Bulletin of the American Physical Society
16th Annual Meeting of the Northwest Section of the APS
Volume 60, Number 6
Thursday–Saturday, May 14–16, 2015; Pullman, Washington
Session E4: Condensed Matter Physics II |
Hide Abstracts |
Chair: Alan Greer, Gonzaga University Room: Webster Physical Sciences 16 |
Saturday, May 16, 2015 1:30PM - 2:00PM |
E4.00001: Transformation of Shock-Compressed Pyrolytic Graphite to Diamond: Role of Microstructure Invited Speaker: J.M. Winey The response of highly-oriented pyrolytic graphite (HOPG) to shock wave compression depends strongly on its microstructure: highly oriented ZYB-grade HOPG showed evidence for rapid transformation to a high density phase; less oriented ZYH-grade HOPG did not [Erskine and Nellis, 1991]. To gain insight into these findings and to understand the rapid phase transformation in ZYB-grade HOPG, theoretical and experimental examinations of shocked HOPG were carried out. Numerical simulations of the previous measurements resulted in the following findings for ZYB-grade HOPG: completion of the transformation in less than 10 ns; the response of the high density phase matches cubic diamond. Also, measured wave profiles for ZYB- and ZYH-grade HOPG, shocked to stresses below the phase transformation onset, revealed significant differences in their elastic-inelastic response. ZYB-grade HOPG exhibits elastic-inelastic waves with large elastic wave amplitudes. In contrast, single overdriven waves for ZYH-grade HOPG suggest negligible elastic limits. These results show that the elastic-inelastic response of shocked pyrolytic graphite depends strongly on the orientational order. They also suggest that rapid phase transformation in shocked ZYB-grade HOPG may be due to large elastic compression.\\[4pt] In collaboration with M. Lucas and Y.M. Gupta, Washington State University. [Preview Abstract] |
Saturday, May 16, 2015 2:00PM - 2:12PM |
E4.00002: Nano-glasses: Non crystalline materials with the controllable atomic structures Lokendra Khanal, William Armour, John McCloy, You Qiang Crystalline materials were used to make the tools in the Stone Age. Today, crystalline nanomaterials are used in steels, semiconductors, superconductors and ferromagnetic materials etc. because of their controllable micro and nanostructures (defect and chemical). Nano-glasses are the amorphous solids with the controllable nanostructures. They are prepared by consolidating the non-crystalline nanoparticles to have melt quenched glassy structures joined together by the interfacial regions with reduced density, reduced neighbor atoms and different electronic structure than the usual glasses. Nano-glasses can be made to show different properties by controlling the volume to surface ratio at the interface and changing the chemical compositions. Some of the nano-glasses e.g. Fe$_{90}$Sc$_{10}$ possesses the ferromagnetic behavior unlike the paramagnetic melt quenched glasses. Nano-glasses also have the properties like ductility, biocompatibility and catalytically active. If we could utilize these kinds of unique properties of the nano-glasses, this may lead the nanotechnology to the new era of the materials use. In this, we will report our first try of silicon based nano-glass too. [Preview Abstract] |
Saturday, May 16, 2015 2:12PM - 2:24PM |
E4.00003: Stability of Molecular Crystals under High Pressure: Tuning of Hydrogen Bonding by Deuterium Substitution Yuchuan Tao, Zbigniew Dreger, Yogendra Gupta Hydrogen bonding (HB) is ubiquitous in many molecular compounds, and is often a key factor in determining the structure and properties of these systems. Because the strength of HB depends on the location of hydrogen atom between two electronegative atoms, external pressure and H/D substitution are very useful tools for controlling and changing the behavior of hydrogen bonding. In this work, we used both factors to gain insight into the role of hydrogen bonding on high-pressure stability of molecular crystals. The pressure effects were examined on two hydrogen-bonded molecular crystals with planar crystal structures, squaric acid (H$_{\mathrm{2}}$C$_{\mathrm{4}}$O$_{\mathrm{4}}$, SQA) and diaminodinitroethylene ([CNO$_{\mathrm{2}}$NH$_{\mathrm{2}}$]$_{\mathrm{2}}$, FOX-7), and their deuterated counterparts. Raman experiments in a diamond anvil cell (DAC) were performed on deuterated SQA and FOX-7 to 25 and 40 GPa, respectively, and compared with our previous results on hydrogenated crystals. We demonstrated that the high-pressure response of these crystals depends on the initial strength of HB, and that the different aspects of high-pressure stability are affected selectively. In particular, we showed that deuteration weakens the hydrogen bonds, affects the pressure onsets for structural phase transitions and lifts vibrational modes coupling. [Preview Abstract] |
