Bulletin of the American Physical Society
13th Annual Meeting of the Northwest Section of the APS
Volume 56, Number 10
Thursday–Saturday, October 20–22, 2011; Corvallis, Oregon
Session C2: Condensed Matter I (MuSR, etc.) |
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Chair: Robert Kiefl, University of British Columbia Room: LaSells Stewart Center Agriculture Leaders Room |
Friday, October 21, 2011 1:30PM - 2:06PM |
C2.00001: Low Energy $\mu$SR and Physics at Interfaces Invited Speaker: The physical properties of an interface between two different materials is usually different from the bulk properties of both. Few experimental methods are capable of examining the local properties of such interfaces or near the surface as a function of depth on the nm scale. The low energy muons spin relaxation/rotation (LE-$\mu$SR) technique at the Paul Scherrer Institute is perfectly suited for such studies. By implanting fully polarized $\mu^+$ particles at a tunable energy, one can investigate the local magnetic properties in a depth resolved manner in thin films and hetero-structures. In this talk I will discuss some details of the technique and present a few examples from applications in superconductivity, magnetism and dimensional effects at interfaces. [Preview Abstract] |
Friday, October 21, 2011 2:06PM - 2:42PM |
C2.00002: Beta Detected NMR a New Probe for Nanoscience Invited Speaker: TRIUMF's ISAC facility in Vancouver, Canada produces intense beams of shortlived radioactive ions for research in nuclear and materials science. The latter uses $^8$Li$^+$ as an implanted spin-polarized radioactive probe. Using the technique of beta-detected Nuclear Magnetic Resonance, local magnetic information is extracted via detection based on the asymmetric property of nuclear beta-decay. The probe ion implantation energy can be varied from $\sim 30$ keV down to $\sim 100$ eV, allowing the implantation depth to be varied from a few hundred nanometers down to a few nanometers. Thus depth-resolved measurements of thin films, heterostructures and near-surface and buried-interface effects can be performed on a wide range of condensed matter systems. The facility and some examples (in superconductors and magnetic materials) will be reviewed, and prospects for the growth of this user-facility within the larger TRIUMF Centre for Molecular and Materials Science will be presented. [Preview Abstract] |
Friday, October 21, 2011 2:42PM - 2:54PM |
C2.00003: Local and distant charge compensation of iron ions in ABO$_{3}$ ferroelectrics Galina Malovichko, Robert Petersen, Valentin Grachev The determination of the lattice sites and charge compensators of non-isovalent impurities are vitally important for both fundamental science and tailoring material properties for various applications. Results of our study of Fe$^{3+}$ centers in LiNbO$_{3}$, LiTaO$_{3}$ and KTaO$_{3}$ (LN, LT and KT) with the help of Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) have shown significant difference in spectroscopic characteristics and, correspondingly, structures of these centers. Two Fe$^{3+}$ dominant centers in cubic KT crystals have axial $<$100$>$ symmetry caused by the presence of local charge compensators (oxygen vacancy for Ta$^{5+}$ substitution and interstitial oxygen for K$^{+}$ substitution). In Li-deficient congruent LT and LN crystals the Fe$^{3+}$ ions substitute only for Li$^{+}$. Since no disorder in the nearest surrounding was found, the distant charge compensation by intrinsic defects in cation sublattice is supposed. In stoichiometric samples of LT and LN the iron ions randomly substitute for both Li$^{+}$ and Ta$^{5+}$ (Nb$^{5+})$ having average self-compensation of excess charges. [Preview Abstract] |
Friday, October 21, 2011 2:54PM - 3:06PM |
C2.00004: Multifrequency EPR/ENDOR/optical study of ytterbium centers in stoichiometric lithium niobate Valentin Grachev, Viktor Bratus, Edward Kokanyan, Galina Malovichko The tremendous narrowing of lines of Electron Paramagnetic Resonance (EPR) in nearly stoichiometric lithium niobate samples, when compared to those in congruent samples, allowed us to distinguish nine non-equivalent centers, as well as line splitting caused by the hyperfine interaction of ytterbium electrons with the nuclear spins of $^{171}$Yb and $^{173}$Yb. Three Yb$^{3+}$ centers have axial C$_{3}$ symmetry; all others have the lowest C$_{1}$ symmetry due to the presence of intrinsic defects and/or charge compensation defects in the near neighborhood of Yb$^{3+}$. Our study of Electron Nuclear Double Resonance (ENDOR) gave direct evidence that Yb$^{3+}$ in the main axial center substitutes for Li$^{+}$ and has no other defects in its surrounding (distant charge compensation mechanism). Possible models for low-symmetry centers are proposed. The obtained numerous characteristics of \textbf{g}-tensors and hyperfine tensors can be used as cornerstones for model calculations of Yb$^{3+}$ centers in lithium niobate. [Preview Abstract] |
