Bulletin of the American Physical Society
2005 7th Annual Meeting of the Northwest Section
Friday–Saturday, May 13–14, 2005; Victoria, BC, Canada
Session B1: Condensed Matter I |
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Chair: Byoung-Chul Choi, University of Victoria Room: MacLaurin D110 |
Friday, May 13, 2005 2:00PM - 2:36PM |
B1.00001: Electron-Nuclear Spin Coupling in Semiconductor Quantum Wells Invited Speaker: The ability to manipulate and detect nuclear spin orientation is the basis for the science of nuclear magnetic resonance (NMR). An exciting application of NMR is three dimensional magnetic resonance imaging (MRI) microscopy, which recently achieved ~$4\mu m\times 3\mu m\times 3\mu m$ resolution with a nuclear spin sensitivity of $~ 3\times10^{12}$ protons. In that application and in most MRI applications nuclear spin is used as a non-invasive probe; the net nuclear polarization does not disturb the characteristics of the sample. Quite the reverse can be true in the spintronic systems studied here (where electron spin dynamics are monitered and manipulated). The electron-nuclear hyperfine coupling means, for example, that nuclear polarization can dominate over external magnetic fields in determining the spin precession frequency of electrons in a GaAs quantum well. This strong coupling suggests that we can use spintronic systems as sensitive probes of nuclear spin polarization. Furthermore by controlling electron spin and electron-nuclear spin coupling we can manipulate local nuclear fields. Working in GaAs/AlGaAs coupled quantum wells (CQWs) and parabolic quantum wells (PQWs), we present several approaches for controlling the electron-nuclear spin coupling. By controlling the position of electron spins in a PQW and by controlling the electron nuclear spin coupling between these electron spins and lattice nuclei we produce thin ($<$10 nm wide) distributions of polarized nuclei. Applying time-varying RF fields across the electrical gates of our samples, we induce local resonant nuclear spin transitions of selected isotopes. Gaining nuclear spin control in these systems may allow us to utilize long nuclear spin coherence times in spintronic application. Also by controlling nuclear spin distributions on nanometer length scales, we can dynamically modify the electron spin environment. [Preview Abstract] |
Friday, May 13, 2005 2:36PM - 2:48PM |
B1.00002: Impact of template morphology on the emission properties of InGaN/GaN multiple quantum wells prepared by molecular beam epitaxy S. Haffouz, H. Tang, T. Riemann, J. Christen, J. Bardwell, J. Webb We report on the impact of template morphology on the emission properties of InGaN/GaN multiple quantum wells (MQWs) deposited by plasma-assisted molecular beam epitaxy (MBE). High-resolution X-ray diffraction of MQWs reveals that the indium incorporation rate was drastically increased using faceted surface template as compared to smooth template. Scanning cathodoluminescence (CL) spectroscopy on MQWs deposited at similar growth temperature (620$^{\circ}$C) reveals the CL peak position of the integral MQW emission is red shifted from 410nm for a smooth template to 560nm for a rough template. In-rich regions of several nanometers size, found to be preferentially forming near the tips of GaN pyramidal grains were observed by CL mapping of the MQWs deposited on a faceted template, whereas a homogeneous distribution of indium atom fraction was obtained using a smooth template. As a result of this study, using a faceted template, MQWs with considerably improved emission efficiency were demonstrated. [Preview Abstract] |
Friday, May 13, 2005 2:48PM - 3:00PM |
B1.00003: W-doped In$_{2}$O$_{3}$ thin films with high electron mobility Paul F. Newhouse, Cheol-Hee Park, Douglas A. Keszler, Janet Tate, Peter S. Nyholm High electron mobility thin films of In$_{2-x}$W$_{x}$O$_{3+y}$ (0 $< x <$ 0.075) were prepared on fused SiO$_{2}$ and yttria-stabilized zirconia (001) single crystal substrates by pulsed laser deposition. Best-case mobilities of 104 and 112 cm$^{2}$/Vs were measured at room temperature for polycrystalline and textured films, respectively. Thin film compositional analysis revealed that the W concentration of the highest mobility films was consistently $x \sim $ 0.03. A slight widening of the band gap was detected from films with increasing electron carrier density, and the electron effective mass calculated from Burstein-Moss theory was 0.3$m_{e}$. In$_{2-x}$W$_{x}$O$_{3+y }$films have high visible transmittance of $\sim $ 80{\%}. [Preview Abstract] |
Friday, May 13, 2005 3:00PM - 3:25PM |
B1.00004: COFFEE BREAK
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Friday, May 13, 2005 3:25PM - 4:01PM |
