Bulletin of the American Physical Society
2007 Joint Spring Meeting of the Texas Sections of APS, AAPT, and SPS
Volume 52, Number 2
Thursday–Saturday, March 22–24, 2007; Abilene, Texas
Session APS2: APS II - Condensed Matter and Nanomaterials |
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Chair: Tikhon Bykov, McMurry University Room: Foster Science Building 458 |
Friday, March 23, 2007 3:30PM - 3:42PM |
APS2.00001: Kinetics Modeling Under Shock-Loading Conditions Steven Valone Shock loading induces complex kinetic processes leading to such macroscopic phenomena as plastic deformation, phase transformations, and spall. The associated rates are typically modeled as first-order processes. The form of any rate constant is then assumed to be of an Arrhenius form [1]. This form of rate law assumes an equilibrium distribution of velocities in the system [2]. Clearly, in a shock-loaded system, the velocity distribution needs to be centered on the particle-velocity, u$_p$ [3]. A revision in the Arrhenius rate model leads to u$_p$-dependent rate constants. At high shock loading, the rate constant naturally switches to a power-law dependence in keeping with observation. \newline [1] D. L. Preston, D. Tonks, and D. C. Wallace, J. Appl. Phys. 93, 211 (2003). \newline [2] H. A. Kramers, Physica (Amsterdam) 7, 284 (1940). \newline [3] S. M. Valone, J. Chem. Phys. 118, 9606 (2003). [Preview Abstract] |
Friday, March 23, 2007 3:42PM - 3:54PM |
APS2.00002: Electronic and vibrational properties of the clathrate ``alloy'' Ba$_{8}$Ga$_{16}$Ge$_{30-x}$Si$_{x}$ Emmanuel Nenghabi, Charles Myles We have used the Generalized Gradient Approximation (GGA) of density functional theory (DFT) to study the electronic band structure and the vibrational spectrum of the Type I clathrate ``alloy'' Ba$_{8}$Ga$_{16}$Ge$_{30-x}$Si$_{x}$ (1$\le $ x $\le $ 15). Our calculations show that Ba$_{8}$Ga$_{16}$Si$_{15}$Ge$_{15}$ is energetically favored and is a narrow gap semiconductor. The trend in the band gap as function of Si concentration is also presented. The vibrational spectrum indicates low frequency rattling modes caused by Ba atoms that are loosely bound in the Si-Ge framework cages. Such modes may scatter the heat carrying acoustic vibrational modes of the framework, potentially reducing the thermal conductivity. [Preview Abstract] |
Friday, March 23, 2007 3:54PM - 4:06PM |
APS2.00003: Vortex Dynamics in the High Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ with In-plane Columnar Defects Heather Quantz, Andra Petrean-Troncalli, Lisa Paulius, Valentina Tobos, Wai-K. Kwok We investigated the vortex dynamics in a single crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ before and after irradiation with high-energy heavy ions. Earlier studies have focused on the effects of irradiation-induced columnar defects parallel to the crystallographic c-axis of the crystal or at relatively large angles off the ab-plane. In our current study, we introduced columnar defects \textit{along the in-plane layered structure} of the crystal. A single crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ was polished down to a narrow width of 27 $\mu $m allowing high energy heavy ions to penetrate the crystal along the ab-plane. The crystal was irradiated with 1.4 GeV $^{208}$Pb$^{56+}$ ions to a dose matching field of 1T. We present analysis of vortex dynamics under various current densities, magnetic field strengths and orientations. This work was supported by the US Department of Energy, under contract DE-AC02-06CH11357 and by National Science~Foundation under grant No. DMR-0072880. [Preview Abstract] |
Friday, March 23, 2007 4:06PM - 4:18PM |
APS2.00004: Are the surfaces of CrO$_{2}$ half-metallic? C.A. Ventrice, Jr., D.R. Borst, H. Geisler, J. van Ek, Y.B. Losovyj, P.S. Robbert, U. Diebold, J.A. Rodriguez, G.X. Miao, A. Gupta Previous photoelectron spectroscopy studies of CrO$_{2}$ have found either no density of states or a very low density of states at the Fermi level, suggesting that CrO$_{2}$ is a semiconductor or a semimetal. This is in contradiction to calculations that predict that CrO$_{2}$ should be a half-metallic ferromagnet. Recently, techniques have been developed to grow high-quality epitaxial films of CrO$_{2}$ on TiO$_{2}$ substrates by chemical vapor deposition. We present photoelectron spectroscopy measurements of epitaxial CrO$_{2}$(110)/TiO$_{2}$(110) and CrO$_{2}$(100)/TiO$_{2}$(100) grown using a CrO$_{3}$ precursor. In addition, measurements of epitaxial Cr$_{2}$O$_{3}$(0001)/Pt(111) films grown by thermal evaporation of Cr in an oxygen atmosphere are presented as a reference for reduced CrO$_{2}$ films. The measurements of the CrO$_{2}$ surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low energy electron diffraction measurements provide evidence that stoichiometric CrO$_{2}$ is present. The consequence of this is that neither surface of CrO$_{2}$ is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces. [Preview Abstract] |
