vastly increased surface-to-volume ratio leads to interesting phenomena. At moderate annealing temperatures (350 C), hydrogen permeates nanoparticles, resulting in a dramatic increase in electrical conductivity, free-carrier absorption, and infrared reflectivity. These results could be relevant to hydrogen sensing and storage applications. [Preview Abstract] |
Friday, May 18, 2007 2:36PM - 2:48PM |
B1.00002: Trace Impurities and Radiation Defects in Optical Materials Galina Malovichko, Valentin Grachev, Martin Meyer, Mark Munro, Vladimir Pankratov Trace impurities and radiation defects lead to inevitable performance degradation of devices based on optical materials. The results of the Electron Paramagnetic Resonance (EPR) study of defects in as grown and irradiated single crystals are reported. Among investigated optical materials are LiNbO$_{3}$, Li$_{2}$B$_{4}$O$_{7}$, KTiOPO$_{4}$ etc. Crystals from different vendors or grown by different ways have different concentrations of non-controlled impurities and, as a result, different physical properties, including radiation resistance. Intrinsic defects (vacancies and antisites), usually present in congruent non-stoichiometric crystals like lithium niobate and tantalate. Many EPR lines of non-controlled impurities were found in KTiOPO$_{4}$ crystals. We found that dominating types of defects formed under visible, UV and gamma photon irradiation are centers created by defects trapped electron or hole. The neutron and high energy electron irradiation creates stable Frenkel pairs - interstitial ions and vacancies. Computer simulation of observed spectra allowed us to determine spectroscopic characteristics and models for more than dozen trace impurities and radiation defects. Obtained data about atomic defects can be used for a selection of materials suitable for various applications. [Preview Abstract] |
Friday, May 18, 2007 2:48PM - 3:00PM |
B1.00003: Where are Nature's missing structures? Gus Hart It has long been claimed that predicting crystal structures of compounds using ``first-principles'' (direct solution of the Schoedinger equation using computers) will fundamentally change the way new materials discoveries are made. However, the ability to reliably predict new materials has been elusive. This ability is elusive for two reasons: the enormous search space of possible crystal structures and the relatively long times required for calculations of candidate materials. In this talk, l will discuss the problem from a different angle: what intermetallic structures are likely based just on simple combinatoric arguments? Some of these likely candidates are indeed well-known intermetallic structures but many other are conspicuously absent. What are the physical reasons that some of these structures exist in Nature and others don't? [Preview Abstract] |
Friday, May 18, 2007 3:00PM - 3:12PM |
B1.00004: Current-Voltage Characteristics of ZnO and MgZnO Nanoparticle Films Chris Berven, Sirisha Chava, Ramey Abdelrahaman, Abby Heieren, John Morrison, Jesse Huso, Leah Bergman We report on initial results on the measurement of the current-voltage (I-V) characteristics of films of ZnO and Mg$_{x}$Zn$_{1-x}$O (x = 0.15) nanoparticles. The nanoparticles were prepared using wet chemical techniques on insulating thermally grown SiO$_{x}$ (15 um thick) Si substrates. Contact to the nanoparticles was by laying down gold wires, about 2 mm apart, across the as-prepared nanoparticle films. On top of the gold wires was put a glass cover slip on the back of which was painted conducting silver paint. On top of the cover-slip was a portion of a glass slide which was compressed down onto the cover slip and wires to ensure good electrical contact to the nanoparticles. This arrangement enabled the application of a gate voltage to the nanoparticle device. Our initial results show that the I-V characteristics are non-linear and gating can modulate the I-V characteristics. The ZnO device shows no hysteresis whereas the MgZnO device shows hysteresis in the I-V characteristics only for negative source-drain bias. The measurements were performed in a environmental chamber, in the dark at 18 mTorr and 2 x 10$^{{\-}7}$ Torr, for the ZnO and MgZnO films, respectively. [Preview Abstract] |
Friday, May 18, 2007 3:12PM - 3:30PM |
B1.00005: Coffee Break
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Friday, May 18, 2007 3:30PM - 3:42PM |
B1.00006: From Stern-Gerlach to Rashba-Dresselhaus Propagators Bailey Hsu, Jean-Francois Van Huele Propagators are used to describe the evolution of quantum systems in space. Schwinger's method, which uses the commutation between the relevant operators to construct the propagators is generally used for spinless particles. We apply Schwinger's method to find propagators for quantum systems with spin in electromagnetic configurations. We first consider neutral particles with magnetic moments in magnetic fields as used in Stern-Gerlach experiments and we construct the propagators and their effect on wave packets. We discuss the extension to charged particles in electromagnetic fields. In spintronics applications, bulk and structure inversion asymmetries lead to spin-orbit coupling interactions. We discuss the dynamics in the special cases of the Rashba and the Dresselhaus effect. [Preview Abstract] |
Friday, May 18, 2007 3:42PM - 3:54PM |
