Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J11: High Pressure II |
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Sponsoring Units: DCMP DMP Chair: Andrea Trave, LLNL Room: LACC 153C |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J11.00001: Magnetism in High Pressure Cobalt from X-ray Spectroscopy Valentin Iota, Jae-Hyun Park, Choong-Shik Yoo, Jonathan Lang, Daniel Haskel, George Srajer We investigate the electronic and magnetic properties of high pressure cobalt using X-ray emission spectroscopy (XES) and magnetic circular dichroism (MCD) measurements in a diamond anvil cell. We ascribe the changes in the line-shape of the K$\beta$ emission as signature of an electronic (spin) transition, similar to those reported in iron and manganese compounds. We further interpret the observed pressure-induced decrease in the MCD signal as evidence of a gradual loss of magnetic order in high-density cobalt. Experiments were conducted at the Advanced Photon Source in Argonne, IL. The work at LLNL was performed under the auspices of the U.S. Department of Energy by University of California, under Contract W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J11.00002: LDA+U Picture of the Moment Collapse under Pressure in MnO Deepa Kasinathan, J. Kunes, W. E. Pickett, Richard. T. Scalettar, B. Maddox, C.S. Yoo, A.K. McMahan The transition metal monoxide MnO crystallizes in the rock-salt structure and is a high-spin antiferromagnetic insulator at low temperatures. Under pressure, experimentally it is observed to undergo a metal-insulator transition along with a structural change to the nickel arsenide phase.[1,2,3] As the first step in a concerted effort to obtain a realistic theory of the pressure behavior of MnO, we have performed full potential local orbital (FPLO) LDA+U calculations in the rock-salt phase. The outcome is a first order moment collapse at reduced volume V/V$_0$ $\approx$ 0.73, whereas within LDA the collapse is smoother and is centered around V/V$_0$ $\approx$ 0.68. The moment collapse is from high spin 4.9 $\mu_B$ to a lower but nonzero spin value of 0.8 $\mu_B$. The strong influence of symmetry-lowering (cubic to rhombohedral) by antiferromagnetism will be discussed. [1]. Kondo et al., J. App. Phys, 87, No.9, 4153 (2000) [2]. C.S. Yoo et al. (preprint 2004) [3]. Patterson et al., PR B 69, 220101 (2004) [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J11.00003: Pressurre-induced magnetic phase transitions in the two-dimensional ferromagnet Cs2CuF4 Isao Yamada, Mamoru Ishizuka, Hirotaka Manaka Since both the interlayer and intralayer ferromagnetic exchange interactions in Cs$_{2}$CuF$_{4}$ are closely correlated with the orbital state of Cu$^{2+}$,we expect pressure-induced magnetic phase transitions in this compound because pressure can change the orbital state of Cu$^{2+}$. To confirm this expectation, we performed magnetic susceptibility measurements at several pressures up to around 25 GPa over the temperature range from 1.5 to 18 K, using a diamond anvil cell and a SQUID vibrating coil magnetometer. The results show successive magnetic transitions around 2 GPa and 23 GPa, which correspond to changes of the interlayer and the intralayer exchange interactions from ferromagnetic to antiferromagnetic, respectively. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J11.00004: Band gap closure in yttrium hydride under high pressure Akihiko Machida, Ayako Ohmura, Tetsu Watanuki, Katsutoshi Aoki, Satoshi Nakano, Kenichi Takemura Trivalent rare-earth metals demonstrate spectacular change in electronic properties by hydrogenation. With increase in hydrogen concentration beyond $x \sim 2.7$, yttrium hydride, YH$_x$, shows metal-insulator phase transition with structural change from the fcc to hcp of yttrium metal lattice. Band gap opening due to orbital hybridization between $1s$ (H) and $4d$ (Y) has theoretically been proposed for the metal- insulator transition. Theoretical studies have also predicted that the volume reduction by applying hydrostatic pressure would lead to metallization in association with band gap closure. We have investigated structural properties of yttrium hydrides by means of x-ray diffraction and infrared absorption beyond a predicted metallization pressure of $\sim 18$~GPa. Hydride specimen was prepared by hydrogenation reaction of yttrium powder or foil with liquid hydrogen in a diamond anvil cell at room temperature. With increase in pressure beyond $\sim 10$~GPa, the hcp lattice of YH$_3$ transforms gradually to a fcc structure. Infrared spectra show peak position change in the hydrogen vibrational region of 450-1500 cm$^{-1}$ above $\sim 11$~GPa, corresponding to the x-ray diffraction results. The H-Y bonding state and expected metallization are discussed on the basis of the high pressure experimental results obtained x-ray diffraction and infrared absorption. