Bulletin of the American Physical Society
10th Annual Meeting of the Northwest Section of APS
Volume 53, Number 6
Thursday–Saturday, May 15–17, 2008; Portland, Oregon
Session G1: Condensed Matter Physics II |
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Chair: Jinke Tang, University of Wyoming Room: Olin 301 |
Saturday, May 17, 2008 2:00PM - 2:36PM |
G1.00001: X-ray Characterization of Oxide-based Magnetic Semiconductors Invited Speaker: Although the evidence for magnetic semiconductors (not simply semiconductors which are ferromagnetic) is compelling, there is much uncertainty in the mechanism for the polarization of the carriers, suggesting that it must be quite novel. Recent experimental evidence suggests that this mechanism is similar to the polaron percolation theory proposed by Kaminski and Das Sarma,\footnote{Kaminski and S. Das Sarma, Physical Review Letters 88, 247202 (2002).} which was recently applied specifically to doped oxides by Coey et al.\footnote{J. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, Nature Materials 4, 173 (2005).} where the ferromagnetism is driven by the percolation of polarons generated by defects or dopants. We have used X-ray absorption spectroscopy at the L-edges and K-edges for low concentrations transition metal (TM) doped magnetic oxides (including TiO$_{2}$, La$_{1-x}$Sr$_{x}$O$_{3}$, HfO$_{2}$, and In$_{2}$O$_{3})$. We have found that in most cases, the transition metal assumes a valence consistent with being at a substitutional, and not interstitial site. We have also measured the X-ray Magnetic Circular Dichroism spectra. Although these materials show strong bulk magnetization, we are unable to detect a robust dichroism feature associated with magnetic elements in the host semiconductor. In the cases where a dichroism signal was observed, it was very weak and could be ascribed to a distinct ferromagnetic phase (TM metal cluster, TM oxide particulate, etc.) separate from the host material. This fascinating absence of a dichroic signal and its significant substantiation of important features of the polaron percolation model may help to finally resolve the issue of ferromagnetism in magnetically doped oxides. [Preview Abstract] |
Saturday, May 17, 2008 2:36PM - 2:48PM |
G1.00002: Effect of Heating and H gas Exposure on MgZnO Nanoparticle Conductivity. Chris Berven, Joseph Dick, Leah Bergman, John Morrison, Jesse Huso Changes in the conductivity of Mg$_{x}$Zn$_{1-x}$O (x = 0.15) nanoparticles as a function gas exposure and temperature are reported. The nanoparticles were prepared using wet chemical techniques on insulating thermally grown SiO$_{x}$ Si substrates. Contact to the nanoparticle film was by gold wires laid across about 2 mm apart. The experiments were performed in a custom-built environmental chamber with the ability to evacuate or introduce various gases. For these experiments, the temperature was varied over a range of about 300 K to 420 K. Our measurements showed history-dependant behavior in changes of the conductance of the nanoparticle film. When the device was heated to $\sim $120 K in vacuum or in an Ar the current increased by the same amount. When repeated with H$_{2}$, the current increase was less. Initially, the effect was quite pronounced with a relative change by a factor of 20. With repetitions of the experiments, the same effect was observed but to a lesser degree suggesting a saturation phenomena. When the experiment was modified so that the H$_{2}$ gas was introduced at a high temperature to an evacuated chamber the current dropped but not by the same degree as before. A similar response to exposure to H$_{2}$ was found for exposure to O$_{2}$. Possible explanations for the observations will be presented. [Preview Abstract] |
Saturday, May 17, 2008 2:48PM - 3:00PM |