Friday, October 21, 2011 3:06PM - 3:18PM |
C2.00005: Fermi liquid theory for thin $^{3}$He films David Li, Roger Anderson, Michael Miller We study the thermodynamic response and collective excitations in thin $^{3}$He films with nonzero polarization. By utilizing results from existing spin susceptibility and specific heat measurements for $^{3}$He adsorbed on graphite substrates and also in thin $^{3}$He -- superfluid $^{4}$He films, we determine $s$-wave and $p$-wave effective interaction components. We can then use Fermi liquid theory to compute state-dependent Landau parameters. We show results for the density and polarization dependence of the effective mass, spin susceptibility, heat capacity, and compressibility for thin $^{3}$He films. We discuss the zero sound and spin-zero sound solutions of Landau's kinetic equation including contributions up to the L = 3 angular momentum components. In particular, we study features in the oscillation amplitudes of the two Fermi surfaces at finite polarization. [Preview Abstract] |
Friday, October 21, 2011 3:18PM - 3:30PM |
C2.00006: Behavioral Model of Spin-Transfer Torque Driven Oscillation in a Nanomagnet Benjamin Buford, Albrecht Jander, Pallavi Dhagat We present a model written in Verilog-A, a behavioral description language, for spin-torque driven oscillations in a nanomagnet. Recent experiments have shown that spin-polarized current passing through a nanomagnet can cause magnetic dynamics from transfer of spin angular momentum. This can result in steady state oscillation of the magnetization at microwave frequencies [1]. Such spin torque oscillators are of interest due to the ability to rapidly tune their operating frequency by adjusting the applied magnetic field and their compatibility with existing CMOS fabrication methods. Our model is based upon the Landau-Lifshitz-Gilbert dynamics of a single- domain nanomagnet [2] and includes thermal agitation. We demonstrate the ability to model small angle, large angle, and out-of-plane precession. Additionally, we characterize the field and current boundaries between these regimes. Our Verilog-A model can be used in industry standard simulation tools alongside CMOS device models to simulate circuits that combine spintronic devices with CMOS control and processing circuitry. \\[4pt] [1] S. I. Kiselev et al., Nature, Vol. 425, pp. 380(3), (2003). \\[0pt] [2] L. Engelbrecht, Ph.D. Dissertation, Dept. Elect. Eng., Oregon State Univ., Corvallis, OR, (2011). [Preview Abstract] |
Friday, October 21, 2011 3:30PM - 3:46PM |
C2.00007: BREAK
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Friday, October 21, 2011 3:46PM - 3:58PM |
C2.00008: Optical and electronic properties of potential solar absorber Cu$_3$PSe$_4$ David H. Foster, Vorranutch Jieratum, Robert Kykyneshi, Douglas A. Keszler, Guenter Schneider We report theoretical investigations of the electronic and optical properties of semiconductor Cu$_3$PSe$_4$. Diffuse reflectance spectroscopy measurements indicate a band gap of 1.40 eV. Our calculations using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional and $GW$-type approximations agree well with the experimental atomic structure and band gap, and reveal that the band gap is direct. The calculated optical spectrum is similar to GaAs in the visible region, with $\alpha > 5 \times 10^4$ cm$^{-1}$ for $\lambda < 630$ nm. We conclude that the optical properties of Cu$_3$PSe$_4$ are within the desired range for a photovoltaic solar absorber material. Of importance to material prediction (\emph{in silico} chemistry), the local density and generalized gradient approximations (LDA/GGA) are found to cause significant error in both the HSE band gap (using HSE with ion positions relaxed in LDA/GGA) and the $GW$ band gap (using $GW$ with fixed LDA/GGA quasi-particle wavefunctions). We attribute this to the different degrees to which HSE and LDA/GGA find the first conduction band states to be anti-bonding (P-s/Se-p$^*$) in character. [Preview Abstract] |
Friday, October 21, 2011 3:58PM - 4:10PM |