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J11.00005: Calculation of PETN molecular crystal vibrational frequencies under hydrostatic pressure Warren Perger, Jijun Zhao, J.M. Winey, Y.M. Gupta First-principles calculations of the effects of hydrostatic pressure on pentaerythritol tetranitrate (PETN) are performed using the all-electron CRYSTAL03 program. The procedure for applying hydrostatic pressure by performing a series of volumetric changes coincident with lattice constant and internal coordinate optimization using various scripts and support programs is described. Once the optimized internal coordinates and lattice constants have been obtained for a given hydrostatic pressure, a separate algorithm consisting of additional scripts and programs is employed for performing a complete normal-mode analysis, with analytic first derivatives and numeric second derivatives of the total energy. The eigenvalues obtained provide the vibrational frequencies and the eigenvectors are used for mode identification. The role of the Gaussian basis sets chosen and the exchange-correlation potential used is discussed. The vibrational frequencies obtained at ambient pressure are shown to compare well with experiment and gas-phase calculations. The shift of the vibrational frequencies under hydrostatic pressure is compared with experiment. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J11.00006: Fluorescence XAFS Study of Heterogeneous Samples and Material Composition. Firouzeh Tannazi, Grant Bunker Fluorescence XAFS spectroscopy is an appropriate technique for in situ study of the dilute and heterogeneous samples. Application of the standard expressions for calculating the fluorescence radiation from homogeneous samples when applied to inhomogeneous samples can introduce significant errors, which limit the accuracy of this spectroscopic method for determination of material speciation and structure. Hence, development of generalized models are of value. In this study we show that the environmental samples must be treated as heterogeneous samples and also investigate heterogeneity effects, of which particle size is the most important, and their impact on determination of material speciation and structure by XAFS. Our work includes the calculation of fluorescence radiation from arbitrary shaped convex particles by Monte Carlo methods, and adapting and expanding early work on particle size effects from X-ray Fluorescence Spectroscopy to XAFS spectroscopy. We have developed several theoretical models to calculate the fluorescence radiation from heterogeneous samples with arbitrary particle size distributions, compared them and discussed the complementary and importance of heterogeneity effects in each of them. The importance of accounting for these physical effects in interpreting experimental data is emphasized. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J11.00007: Crystal Structure Characterization Using High Pressure - Temperature Optical Properties Jeffrey H. Nguyen, J. Reed Patterson, Lorin X. Benedict, John E. Klepeis, Neil C. Holmes Recent developments in tailored dynamic compression techniques have given us the ability to explore the dynamic phase space along prescribed thermodynamic paths. However, our ability to characterize the crystal structure under ultra-fast (sub-ns) and extreme pressure-temperature conditions is lacking. Here, we will report a novel idea of using optical properties to characterize phase transitions and crystal structures under such conditions. Preliminary measurements on three phase transitions will be reported: Fe ($\alpha $--$>\varepsilon )$, Sn and Bi (solid --$>$ liquid). Changes in complex optical constants across these phase transitions have been observed. We will discuss the implications of these observations in emissivities, temperature measurements and on phase diagrams such as iron. We will also discuss the possibility of using this technique to explore the differences between the dynamic and static phase diagrams. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J11.00008: First principle study on high pressure phase of LiH: discovery of B2 phase Tadashi Ogitsu Recent progress in high-pressure experimental method expanded the accessible P-T condition significantly, and it has already been established as a powerful method for material design. However, characterization of the new phase is still challenging especially at high-temperature. For example, melting line of hydrogen under mega bar range is already accessible for laser heated Diamond Anvil Cell (DAC) technique, yet, it is not possible to determine the structural change upon the melt. On the other hand, ab-initio calculation method, in principle, does not have the limitation both in the physical condition and in structural characterization. Lithium hydride is only the alkali hydride, in which B2 phase has not yet been found experimentally. The B1-B2 phase boundary at 0K suggested by previous ab-iniito calculations are around 4 mega bar, which is still out of reach for DAC, however, the temperature axis has not yet been explored yet. We demonstrate, using ab-initio two-phase simulation method, that B1-B2 phase boundary near melting line is as low as 150Gpa, which is accessible with the laser heated DAC method. The detailed discussion will be given at the presentation. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J11.00009: Melting mechanisms and shocked superheated alkali halide single crystals David A. Boness Even after decades of experimental work, increasingly detailed MD simulations, and refined theoretical approaches, our understanding of the physics of melting remains incomplete at best. Consideration of superheating of solids has been important in studying the melting process through its frustration by means of surface constraint, lack of nucleation sites, and dynamic process. Mechanisms of Lindemann, of Born, of Mott, of Fecht, of Tallon, and of others have been investigated recently by MD simulations of Jin et al. [1] and by Luo et al. [2]. Reanalysis of experimental results on shocked single crystals of the alkali halides KBr and CsBr [3] (with our unpublished data on NaCl and KCl), in light of the recent MD superheating computations [1, 2], leads to a more complete picture of melting mechanisms on superheated dynamically compressed single crystals in the [100] orientation. These shock experiments combine temperature measurement of the region just behind the shock front with elastic wave propagation sampling of the entire shocked region of the crystal. $^1$Z. H. Jin et al., Phys. Rev. Lett. 87, 055703 (2001). $^2 $S.