B1.00007: Electrical spin measurements of diffused phosphorous donors in crystalline silicon Heather Seipel, Thomas Herring, Christoph Boehme With recent experimental demonstration of the electrical detection of electron spins of phosphorous donors as well as their hyperfine coupling to the $^{31}$P phosphorous nuclear spin [Stegner et al., Nature Physics, doi:10.1038/nphys465, (2006).], a potential mechanism for a $^{31}$P in crystalline silicon (c-Si) nuclear spin readout based on spin-dependent $^ {31}$P-P$_{b}$ recombination is available. To further investigate the properties of this mechanism, we present pulsed electrically detected magnetic resonance (pEDMR) measurements on diffusion doped silicon samples. For their preparation, c- Si (100) wafers are diffused with a profile whose concentration at the surface leads to degenerately doped c-Si before it then drops off into the semiconducting region. Deep trenches are made with a plasma enhanced reactive ion etch where the choice of the trench depth determines the dopant concentration of the sample without changing any other sample preparation parameters. A study of the qualitative and quantitative nature of the observed pEDMR signals is presented for different etch depths taken close to the metal-insulator transition. [Preview Abstract] |
Friday, May 18, 2007 3:54PM - 4:06PM |
B1.00008: Investigation of Exciton States in CdSe Quantum Dots via Hamiltonian Diagonalization Method Zachary Schultz, John Essick We analyze the electron-hole (``exciton'') states involved in visible-light absorption by a spherical CdSe quantum dot. Working within the effective mass approximation and assuming a dot size on the order of the bulk exciton Bohr radius $a_B $, we exploit a Hamiltonian diagonalization method, which accounts for the exciton's kinetic $T$, direct Coulomb $U_{eh} $, and surface polarization $U_p $ energies. Using a basis set composed of 54 composite infinite spherical-well wavefunctions, we obtain representations of exciton wavefunctions and their corresponding energies. For a dot radius $R=0.80a_B $, we find that the energies associated with $T$, $U_{eh} $, and $U_p $ are $+15.4E_R $, $-4.7E_R $, and $+1.4E_R $, where $E_R $ is the bulk CdSe exciton Rydberg energy. Our theoretical results are then used to predict the size-dependent visible-light absorption spectra of CdSe quantum dots. Comparisons of the theoretical spectra are made to absorption data we have taken on CdSe dots with known radii. We find that our theoretical model accurately describes the experimental data for dots with radii $R\ge 0.6a_B $. Finally, we explain why our model breaks down for dots with $R\le 0.6a_B $ and comment on the accuracy obtained with use of a more limited basis set. [Preview Abstract] |
Friday, May 18, 2007 4:06PM - 4:18PM |
B1.00009: Impedance studies of mixed ionic-electronic conjugated polymers Yongjun Wang, Fuding Lin, Mark Lonergan A polymer fabricated by sandwiching an ionically functionalized polyacetylene between two metal electrodes, is investigated with impedance analysis at temperatures ranging from 308K to 398K. Three processes-namely geometric capacitance charging, ion hopping and interfacial polarization-can be identified in the frequency region of 0.01 Hz to 1MHz. The temperature dependence of the conductivity and the hopping frequencies shows Arrhenius behavior with activation energies of 0.97 eV and 0.98 eV respectively. The similarity of these activation energies implies that the concentration of mobile charge carriers is independent of temperature and the conductivity is determined primarily by the charge carrier mobility. Ionic conductivity is found in the range 10$^{-12}$ to 10$^{-10}$ S/cm in the temperature span studied. These experiments lay the foundation for further investigations of charge transport at conjugated polymer interfaces with ionic functionality. [Preview Abstract] |
Friday, May 18, 2007 4:18PM - 4:30PM |
B1.00010: Ferromagnetism enhanced by double exchange interactions in doped ZnO DMS Nanoclusters You Qiang, Jiji Antony, Amit Sharma, Daniel Meyer, M. Faheem, Alan Mcconnaughey, Jamie Hass, Ryan Souza Diluted magnetic semiconductor (DMS) of doped ZnO is of great interest to current research due to its wide variety of applications in spintronics and sensors. We synthesized Ti, V, Co or Ni-doped ZnO nanoclusters using a third generation nanocluster source that utilize a combination of magnetron sputtering and gas-aggregation technique. High-resolution TEM images show that the nanoclusters are monodispersive with a nanocrystalline size $<$ 10 nm. XRD patterns are identical to the bulk ZnO wurtzite structure. XPS detected the dopant elements, which are uniformly distributed in doped nanoclusters. High-resolution XPS showed oxidation states of dopant Ti in +4, and Co in +2 with isovalance, while V in +4 and +5, and Ni in +2 and +3 with mixed valances. These analyses indicate that dopant elements do not exist as independent aggregates but are incorporated into the ZnO structure. All the doped ZnO nanoclusters are ferromagnetic above room temperature. Magnetic moments of Ni and V-doped ZnO (1.5$\mu_{B}$ or 3.5$\mu_{B}$ per dopant atom) are much larger than Ti or Co doped ZnO clusters (0.2$\mu_{B}$ or 0.6$\mu _{B}$ per dopant atom). Double exchange interactions due to the mixed valance states are the reason that Ni or V-doped ZnO clusters have much larger magnetic moments than the Ti or Co-doped clusters. Both magnetic and UV optical properties of doped ZnO nanoclusters are dopant concentration dependent. [Preview Abstract] |
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