G1.00003: Probing Magnetic Exchange in Dilute Magnetic Semiconductors by Neutron Scattering Zachary Wiren, Tomasz Giebultowicz, Henryk Kepa, Craig Brown, Juscelino Leao There is presently much interest in magnetic transition metal doped group II-VI semiconductors (A$^{\mathrm{II}}_{1-x}$Mn$_{x}$B$^{\mathrm{VI}}$). It is theorized making them strongly p-type would allow materials to remain ferromagnetic at room temperature. Since this interaction must compete with the intrinsic antiferromagnetism of the material, neutron scattering experiments were undertaken to better understand the antiferromagnetic interactions in these materials. Primarily, the study focused on the effects altering the distance between magnetic atom has on the exchange interaction. Inelastic neutron scattering was undertaken to determine the exchange constant in various materials while neutron diffraction was used to determine the distances involved in the exchange interaction. There appears to be an overall distance dependance in the materials, but high pressure studies undertaken show the functionality differers between the overall trend and the individual materials. Results will be discussed for Zn(Mn)O, Zn(Mn)S, Zn(Mn)Se, Zn(Mn)Te, and Cd(Mn)Te. The data taken is also analyzed to determine more distant interactions. [Preview Abstract] |
Saturday, May 17, 2008 3:00PM - 3:12PM |
G1.00004: Wide band gap $p$-type semiconductors Cu$_{3}$TaQ$_{4}$ (Q = S or Se) Paul Newhouse, Peter Hersh, Andriy Zakutayev, Annette Richard, Heather Platt, Douglas Keszler, Janet Tate The structural and optical properties of thin films, powders, and single crystals of two wide band gap ($E_{g} \quad >$ 2.4 eV) $p$-type semiconducting materials Cu$_{3}$TaQ$_{4}$ (Q = S, Se) are presented. These materials exhibit a cubic crystal structure, which is a favorable processing characteristic and unusual among Cu-based wide-gap $p$-type materials. Thin films have been prepared using a two-step growth process involving pulsed laser deposition of ceramic Cu$_{3}$TaQ$_{4}$ targets and \textit{ex-situ} annealing of the as-deposited films in a chalcogenide-containing atmosphere. The resultant films exhibit mixed polycrystalline and (100)-oriented structure when grown on amorphous SiO$_{2}$ substrates and show strong (100) preferential orientation when prepared on single-crystal yttria-stabilized zirconia substrates. Powders and thin films of Cu$_{3}$TaS$_{4}$ exhibit intense visible photoemission when irradiated by UV light, and the wavelength of the photoemission can be modulated by doping. [Preview Abstract] |
Saturday, May 17, 2008 3:12PM - 3:24PM |
G1.00005: Metal-insulator phase boundary in VO$_{2}$ nanobeams Jiang Wei, Wei Chen, Zenghui Wang, David Cobden Vanadium dioxide nanobeams show the same dramatic metal-insulator transition as does bulk VO$_2$, occuring at about 67 degrees C under ambient conditions. The transition is first-order, accompanied by an abrupt and rapid changes in the electronic and optical properties, a latent heat, and a lattice distortion.~ In the bulk the transition is frustrated, leading to sample degradation, but in nanobeams this is not the case. As a result, in end-clamped nanobeams under tension we are able to investigate a regime of coexistence of the metallic and insulating phases. We find that the resistivity of the insulating phase along the phase boundary is independent of temperature.~ Furthermore the MIT occurs from the intermediate M2 insulating phase, which we detect near the transition by its higher resistivity, but not directly from the low-temperature M1 phase. These results imply that the MIT is triggered by carrier density and therefore involves electron correlations, and suggest that it takes place in the undimerized vanadium chains present in M2 but not in M1. More generally, these studies illustrate the scientific and technological potential of strongly correlated materials in nanoscale form. [Preview Abstract] |
Saturday, May 17, 2008 3:24PM - 3:36PM |