C2.00009: Ultra-Smooth ZnS Films Grown on Silicon via Pulsed Laser Deposition Christopher Reidy, Janet Tate Ultra-smooth, high quality ZnS films were grown on (100) and (111) oriented Si wafers via pulsed laser deposition with a KrF excimer laser in UHV (10$^{-9}$ Torr). The resultant films were examined with optical spectroscopy, electron diffraction, and electron probe microanalysis. The films have an rms roughness of $\sim $1.5 nm, and the film stoichiometry is approximately Zn:S :: 1:0.87. Additionally, each film exhibits an optical interference pattern which is not a function of probing location on the sample, indicating excellent film thickness uniformity. Motivation for high-quality ZnS films comes from a proposed experiment to measure carrier amplification via impact ionization at the boundary between a wide-gap and a narrow-gap semiconductor. If excited charge carriers in a sufficiently wide-gap harvester can be extracted into a narrow-gap host material, impact ionization may occur. We seek near-perfect interfaces between ZnS, with a direct gap between 3.3 and 3.7 eV, and Si, with an indirect gap of 1.1 eV. [Preview Abstract] |
Friday, October 21, 2011 4:10PM - 4:22PM |
C2.00010: Soft x-ray spectroscopy of Ca-doped BiCuOSe thin films grown by pulsed laser deposition Jason Francis, Janet Tate, Shawn Sallis, Louis Piper Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO$_{3}$ substrates via pulsed laser deposition. X-ray absorption and emission spectroscopy were used to analyze the O $K$-edge and Cu $L_{3,2}$-edge in order to determine the bulk electronic structure of BiCuOSe. Analysis of the O $K$-edge XAS/XES spectra yield a band gap of $\sim $1 eV, consistent with optical measurements on thin films. Optical measurements on single crystals show a band gap of $\sim $0.83 eV. XAS/XES results show the presence of strongly hybridized Bi 6$s$ - O 2$p$ orbitals in undoped samples. This hybridization is diminished in doped samples, providing strong evidence of Ca$^{2+}$ on the Bi$^{3+}$ site. X-ray diffraction measurements show that the films are highly oriented, with rocking curves around the (003) peak having a FWHM of 1\r{ }. Expansion of the c-axis is observed as Ca concentration is increased. All films show $p$-type conductivity and develop more metallic character as calcium doping increased. Ca concentration was determined by EPMA, which shows non-stoichiometric transfer of Ca from the target into the films. [Preview Abstract] |
Friday, October 21, 2011 4:22PM - 4:34PM |
C2.00011: Ultrafast Exciton-Polariton Bleaching and Recovery in a Quantum-Well Microcavity Induced by Strong Terahertz Pulses Andrew Jameson, Joseph Tomaino, Yun-Shik Lee, Galina Khitrova, Hyatt Gibbs, A.C. Klettke, Mack Kira, Stephan Koch We present a study of the quantum coherent transients of exciton-polaritons in a quantum-well (QW) microcavity driven by strong THz pulses. The optical response of a QW microcavity exhibits pronounced cavity polariton modes in the strong-coupling regime. We observed the time-resolved optical reflectivity of the lower and higher exciton-polariton (LEP and HEP) modes in the presence of strong THz fields. The two polariton modes are a manifestation of the periodic process in which excitons emit and reabsorb photons a number of times during the cavity lifetime, typically $\sim $10 ps. Consequently, THz-induced excitonic nonlinear effects can be multiplied during this process. The small mass and the large size of these quasi-particles imply that polaritons are susceptible to THz fields. Our experiments uncover the nature of the THz interaction with this system showing heavily modulated polariton modes and revealing the time resolved dynamics of the quantum coherence between the LEP and HEP modes. [Preview Abstract] |
Friday, October 21, 2011 4:34PM - 4:46PM |
C2.00012: Large Area Graphene Growth and Characterization Jenna Wardini, Joshua Kevek, Tristan DeBorde, Ethan Minot Graphene is an exciting new material with exceptional electronic properties. For graphene to become widely used in future electronics applications, optimization of the graphene growth processes is essential. At Oregon State University we have installed a new chemical vapor deposition (CVD) system to improve the quality of large-area (12" x 36") graphene films. Our CVD system has a number of unique capabilities including operating pressures below $10^{-6}$Torr and water vapor cleaning to refresh the chamber walls. We will present an overview of the CVD process, the design of our system, and methods we use to characterize graphene surface coverage, grain size and atomic defects. Preliminary results from our growth system will be discussed. [Preview Abstract] |
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