-N. Luo et al., Phys. Rev. B 68, 134206 (2003). $^3$D. A. Boness and J. M. Brown., Phys. Rev. Lett. 71, 2931 (1993). [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J11.00010: RIXS across the volume collapse transition in Gd metal at high pressures Brian Maddox, Warren Pickett, Richard Scalettar, Choong-Shik Yoo, Andrew McMahan, Paul Chou, Michael Hu Gadolinium (Gd) undergoes a volume-collapse phase transition at 59 GPa as a result of pressure-induced change in 4f-electron correlation. Similar correlation-driven electronic phase transitions occur in many other rare-earth metals, some (Ce, Pr, Dy, for example) with large volume collapses and others (Nd, Pm, Sm) without. The exact relationships between crystal structure, volume collapse, and electronic correlation in these materials, however, are not well understood. In this study, we have investigated the electronic structure change of Gd to 113 GPa in a diamond anvil cell, by using resonant inelastic x-ray scattering(RIXS) at the HPCAT/APS. Utilizing the resonance at the LIII-absorption edge of Gd, we were able to probe the dipole allowed 3d-5d transition as well as the quadrupole 3d-4f transition as a function of pressure where the degree of 4f electron correlation should manifest as a change in relative intensity of 4f and 5d transition peaks. This work has been supported by the LDRD-04-ERD-020 at the LLNL, University of California, under the auspices of the U.S. DOE under Contract No. W- 7405-ENG-48. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J11.00011: First-principles phase diagram for Ce-Th system Alexander Landa, Per Soderlind, Andrei Ruban, Levente Vitos, Leonid Pourovskii {\it Ab initio} total energy calculations based on the exact muffin-tin orbitals (EMTO) theory are used to determine the high pressure and low temperature phase diagram of Ce and Th metals as well as the Ce$_{43}$Th$_{57}$ disordered alloy. The compositional disorder for the alloy is treated in the framework of the coherent potential approximation (CPA). Equation of state for Ce, Th and Ce$_{43}$Th$_{57}$ has been calculated up to 1 Mbar in good comparison with experimental data: upon compression the Ce-Th system undergoes crystallographic phase transformation from an fcc to a bct structure and the transition pressure increases with Th content in the alloy. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-Eng-48. A.R. and L.V. are grateful to the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Royal Swedish Academy of Sciences. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J11.00012: Optical Studies of f-electron Materials Under High Pressure Erik Emmons, Thomas Cowan, Srividya Duvvuri, Gunes Kaplan, Richard Kraus, Nathalie LeGalloudec, Katherine McCall, Gunjan Mishra, Jeffrey Thompson, T.J. Ulrich, Aaron Covington We are currently developing optical techniques for the study of f-electron materials under high static and dynamic pressures. Static pressures will be obtained using a diamond anvil cell and dynamic high pressures will be obtained using a pulsed laser system. Laser induced fluorescence using continuous wave ultraviolet and visible laser lines will be used for the purpose of characterizing the electronic structure of the materials and to obtain signatures of phase transitions. Preliminary data will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J11.00013: Localization of 5f electrons and phase transitions in americium Michel Penicaud Density-functional electronic calculations have been used to investigate the high-pressure behavior of americium. The phase transitions calculated agree with the recent sequence obtained experimentally under pressure; double hexagonal close packed $\rightarrow$ face centered cubic $\rightarrow$ face centered orthorhombic $\rightarrow$ primitive orthorhombic. In the first three phases the 5f electrons are found localized, only in the fourth phase (Am IV) the 5f electrons are found delocalized. The localization of the 5f electrons is modeled by an anti-ferromagnetic configuration which has a lower energy than the ferromagnetic ones. In this study the complex crystal structures have been fully relaxed. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J11.00014: Effect of pressure on the superconducting transition temperature of doped and neutron-damaged MgB$_2$ Sergey Bud'ko, Rudeger H. T. Wilke, N. Manuel Angst, Paul Canfield Measurements of the superconducting transition temperature for Al-doped, C-doped and neutron-damaged-annealed MgB$_2$ samples under pressure up to $\sim 8$ kbar are presented. The $dT_c/dP$ values change systematically with the decrease of the ambient pressure $T_c$ in a regular fashion for each group of the samples. The evolution of the pressure derivatives can be understood assuming that the change in phonon spectrum is a dominant contribution to $dT_c/dP$. [Preview Abstract] |
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