G1.00006: Phase transitions and ferroelectricity in NaSb$_{3}$F$_{10}$ Peter Wu, Panos Photinos, Sidney Abraham, Jason Matthews, R. Christie The structural prediction that NaSb$_3$F$_{10}$ is a new ferroelectric has been confirmed experimentally. The mean phase transition temperature $T_{\mbox{c}}$ $\approx$ 461 K with an associated entropy change $\sim$6 J mol$^{-1}$ K$^{-1}$. The colorless crystals melt at $T_{\mbox{m}}$ $\sim$515 K with decomposition starting at $\sim$600 K. A thermal hysteresis in $T_{\mbox{c}}$ of $\sim$35 K between heating and cooling at 25 K min$^{-1}$ is typical of a first order phase transition. The space group in ferroelectric phase III is $P6_3$, that in the predicted antiferroelectric phase II is $P6_3$22, a supergroup of $P6_3$. The space group of prototypic nonferroic phase I is supergroup $P6_3/mmc$, of which the space group of phase III is not a subgroup. The dielectric permittivity at 100 Hz increases more than an order of magnitude from 350 K before undergoing a major inflection at $T_{\mbox{c}}$ = 460(10) K; it increases thereafter to $T_{\mbox{m}}$. The dielectric loss at 100 Hz is low but increases an order of magnitude from its value at $\sim$350 K before undergoing an inflection at $\sim$460 K, also rising steadily thereafter to $T_{\mbox{m}}$. The reproducible dielectric hysteresis loop, with $P_{\mbox{s}}$ $\approx$ 20$\mu$ C m$^{-2}$ at room temperature under the application of 0.3 MV m$^{-1}$ a.c. or greater, unambiguously verifies the predicted ferroelectric property. The pyroelectric coefficient $\langle{p}\rangle$ = 17(5)$\mu$ C m$^{-2}$ K$^{-1}$ at 298 K. [Preview Abstract] |
Saturday, May 17, 2008 3:36PM - 3:50PM |
G1.00007: BREAK
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Saturday, May 17, 2008 3:50PM - 4:26PM |
G1.00008: Luttinger Liquid Behavior in a Quasi-One-Dimensional Transition Metal Oxide Invited Speaker: One-dimensional solids are attractive because of their inherent simplicity. However, theory shows that even weak Coulomb interactions in one dimension lead to strong perturbations, which can cause unusual physics, such as the separation of spin and charge and simple power-law dependencies of important physical properties. In this talk, I will provide an overview of the Lithium Purple Bronze, Li$_{0.9}$Mo$_{6}$O$_{17}$ - a compound discovered over 20 years ago. Our recent work helps to understand the transition to superconductivity at 1.9 K as the result of a crossover to higher dimension, as predicted by theories for Luttinger Liquids. In addition, our group has shown that the introduction of defects causes Bose Metal behavior, where a lack of phase coherence leads to a metallic state instead of superconductivity. Possible physical connections to high temperature superconductors will be highlighted in the presentation. [Preview Abstract] |
Saturday, May 17, 2008 4:26PM - 4:38PM |
G1.00009: Universal Behavior in Frequency- and Temperature- Dependent Hysteresis Loops of Compacted Cobalt-rich Magnetic Nanoparticles K.M. Chowdary We use the polyol process to synthesize cobalt-rich spherical nanoparticles with average particle size 200 nm and average grain size 20 nm that were consolidated via plasma pressure compaction to greater than 90\% theoretical full density. Measurements of the minor hysteresis loops of the compacted magnetic nanoparticles over a wide range of frequencies (0.1 to 100 kHz) and temperatures (77$^{\circ}$ to 873$^{\circ}$ K) indicate universal behavior in the height, width, and area of the loops. Simulated hysteresis loops determined through numerical solutions of the N\'{e}el-Brown thermal after-effect show similar frequency and temperature dependence. [Preview Abstract] |
Saturday, May 17, 2008 4:38PM - 4:50PM |
G1.00010: Path-integral quantum Monte Carlo studies of lattice polarons and bipolarons Pavel Kornilovitch A path-integral Quantum Monte Carlo method for lattice polaron and bipolaron is presented. Analytical integration eliminates the phonons exactly leading to one or two self-interacting imaginary-time trajectories, which are then simulated by Metropolis Monte Carlo. Projection operators separate states of different symmetry, which provides access to various excited states such as the polaron energy, spectrum, effective mass, density of states and singlet-triplet bipolaron splitting. Monte Carlo updates are formulated in continuous imaginary time on infinite lattices and as such provide statistically unbiased results without finite-size and finite time-step errors. Numerical data are presented for models with short-range and long-range electron--phonon interactions. It is shown that a long-range electron-phonon interaction dramatically reduces the polaron and bipolaron mass, potentially leading to a high critical temperature of the bipolaronic superconductor. [Preview Abstract] |