Saturday, May 16, 2015 2:24PM - 2:36PM |
E4.00004: Low-Temperature and High-Pressure $^{31}$P-NMR Study of the Magnetic Properties of the 2D Frustrated Square-Lattice Compound BaCdVO(PO$_{4})_{2}$ Beas Roy, Ramesh Nath, David C. Johnston, Yuji Furukawa BaCdVO(PO$_{4})_{2}$ is a spin $S = $ 1/2 frustrated square-lattice compound with a nearest-neighbor exchange coupling $J_{1} = -$3.62 K and a next-nearest-neighbor exchange coupling $J_{2} = $ 3.18 K yielding \textbar $J_{2}$/$J_{1}$\textbar $=$ 0.88. A transition to an antiferromagnetic (AFM) ground state occurs below a temperature $T_{\mathrm{N}} = $ 1.0 K under ambient pressure $p$. Based on the $J_{2}$/$J_{1}$ ratio, the system is located close to the disordered ground state regime of the phase diagram. We carried out $^{31}$P-NMR measurements under high $p$ up to 1.88 GPa, and at low temperatures $T$ down to 100 mK, to investigate the effects of $p$ on the magnetic properties of the system. We established the first $H$--$p$--$T$ phase diagram for this system. With increasing $p$, the magnetization saturation field is suppressed from $H_{\mathrm{S}} = $ 4.2 T at ambient $p$ to $H_{\mathrm{S}} = $ 0.55 T at $p = $ 1.88 GPa and \textbar $J_{2}$/$J_{1}$\textbar decreases to 0.62 at $p = $ 1.45 GPa, thus moving towards the disordered region of the phase diagram. [Preview Abstract] |
Saturday, May 16, 2015 2:36PM - 2:48PM |
E4.00005: X-ray diffraction measurements to examine structural changes in shock compressed solids at the Dynamic Compression Sector Stefan Turneaure, Nicholas Sinclair, Kurt Zimmerman, Yogendra Gupta The Dynamic Compression Sector (DCS) is a new facility at the Advanced Photon Source (APS) designed for real-time examination of dynamically compressed condensed matter using hard x-rays as probes. The pulsed (153.4 ns period), short duration (about 100 ps), hard x-rays (8-35 keV) at the APS are ideal for examining the real-time response of condensed matter to dynamic loading. The DCS has several impact launchers allowing half-inch diameter flat-faced projectiles to be accelerated to velocities ranging from several hundred m/s to over 5 km/s. The projectile impacts a disk-shaped sample resulting in high dynamic stresses ranging from a few kilobars to over 1 megabar. A two-dimensional pixelated x-ray detector developed for use at the DCS can capture 4 successive x-ray diffraction frames with 153.4 ns interframe time during the impact event. Representative results of x-ray diffraction measurements on shock compressed solids undergoing stress-induced structural changes will be presented. The DCS, a national user facility, will be available to general users in CY2016 through the APS proposal process. Work supported by DOE/NNSA. [Preview Abstract] |
Saturday, May 16, 2015 2:48PM - 3:00PM |
E4.00006: Spin Hall effect in disordered organic solids Zhi-Gang Yu We study the spin Hall effect (SHE) in disordered $\pi$-conjugated organic solids, where individual molecules are oriented randomly and electrical conduction is via carrier hopping. The SHE, which arises from interference between direct ($i\to j$) and indirect ($i\to k\to j$) hoppings in a triad consisting of three molecules $i$, $j$ and $k$, is found to be proportional to $\lambda ({\mathbf n}_i\times {\mathbf n}_j+{\mathbf n}_j\times {\mathbf n}_k + {\mathbf n}_k\times {\mathbf n}_i)$, where $\lambda$ is the spin admixture of $\pi$ electrons due to the spin-orbit coupling and ${\mathbf n}_i$ is the orientation vector of molecule $i$. Electrical conductivity $\sigma_{qq}$ ($q=x,y,z$) and spin-Hall conductivity $\sigma_{\rm sh}$ are computed by numerically solving the mater equations of a system containing $32\times32\times32$ molecules and summing over contributions from all triads in the system. The obtained value of spin Hall angle, $\Theta_{\rm sh}\equiv \sigma_{\rm sh}/\sigma_{qq}$, is consistent with experimental data in PEDOT:PSS, with a predicted temperature dependence as $\log \Theta_{\rm sh} \sim T^{-1/4}$. [Preview Abstract] |
Saturday, May 16, 2015 3:00PM - 3:12PM |