Friday, March 23, 2007 4:18PM - 4:30PM |
APS2.00005: Muonium in Silicon Germanium Alloys Brent Carroll, Roger Lichti, Philip King, Gurkan Celebi We report observations of muonium defect centers in bulk, single crystalline Silicon Germanium alloys. Analysis of both bond-centered and interstitial T-site muonium gives a test for predictions of isolated hydrogen defect levels in Si$_{1-x}$Ge$_{x}$ alloys across the full alloy composition range. Temperature dependent amplitudes for neutral Mu$_{BC}$ and Mu$_{T}$ from high transverse field muon spin rotation (HTF-$\mu $SR) measurements yield ionization energies for these muonium centers. The hyperfine parameter for Mu$_{BC}$ varies linearly with alloy composition, whereas for Mu$_{T}$ it varies non-linearly, perhaps due to faster motion among germanium T-sites compared to silicon. Our HTF-$\mu $SR analysis of Si$_{0.16}$Ge$_{0.84}$ shows that the diamagnetic muonium species grows around 130 K with an activation energy of 102(1) meV. Similar data for Si$_{0.10}$Ge$_{0.90}$ give an activation energy of 67(3) meV. Both HTF-$\mu $SR and RF resonance results show two distinct Mu$_{T}$ signals for x $\ge $ 0.84. These states are tentatively assigned to T-sites with all Ge neighbors versus those with a Si neighbor. [Preview Abstract] |
Friday, March 23, 2007 4:30PM - 4:42PM |
APS2.00006: Stacking faults in SiC nanowires Kendra L. Wallis, Mirza Hasan, Monika Wieligor, Waldek Zerda Nanostructured silicon carbide exhibits a wide-range of enhanced properties, including mechanical, electrical, thermal, and optical, over silicon carbide of larger grain size. Thus nanostructured silicon carbide offers the prospect of being a very useful material for many applications. We have produced pure SiC nanowires by silicon vaporization reaction with carbon multiwall nanotubes. These nanowires range in diameter from 20 nm to almost 600 nm and reach lengths up to 30 $\mu $m. We will show SEM and HRTEM images of the produced nanowires, along with x-ray diffractograms, Raman spectra, and FTIR spectra, all of which have been analyzed to study the structure and growth of pure SiC nanowires. We have found some wires with no stacking faults and some wires with stacking faults. The stacking faults and interesting information about them will be discussed. We show that population of structural defects depends on quality of carbon nanotubes. [Preview Abstract] |
Friday, March 23, 2007 4:42PM - 4:54PM |
APS2.00007: Structural and Magnetic Characterizations of Gold and Silica Doubly Coated $\gamma $-Fe$_{2}$O$_{3}$ Nanoparticles Gan Liang, Keeseong Park, John Markert, Zhi-Ping Luo, Mark Croft, Xiaojun Ji, Chun Li An extensive structural and magnetic characterization has been carried out on the gold (Au) and silica (SiO$_{2})$ doubly coated $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles. The phase of the uncoated and Au/SiO$_{2}$ coated nanoparticles were studied by powder x-ray diffraction (XRD) and x-tray absorption spectroscopy (XAS), which show that the iron oxide particles before and after coating have the phase of $\gamma $-Fe$_{2}$O$_{3. }$The transmission electron microscopy (TEM) results indicate that the average sizes of the $\gamma $-Fe$_{2}$O$_{3.}$cores, the Au/SiO$_{2}$ coated Fe$_{2}$O$_{3 }$ nanoparticles, and the Au nanoparticles in the Au nanoshells are about 12 nm, 92 nm, and 7 nm, respectively. We have the following observations from the magnetization measurements: (i) both the uncoated and coated nanoparticles are superparamagnetic at room temperature (T), and the Au nanoparticles are diamagnetic at all temperatures, (ii) the blocking temperature is decreased by the Au/SiO$_{2}$ double coating, and (iii) the coercivity for both the uncoated and coated nanoparticles decreases almost linearly with T$^{ \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$ with the latter decreasing faster than the former. [Preview Abstract] |
Friday, March 23, 2007 4:54PM - 5:06PM |