Saturday, May 17, 2008 4:50PM - 5:02PM |
G1.00011: Detection of adsorbed gas atoms on a suspended single-walled carbon nanotube balance Zenghui Wang, Jiang Wei, Oscar Vilches, David Cobden A monolayer of gas atoms or molecules adsorbed on a suspended single-walled carbon nanotube offers the opportunity to study the phases and phase transitions of a unique low dimensional system. The adsorbed phases are expected to resemble the well studied 2D monolayers on planar graphite, but with modified binding energies, curvature effects, reduced grain size effects, and cylindrical boundary conditions imposed. The adsorbed mass can be detected by using the nanotube itself as a vibrating balance whose resonant frequency shifts when adsorbates add mass to it. We have developed a vibrating nanotube balance with which we have measured the mass adsorption vs equilibrium vapor pressure for Ar at 77 K. We will describe the development of the nanotube balance, the measurement technique, and the sensitivity of the method (of the order of 100 atoms or less), and compare our results with the well known adsorption isotherm of Ar on exfoliated graphite at this temperature. Comparison with other gases (Kr and N2) at this temperature is in progress. [Preview Abstract] |
Saturday, May 17, 2008 5:02PM - 5:14PM |
G1.00012: Identifying the mechanism of biosensing with carbon nanotube transistors Ethan Minot Carbon nanotube transistors have outstanding potential for electronic detection of biomolecules in solution. The physical mechanism underlying sensing however remains controversial, which hampers full exploitation of these promising nanosensors. Previously suggested mechanisms are electrostatic gating, changes in gate coupling, carrier mobility changes, and Schottky barrier effects. Each mechanism has its characteristic effect on the liquid-gate potential dependence of the device conductance. For devices that reveal ambipolar conduction, the sensing mechanisms can be unambiguously identified. Extensive protein-adsorption experiments on such devices show that electrostatic gating and Schottky barrier effects are the two relevant mechanisms, with electrostatic gating being most reproducible. If the contact region is passivated, sensing is dominated by electrostatic gating, which demonstrates that the sensitive part of a nanotube transistor is not limited to the contact region, as previously suggested. Such a layout provides a reliable platform for biosensing with nanotubes. [Preview Abstract] |
Saturday, May 17, 2008 5:14PM - 5:26PM |
G1.00013: Heat Transfer Dynamics at Carbon Nanotube Thermal Switch and Silicon Interface Taejin Kim, Mohamed Osman Carbon nanotubes are very attractive for interconnect and thermal management due to their very high thermal and electrical conductivities. However, the properties of the interface between CNT and other materials influence the overall performance of the CNT interconnects and thermal materials. For example, a thermal switch based vertical carbon nanotube exhibited higher thermal resistance compared to one using mercury drops even though mercury's thermal conductivity is lower than that of carbon nanotubes. In this contribution, we describe a molecular dynamics model for examining heat transfer between a carbon nanotube thermal switch and silicon. The MD model takes into account the 2x2 reconstruction at the silicon surface and use the optimum sites for bonding between silicon and carbon atoms. In the MD simulations, both silicon and nanotubes are first heated to 300K followed by applying a 10 picoseconds heat pulse to the CNT only. As a result of heat diffusion to silicon, the temperature difference between the CNT and silicon exhibits an exponential decay with a time constant $\tau =38$ps (when a single time constant is assumed) which is used to estimate the interface thermal resistance. We will report on the results of our simulations, details of the MD model, role of the phonon coupling at silicon-CNT interface, and compare the results to those obtained from nonequilibrium MD approach. [Preview Abstract] |