E4.00007: Vortex-Bound States in Chiral d-Wave Superconductors Darrick Lee, Andreas Schnyder Superconductors have a full energy gap in their bulk spectrum, but subgap bound states can occur at magnetic impurities, at the surface, or inside magnetic vortices. The latter are called vortex bound states. In this talk, we discuss how quasiparticle excitations in a superconductor with vortices can be modelled using a set of equations called the Bogoliubov de Gennes equations. In addition, we discuss how these equations are numerically and analytically solved for chiral $d$-wave superconductors to obtain the structure of their vortex bound states. This structure refers to a combination of two properties: the energy spectrum of the vortex bound states and the local density of states about the vortices. We find that in some cases, the vortex bound states form zero energy Majorana states, whereas in other cases, the bound states only occur at finite energies. In addition, we find that in all cases of chiral $d$-wave superconductors, the local density of states about the vortex is different than the local density of states about the antivortex. This difference remains present after temperature broadening of the local density of states, modeling a more realistic experimental environment, and can thus be used to characterize chiral $d$-wave superconductors. [Preview Abstract] |
Saturday, May 16, 2015 3:12PM - 3:30PM |
E4.00008: Break
|
Saturday, May 16, 2015 3:30PM - 4:00PM |
E4.00009: Acceptors in zinc oxide: Nitrogen will not work Invited Speaker: Matthew McCluskey Zinc oxide (ZnO) is a new/old semiconductor with potential applications in optoelectronics, spintronics, and transparent electronics. There is an ongoing quest for an acceptor dopant that can yield reliable $p$-type conductivity. Early work suggested that nitrogen acceptors could fulfill that role. However, our experimental results verify recent theoretical predictions that the N acceptor level is $\sim$ 1.5 eV above the valence band, too deep for any practical application requiring free holes. All is not lost, though. Potential alternatives will be discussed. [Preview Abstract] |
Saturday, May 16, 2015 4:00PM - 4:12PM |
E4.00010: Sodium acceptor doping of ZnO crystals Narendra Parmar, Kelvin Lynn ZnO bulk single crystals were doped with sodium by thermal diffusion. Positron annihilations spectroscopy was employed to confirm the filling of zinc vacancies, to \textgreater 6 $\mu $m deep in the bulk. SIMS measurement shows the diffusion of sodium up to 8 $\mu $m with concentration (1 --- 3.5) $\times$ 10$^{17}$ cm$^{-3}$. Broad photoluminescence excitation peak at 3.1 eV, with onset appearance at 3.15 eV in Na:ZnO is attributed to an electronic transition from a Na$_{\mathrm{Zn}}$ level at $\sim$ (220 -- 270) meV to the conduction band. For electrical measurements, Ohmic contacts were made using a MoO$_{2}$ solution (ethyl glycol:H$_{2}$O:: 60:40) on a hot plate at 200 $^{\circ}$C for 10 min. MoO$_{2}$ has a work function of 6.5 eV and can be useful for electrical Ohmic contacts for wide band gap semiconductors. Resistivity in Na doped ZnO crystals increases up to (4 --- 5) orders of magnitude at room temperature.\\[4pt] [1] N. S. Parmar and K. G. Lynn, \textbf{\textit{Applied Physics Letters }}\textbf{106, }022101 (2015). [Preview Abstract] |
Saturday, May 16, 2015 4:12PM - 4:24PM |
E4.00011: Lithium Acceptors and Hydrogen in Zinc Oxide Caleb Corolewski, Matthew McCluskey Zinc Oxide, a wide-direct-bandgap semiconductor, is predominantly $n$-type. Realization of $p$-type material coupled with high efficiency UV emission could provide efficient solid state lighting. In this study we developed a diffusion process to maximize the Li acceptor concentration in ZnO crystals. We provide evidence for weak $p$-type conduction in Li-doped ZnO. SIMS measurements show the Li concentration increases from 5 $\times$ 10$^{17}$ to 3 $\times$ 10$^{19}$ cm$^{-3}$ and is constant for the 16 $\mu$m measured, indicating bulk doping was achieved. Photoluminescence (PL) spectra show the characteristic yellow band for an 800 meV Li acceptor, and an additional blue band. The 800 meV Li acceptor has a 3.1 eV onset for optical excitation. Fourier transform infrared spectroscopy measurements show defect local vibrational modes corresponding to a 3326.8 cm$^{-1}$ line, the 3577 cm$^{-1}$ Li-OH, and the 3677 cm$^{-1}$ surface OH monolayer. The intensity ratios of the 3326.8 cm$^{-1}$ to the 3677 cm$^{-1}$ lines and of the blue to yellow PL bands depends on the presence of Zn in dopant source and are therefore Zn vacancy related. Van der Pauw resistivity measurements show Li-doped crystals to be insulating (10$^{9}$ $\Omega\cdot$ cm) at room temperature. Above 400 K the resistivity drops due to the increased concentration of free holes, consistent with thermal ionization of Li acceptors. Modeling the data determined a defect ionization energy of 880 meV. The magnitude and behavior of the resistivity with temperature is consistent with highly resistive and weakly $p$-type ZnO. [Preview Abstract] |