B1.00005: The phase transition to superconductivity--what information can thermal expansion measurements provide? Invited Speaker: At the phase transition from the normal (non-superconducting) to superconducting state, thermodynamics tells us that a change in slope of the sample volume is expected [1]. This transition is known as a second-order, or continuous, phase transition. However, the change of slope is very small, roughly 1 part in 10 million. Therefore, the study of phase transitions such as this requires measurements with outstanding resolution and stability. In addition, many of the exciting single crystalline materials at the forefront of physics, are only available in the millimeter-size range. This adds another complication. In this presentation, a novel thermal expansion cell will be described. It is constructed entirely of quartz, which has a very small thermal expansion coefficient. The thermodynamics associated with the normal to superconducting phase transition will be explained, and some recent results on the 39 Kelvin superconductor MgB$_{2}$ will be presented. \newline \newline [1] A. B. Pippard, \textit{The Elements of Classical Thermodynamics} (Cambridge University Press, New York, 1957) [Preview Abstract] |
Friday, May 13, 2005 4:01PM - 4:13PM |
B1.00006: Thermoelectric Power in the Transparent Conductive Oxide (TCO) CuSc$_{1-x}$Mg$_{x}$O$_{2+y}$ Dara L. Easley, Allen L. Wasserman In the final process of fabricating the p-type thin film transparent conductive oxide (TCO) CuSc$_{1-x}$Mg$_{x}$O$_{2+y}$, the material is exposed to oxygen under varying pressures. This step intercalates oxygen into the lattice structure, which has the effect of introducing acceptor-like impurities (holes). In order to extract more information about this material a thermodynamic theory is applied to recently measured Seebeck coefficients. The theory assumes that intercalated oxygen introduces holes that thermally migrate to copper sites, providing the system with transport carriers that respond to the influence of electric fields and/or temperature gradients. The calculated Seebeck coefficient is a function of temperature, an intercalated atom concentration for the material and a concentration dependent hole activation energy. When used to fit the Seebeck data it yields an intercalated oxygen concentration dependence which corresponds well with a simple Langmuir model of surface adsorption of oxygen under pressure, thereby providing an independent context for interpreting and validating the model parameters extracted from the Seebeck measurements. [Preview Abstract] |
Friday, May 13, 2005 4:13PM - 4:25PM |
B1.00007: Novel Manifestations of Defects in Cobalt-doped TiO$_{2}$ Alexandre Lussier, E. Negusse, J. Holroyd, J. Dvorak, Y.U. Idzerda, E. Arenholz, S.R. Shinde, S.B. Ogale, T. Venkatesan Room temperature ferromagnetism in magnetic-impurity-doped metal oxides offers the hope of integrating magnetism in conventional semiconductor technology, resulting in a new and more powerful generation of electronics. New models have been proposed to explain the still unresolved ferromagnetic mechanisms and experimental data is required to confirm or disprove them. Our experimental measurements by X-ray Absorption Spectroscopy (XAS) revealed a surprising new spectral feature at the L$_{2}$ and L$_{3}$ edges of cobalt in cobalt-doped rutile TiO$_{2}$, anatase TiO$_{2}$, and La$_{0.5}$Sr$_{0.5}$TiO$_{3}$. Our observations, and supporting Monte Carlo simulations, show that the feature is associated with isolated defects consisting of cobalt atoms and accompanying oxygen vacancies. We will reveal the striking similarities between our model and a recently proposed theoretical model for ferromagnetism. [Preview Abstract] |
Friday, May 13, 2005 4:25PM - 4:37PM |
B1.00008: How does an electron flip a magnetic moment? Frank Marsiglio, Wonkee Kim A magnetic moment is incapable of flipping a magnetic moment unless some damping mechanism is present. On the other hand a spin current can flip a moment, as we will demonstrate in this talk. A variety of quantum effects will be illustrated, using an easy-to-understand semiclassical treatment. Finally, resorting to strictly quantum mechanics, the time evolution of a spin current/magnetic moment system can be determined; the problem is similar to the standard 4th year textbook problem of a particle scattering off a square barrier/well. In this case the ``barrier/well'' responds, i.e. the moment `flips.' We outline how one can monitor the `flip' as a function of time. [Preview Abstract] |
Friday, May 13, 2005 4:37PM - 4:49PM |
B1.00009: Analytical Coarse-Grained Description of Polymer Melts and Blends Edward Sambriski, Galina Yatsenko, Maria Nemirovskaya, Marina Guenza We present an analytical coarse-grained description that maps polymer melts and blends onto fluids of soft colloidal particles. From liquid state theory, we derive the center-of-mass total pair correlation functions, $h(r)$, and the efective pair potentials, $v(r)$. The $v(r)$ serve as input to mesoscale simulations where polymer fluids are modeled as interacting soft colloidal particles. The $h(r)$ from theory and mesoscale simulations agree with united-atom simulation results with no need for fitting parameters. In this way, our approach accurately bridges between microscopic and mesoscopic descriptions of the polymer fluid structure. [Preview Abstract] |
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