APS2.00008: Spectroscopic characterizations of CdS polymer nanocomposites Felicia Manciu, Jayesh Govani, Eric Hagedorn, Ronald Ziolo We explore the optically active phonon modes of CdS nanoparticles synthesized in poly(styrenesufonate) ion exchange resin and in free standing, uncrosslinked films to obtained information about the morphology, crystallinity, and surface interactions. The infrared (IR) transmission measurements were carried out with a Bruker IFS 66v Fourier Transform Spectrometer. The micro-probe and FT-Raman analysis were performed with a Jobin-Yvon and a Bruker FRA 106 system, respectively. Two excitations of 1064 nm and 632.8 nm were used. Both IR and Raman results demonstrate the existence of crystalline CdS. The dominant feature in the far-IR spectrum of the CdS/resin nanocomposites is a sharp peak centered at 255 cm$^{-1}$. This feature, based on a core-shell dielectric model calculation, is attributed to the presence of a very thin layer of CdS. The IR spectrum of CdS/ film nanocomposites reveals the presence of a broad absorption, which is a combination of transversal and surface optical phonon modes of CdS nanoparticles. HRTEM images of the CdS nanocomposites show CdS nanoparticles of about 2.5 nm aligned in rows or strings on the polymer surface. Amorphous CdS is also present and may be seen surrounding the nanocrystalline regions. [Preview Abstract] |
Friday, March 23, 2007 5:06PM - 5:18PM |
APS2.00009: On the Limiting Cases of Nonextensive Thermostatistics Michele Campisi We investigate the limiting cases of Tsallis statistics. The viewpoint adopted is not the standard information-theoretic one, where one derives the distribution from a given measure of information. Instead the mechanical approach recently proposed in [M. Campisi, G.B. Bagci, Phys. Lett. A, 362(1):11-15 (2007)], is adopted, where the distribution is given and one looks for the associated physical entropy. We show that, not only the canonical ensemble is recovered in the limit of $q$ tending to one, as one expects, but also the microcanonical ensemble is recovered in the limit of $q$ tending to minus infinity. The physical entropy associated with Tsallis ensemble recovers the microcanonical entropy as well and we note that the microcanonical equipartition theorem is recovered too. We are so led to interpret the extensivity parameter q as a measure of the thermal bath heat capacity: $q=1$ (i.e. canonical) corresponds to an infinite bath (thermalised case, temperature is fixed), $q=-\infty$ (microcanonical) corresponds to a bath with null heat capacity (isolated case, energy is fixed), intermediate $q's$ (i.e. Tsallis) correspond to the realistic cases of finite heat capacity (both temperature and energy fluctuate). [Preview Abstract] |
Friday, March 23, 2007 5:18PM - 5:30PM |
APS2.00010: Scientific Investigation over the Interactions of Polar and Non-Polar Gases with Carbon Nanotubes using Microwave Spectroscopy James Roberts, Aman Anand, Jai N. Dahiya A resonant cavity in the TE$_{011}$ mode is used as a probe to study the gas absorption in loaded nanomaterials with high surface area to mass ratio. The microwave Network Analyzer was used to study the gas absorption effects for gas pressure and frequency at room temperature. Multiple runs for polar and non-polar gases were studied for the comparing the adsorption strengths. Polynomial plots were used to measure the complex dielectric constant of the loading medium. Standard perturbation techniques were used to study the E and B field vector responses inside the cavity. The Interaction of the load and the fields can be described by the equation, Z = f1($\mu $e, E)-f2($\mu $m, H), where f1($\mu $e, E) is a function of the electric permeability $\mu $e and the electric field, while f2($\mu $m,H) is a function of the magnetic permeability $\mu $m and the magnetic field. Gas absorption during irradiation of the CNTs affects the load in the resonant cavity, produces the frequency shifts and also changes the Q factor of the cavity. The quantitative measurement of hysteresis between pressurizing and de-pressurizing gave the relativistic adsorption characteristics of the samples inside the cavity. [Preview Abstract] |
Friday, March 23, 2007 5:30PM - 5:42PM |
APS2.00011: A computational model of organic semiconductor spin valves Armando Barranon, F. de L. Castillo-Alvarado A computational model has been developed of a spin valve where a spin sandwich is formed by two ferromagnetic electrodes and an organic semiconductor in the middle of the sandwich. Monomers are randomly placed in the central region of the sandwich and a genetic algorithm simulates the random movement of the monomers until they reach a polymer linear chain, optimizing a screened Coulomb Potential in energy space. Afterwards, spontaneous magnetization is simulated by a genetic algorithm that optimizes an Ising Hamiltonian in energy space. This way a remarkable enhancement of about 4300\% in magnetization is obtained when electrode spins are parallel compared to the magnetization of the antiparallel array. This is in qualitative agreement with a experimental study of pyrochlore oxide superconductor KOs$_2$O$_6$. A.B. acknowledges funding from CONACYT Project 58939. [Preview Abstract] |
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