Saturday, May 16, 2015 4:24PM - 4:36PM |
E4.00012: Defect Dynamics and optical properties in ZnO films Dinesh Thapa, Jesse Huso, Hui Che, Amrah Canul, John Morrison, Caleb Corolewski, M.D. McCluskey, Leah Bergman ZnO is an environmentally-friendly material with a wide range of applications in optoelectronic devices for which an enhanced UV photoluminescence (PL) of ZnO is a key feature. However, many as-grown films are observed to contain some intrinsic defects which can reduce the UV PL efficiency, limiting their practical usefulness. This study presents a route to achieve an enhanced UV PL from the sputtered film and examines the origin of the enhanced UV PL. As-grown films reveal a weak UV emission and a visible emission due to zinc interstitial (Zni) related defects. To understand the route toward enhanced UV PL, annealing was performed on two sets of as-grown films each separately under Ar and O2 atmospheres. The PL spectra of O2 annealed samples display an enhanced UV peak with elimination of the Zni related emission peak, however an Oi-related defect emission was introduced. In contrast, Ar annealed films showed a significantly enhanced UV PL with nearly quenched visible emission. The origin of enhanced UV PL was investigated via low temperature PL measurements which indicate that dominant emission in the UV region is related to structural defects. We acknowledge the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering under Grant No. DE-FG02-07ER46386. J.H. who contributed to data interpretation acknowledges the National Science Foundation under Grant No. DMR 1202532. [Preview Abstract] |
Saturday, May 16, 2015 4:36PM - 4:48PM |
E4.00013: Microgravity experiments on phase separation using helically-supported capillary channels Maverick Terrazas, David Thiessen In microgravity environments fluid phase separation must occur without gravity. Capillarity plays a larger role in fluid transport in space, and as a passive force, it makes a great candidate for the replacement of gravity as a phase separator. This study uses helically-supported capillary channels at low-pressure for their stability over a range of pressures and ability to remove droplets from two-phase flows. Channel priming and subsequent droplet absorption by primed capillary channels is studied under normal gravity, in a drop tower (2 sec zero-g), and on a parabolic aircraft (20-25 sec zero-g). The dynamic capillary channel response to droplet impact depends on the fluidic capacitance of the channel and flow resistance to the reservoir. [Preview Abstract] |
Saturday, May 16, 2015 4:48PM - 5:00PM |
E4.00014: On the Role of Point Defects in the Persistent Photoconductivity of Strontium Titanate Violet Poole, Matt McCluskey Strontium titanate (STO) is a wide band gap transparent oxide semiconductor. STO often serves as a substrate for high temperature superconductors and is also used in oxygen sensors. STO was recently found to exhibit persistent photoconductivity (PPC) upon annealing, which means that the sample goes from insulating to conductive upon light exposure. The conductivity persists even after the light is turned off. As with any semiconductor, the electronic properties of STO are strongly affected by the presence of defects and impurities, which can be altered by annealing treatments. Oxygen vacancies have long been thought to be the dominant factor controlling the conductivity. However, hydrogen is usually present in oxide materials and can greatly influence the electronic properties. In this research, we investigate the roles hydrogen and oxygen vacancies play in the PPC behavior observed in STO. Spectroscopic methods in the visible and infrared regions of the electromagnetic spectrum were used to identify these elements and their behaviors under various conditions. We found that oxygen vacancies and hydrogen impurities appear to be important actors in this phenomenon. Refinements to the annealing procedure are suggested based on these results. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700