Bulletin of the American Physical Society
2008 Joint Fall Meeting of the Texas and Four Corners Sections of APS, AAPT, and Zones 13 and 16 of SPS, and the Societies of Hispanic & Black Physicists
Volume 53, Number 11
Friday–Saturday, October 17–18, 2008; El Paso, Texas
Session J5: Materials: General |
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Chair: Bill Durrer, The University of Texas at El Paso Room: Union East, 3rd Floor Elkins |
Saturday, October 18, 2008 1:30PM - 1:42PM |
J5.00001: Hyperpolarization Of Phosphorus Donors In Silicon Eric Sorte, William Baker, Dane McCamey, Gernot Laicher, Christoph Boehme, Brian Saam Silicon phosphorus (Si:P) is a model system for investigating spin effects in solid state materials. Recently, members of this group demonstrated a simple method for optically inducing a non-equilibrium state of spin hyperpolarization in phosphorus doped silicon by exploiting a modified Overhauser process. The ability to pump high nuclear spin polarizations in this system could have far reaching technological implications for many fields. For example, hyperpolarized silicon nanoparticles have the potential to improve contrast in magnetic resonance imaging. Additionally, well-characterized quantum spin states have the potential to be useful as quantum qubits. Our current work attempts to extend these recent electron paramagnetic resonance (EPR) and electrically detected magnetic resonance (EDMR) measurements to direct nuclear magnetic resonance measurement of the hyperpolarized phosphorus nuclei. In this talk we will report on our current efforts to measure $^{31}$P spin hyper-antipolarization after the sample is briefly exposed to an inert room temperature environment. We demonstrate the procedure of $^{31}$P polarization measurement through low field electron spin resonance as a precursor to direct NMR measurement. [Preview Abstract] |
Saturday, October 18, 2008 1:42PM - 1:54PM |
J5.00002: Adiabatic Electron-Phonon Interaction at High Temperature in A15 Compounds Jorge Munoz, Olivier Delaire, Matthew Lucas, Max Kresch, Brent Fultz Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V$_{3}$Si, V$_{3}$Ge, and V$_{3}$Co at temperatures from 10 to 1273 K. It was found that phonons in V$_{3}$Si and V$_{3}$Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V$_{3}$Co have a normal softening behavior. First-principles calculations show that this anomalous increase in phonon frequencies at high temperatures originates with an adiabatic electron-phonon coupling mechanism. The anomaly is caused by the thermally induced broadening of sharp peaks in the electronic density of states of V$_{3}$Si and V$_{3}$Ge, which tends to decrease the electronic density at the Fermi level. These results show that the adiabatic electron-phonon coupling can influence the phonon thermodynamics at temperatures exceeding 1000 K. [Preview Abstract] |
Saturday, October 18, 2008 1:54PM - 2:06PM |
J5.00003: Jump frequencies of Cd tracer atoms in L1$_{2}$ lanthanide stannides Megan Lockwood, Gary S. Collins Diffusional jump frequencies of Cd tracer atoms were measured as functions of temperature in four lanthanide stannides, Sn$_{3}$Pr, Sn$_{3}$Nd, Sn$_{3}$Sm, and Sn$_{3}$Gd, using perturbed angular correlation of gamma rays (PAC). Jumps of tracer atoms on the Sn sublattice lead to fluctuating electric field gradients (EFG) that cause damping of the PAC quadrupole perturbation function. Large jump frequencies were observed as in previous studies on Sn$_{3}$La and Sn$_{3}$Ce and on lanthanide indides and gallides. Between 800-1100 K, jump frequencies in all stannides were in the range 1-30 MHz. Jump frequencies were thermally activated with fitted activation enthalpies in the range 0.73-1.46 eV and jump-frequency prefactors in the range $\sim $0.1-50 THz. For some alloys, measurements were made both for Sn-rich and Sn-poor samples. It was found that the jump frequencies in Sn$_{3}$Sm and Sn$_{3}$Gd were greater at the Sn-poor boundary compositions, as previously found also for Sn$_{3}$La and Sn$_{3}$Ce. Detailed trends will be discussed. [Preview Abstract] |
Saturday, October 18, 2008 2:06PM - 2:18PM |
J5.00004: Pressure and magnetic field effects in heavy-fermion UCu$_{3.5}$Al$_{1.5}$ Farzana Nasreen, Karunakar Kothapalli, Heinz Nakotte, Abdel Alsmadi, Vivien Zapf, Frederik Fabris, Alex Lacerda, Jiri Kamarad UCu$_{3.5}$Al$_{1.5}$ is a heavy-fermion compound which crystallizes in the hexagonal CaCu$_{5}$ structure, and it was reported to exhibit non-Fermi-liquid scaling at low temperatures. We report on the measurements of the electrical resistivity and magnetoresistance as a function of temperature (2-150 K), pressure (0-10kbar) and applied magnetic field (0-18T). The results provide evidence that, for UCu$_{3.5}$Al$_{1.5}$, application of pressure and/or magnetic field tends to suppress non-Fermi-liquid scaling and a tendency toward long-range magnetic correlations is observed for temperatures below 20 K, although there is no clear evidence of long-range order in the available pressure and field range. [Preview Abstract] |
Saturday, October 18, 2008 2:18PM - 2:30PM |