Saturday, May 17, 2008 5:26PM - 5:38PM |
G1.00014: Combined density functional and Hartree Fock calculations of the total electronic energy of atoms Kenneth Walsh, Henri Jansen The total electronic energy of an atom has been calculated using unrestricted Hartree Fock methods. In a first step, we perform density functional calculations and obtain the ground state energies and wave functions for spherically symmetric potentials. These wave functions are then used as basis functions for solving the unrestricted Hartree Fock equations. Spherical symmetry is broken by expanding the basis set into spherical harmonic states and by complete angular consideration in the coulomb potential. The wave functions generated by the density functional calculations provide nearly converged basis sets. Breaking the spherical symmetry allows splitting degeneracies in the electronic contribution to the total energy. The resulting Hartree Fock energies are compared with experiment. [Preview Abstract] |
Saturday, May 17, 2008 5:38PM - 5:50PM |
G1.00015: A new quartz capacitance dilatometer for magnetostriction measurements Mario S. da Luz, David P. Cebulla, John J. Neumeier A new dilatometer cell has been developed for measuring magnetostriction. It is constructed entirely of fused quartz and consists of two rectangular metallized plates. This is a capacitive technique because the space between these plates changes when the sample length changes in magnetic field. In this work the construction and performance of the cell are discussed. In addition, we show preliminary results for magnetostriction of the low dimensional conductor Li$_{0.9} $Mo${_6}$O$_{17}$. This system is investigated because prior measurements have shown the electrical resistivity to be strongly dependent on magnetic field. Our goal is to determine if the sample dilation caused by the magnetic field plays a role in the magnetoresistance. Measurements of the magnetoresistance and magnetostriction will be done in magnetic fields up to 9 T. This material is based upon work supported by the National Science Foundation (DMR $-$ 050476 and DMR $-$ 0552458) and Department of Energy Office Basic of Energy Sciences (DE $-$ FG $-$ 06ER46269). [Preview Abstract] |
Saturday, May 17, 2008 5:50PM - 6:02PM |
G1.00016: Magnetocaloric Effect and Thermal Expansion in Mn$_{1-x}$Fe$_x$As Ariana de Campos, Sergio Gama, Brandom McGuire, John J. Neumeier The magnetocaloric effect (MCE) is an active research area because of the prospect of replacement of conventional gas compression technology for refrigeration by magnetic devices based on MCE. The compound MnAs exhibits an extremely large MCE called the colossal magnetocaloric effect (CMCE) [1], but it appears only under high pressures. In this work we report the magnetocaloric effect in MnAs obtained by different methods. We will show results in compounds of Mn$_{1-x}$Fe$_x$As [2] which exhibits CMCE at ambient pressure tuned by the Fe substitution. We will also report preliminary results of thermal expansion measurements for some of these materials. \newline \newline [1] S. Gama, et al., PRL 93, 237202 (2004). \newline [2] A. de Campos, et al., Nature Materials 5, 802 (2006). [Preview Abstract] |
Saturday, May 17, 2008 6:02PM - 6:14PM |
G1.00017: Energy Spread Measurements of Cold Field Emitting HfC(310) Shawn Pollard, Kevin Kagarice, William Mackie Cold field emitting HfC(310) has been researched as a potential next generation electron source for high resolution electron microscopy. Energy spread measurements were taken using a retarding potential energy analyzer. Measurements were taken over a range of angular intensities, varying from 5 uA/Sr to 100 uA/Sr, and compared to theoretical values determined numerically using a software program. Experimental data was compared with cold field W(100), a commercially available cold field emission source currently used in high resolution electron microscopy. [Preview Abstract] |
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