J5.00005: Effects of Radiation Induced Conductivity on Samples Charged by Particle Beams J.R. Dennison, Ryan Hoffmann, Jennifer Roth, Alec Sim The electron emission and electron transport properties of highly insulating materials depend on both the fluence and flux of incident charged particle beams. These properties are affected by the deposition, accumulation and dissipation of both charge and energy in the material. We describe the effects of these processes on three distinct experiments: (i) the decay of charge deposited within thin film samples by high energy electron and proton beams; (ii) internal arcing of highly charged, thin film insulators by medium- to high-energy electron beams; and (iii) the modification of total electron emission yields as charge is accumulated by low- to medium-energy electron probe pulses In each case, the radiation induced conductivity as a result of absorbed energy dose from the particle beam enhances electron transport and alters the effects of charging. [Preview Abstract] |
Saturday, October 18, 2008 2:30PM - 2:42PM |
J5.00006: Measuring the Surface Charge on an Insulated Sample in Vacuum Jesse Hayes, Joshua Hodges, Ryan Hoffman, J.R. Dennison We describe the results of a project to design, build, calibrate and test a system to measure the surface charge on an insulator as a function of time and position in situ in a vacuum chamber. In our system, surface charge is created when an insulator is bombarded by fast moving electrons, creating secondary electrons which are emitted from the material. Deposited charge dissipates on relatively long time scales by charge transport through highly resistive materials. The ultrahigh vacuum system uses a capacitive sensor acting as an electrostatic field transfer probe and an external electrostatic field probe to measure the surface voltage. The probe can measure a wide range of surface voltage, is very compact, and can be swept across the sample using an in vacuo stepper motor to measure charge distribution. We describe measurements to characterize the stability, sensitivity, accuracy, range, spatial resolution and temporal response of the surface charge measurable by our system. Applications to studies by our group of electron emission from insulators and conductivity, as well as spacecraft charging, are also discussed. [Preview Abstract] |
Saturday, October 18, 2008 2:42PM - 2:54PM |
J5.00007: Observation and Characterization of Electrostatic Discharge in Insulating Materials Induced by Electron Beam Bombardment Jennifer Roth, Ryan Hoffmann, J.R. Dennison Understanding the characteristics of electron beam bombardment that induce electrostatic discharge (ESD) of insulating materials is crucial to constructing an electrically stable spacecraft. A measurement system has been designed to determine the beam energy and charge flux densities at which spacecraft materials (bare and conductor-coated polymeric thin-film Kapton E) undergo ESD. Because ESD events can occur over very short time intervals multiple, simultaneous detection methods were employed as charge was accumulated on a sample surface; these included monitoring of sample current and optical emissions from the sample surface. Data from each of these detection methods were analyzed to determine the incident beam conditions which induced ESD, as well as the magnitude of the discharge events. Each sample was also examined with optical microscopy before and after testing to determine permanent changes in the materials due to ESD. We also consider the effects of electron emission, dark current conductivity, and radiation induced conductivity on the accumulation, dissipation and discharge of the accumulated charge. [Preview Abstract] |
Saturday, October 18, 2008 2:54PM - 3:06PM |
J5.00008: Computational modeling on molybdenum sulfide grain boundary interfaces Manuel Ramos Since discovery of the CoMoS phase in catalysis, many investigations had been conducted in order to understand its intrinsic chemical-physical properties. The CoMoS phase is important because it posses strong catalytical properties. Up to now some models exist in order to explain a mechanism of why this phase is created on a catalyst. Those models are ``cherry model'' which assumes that Co atoms start nucleating on MoS$_{2}$ at the edges, other model like ``decoration'' states that Co atoms are added on the MoS$_{2}$ edges in a decoration form, however even when those two mentioned models are based on density functional calculations (DFT), they lack of explain how two different interfaces meet (MoS$_{2}$/Co$_{9}$S$_{8}$, or MoS$_{2}$/Ni$_{3}$Co$_{6})$ in a bulk catalyst, this lacking of information provided by the models could be attribute to the fact that they are made to explain specific cases, such as small particles (\textit{Hexagonal truncated, Nano-octahedral and Triangular prism}) containing around 10$^{5}$ atoms as much. This work presents computational calculations using Cerius2 molecular software to model based on DFT methods the CoMoS and NiMoS interfaces. Obtain information will be compared with results obtained by HRTEM. [Preview Abstract] |
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