Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V21: Materials at Extremes: Warm Dense MatterFocus
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Sponsoring Units: GSCCM DCOMP DMP Chair: Arianna Gleason, Los Alamos National Laboratory Room: 320 |
Thursday, March 17, 2016 2:30PM - 3:06PM |
V21.00001: Study of the Warm Dense Matter with XANES spectroscopy - Applications to planetary interiors Invited Speaker: Adrien Denoeud With the recent discovery of many exoplanets, modelling the interior of these celestial bodies is becoming a fascinating scientific challenge. In this context, it is crucial to accurately know the equations of state and the macroscopic and microscopic physical properties of their constituent materials in the Warm Dense Matter regime (WDM). Moreover, planetary models rely almost exclusively on physical properties obtained using first principles simulations based on density functional theory (DFT) predictions. It is thus of paramount importance to validate the basic underlying mechanisms occurring for key planetary constituents (metallization, dissociation, structural modifications, phase transitions, etc....) as pressure and temperature both increase\footnote{K. Umemoto et al., Science 311, 983 (2006)}$^{,}$\footnote{D. Hicks et al., Phys. Rev. Lett, 97, 025502 (2006)}. \\In this work, we were interested in two materials that can be mainly found in the Earth-like planets: silica, or SiO2, as a model compound of the silicates that constitute the major part of their mantles, and iron, which is found in abundance in their cores. These two materials were compressed and brought to the WDM regime by using strong shock created by laser pulses during various experiments performed on the LULI2000 (Palaiseau, France) and the JLF (Livermore, US) laser facilities and on the LCLS XFEL (Stanford, US). In order to penetrate this dense matter and to have access to its both ionic and electronic structures, we have probed silica and iron with time-resolved X-ray Absorption Near Edge Structure (XANES)\footnote{A. Benuzzi-Mounaix et al., Phys. Rev. Lett, 107, 165006 (2011)}. In parallel with these experiments, we performed quantum molecular dynamics simulations based on DFT at conditions representative of the region investigated experimentally so as to extract the interesting physical processes and comprehend the limits of the implemented models\footnote{V. Recoules et al., Phys. Rev. B, 80, 064110 (2009)}. In particular, these works allowed us to highlight the metallization processes of silica in temperature\footnote{A. Denoeud et al., Phys. Rev. Lett, 113, 116404 (2014)} and the structural changes of its liquid in density, as well as to more constrain the melting curve of iron at very high pressures. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V21.00002: Free-electron x-ray laser measurements in isochorically heated warm dense matter Philipp Sperling, Hyun Chung, Luke Fletcher, Eric Galtier, Eliseo Gamboa, Hae Ja Lee, Yultuz Omarbakiyeva, Heidi Reinholz, Gerd Röpke, Ulf Zastrau, Siegfried Glenzer We present the highly-resolved measurements of inelastic x-ray scattering spectra in an ultrafast heated solid. The obtained spectra from the isochorically heated foils permit a direct temperature dependent determination of plasma properties, e.g. transport coefficient. X-ray pulses from the seeded Linac Coherent Light Source delivering an average of $0.3$~mJ of $8$~keV x-ray photons in a $0.005$\% bandwidth pulse, have been focused to micrometer diameter focal spots isochorically heating solid materials to temperatures up to several eV. The inelastic forward scattering spectra resolve electronic plasma oscillations that directly allow an accurate determination of the electron temperature and density indicating a warm dense matter state. This accuracy enable us to extract plasma properties, e.g. the electrical conductivity, and enables the validation of existing theories. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V21.00003: Hot-dense hydrogen study up to 300 GPa Chang-Sheng Zha Hydrogen study under extreme pressure-temperature conditions has fundamental importance for the development of condensed physics. The prediction of insulator to metallic state transition at sufficient high pressure has been a long-standing open question for the high pressure physics community. Recently, more experimental and theoretical interests were focused on the hot-dense state of hydrogen. A numerous investigations indicated a turnover melting line with a maximum point around \textasciitilde 100 GPa. First-principle theoretical models indicate that the metallization could be a liquid-liquid transition just above the melting line. Experiments for these studies were mostly conducted in shock compression or pulsed laser heating in static compression resulted in large controversy observations. Hydrogen study also has been one of the engines driving the advance of static pressure-temperature technologies. New developments in hydrogen study have brought static pressure generation and signal probing technique into 300 \textasciitilde 400 GPa range, leading to more new phases found. New experimental results using static pressure-temperature DAC techniques demonstrate that hydrogen has much more complicated phase behaviors at multiple megabar pressure range than that expected previously. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V21.00004: Hydrogen Deuteride to 3.4 Megabar Mixed Isotopes and New Phases Ranga Dias, Ori Noked, Isaac Silvera We present infrared absorption studies of solid hydrogen deuteride to pressures as high as 3.4 megabar in a diamond anvil cell and temperatures in the range 5 to 295 K. Above 198 GPa the sample transforms to a mixture of $_{,}$ and, interpreted as a process of dissociation and recombination.$_{\, \, }$Three new phases-lines are observed, two of which differ remarkably from those of the high-pressure homonuclear species, but none are metallic. The time-dependent spectral changes are analyzed to determine the molecular concentrations as a function of time.y. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V21.00005: A Transition to Metallic Hydrogen: Evidence of the Plasma Phase Transition Isaac Silvera, Mohamed Zaghoo, Ashkan Salamat The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from T$=$0 K to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1-1.7 Mbar and up to 2200 K in a diamond anvil cell using pulsed laser heating of the sample. We present evidence in two forms: a plateau in the heating curves (average laser power vs temperature) characteristic of a first-order phase transition with latent heat, and changes in transmittance and reflectance characteristic of a metal for temperatures above the plateau temperature. For thick films the reflectance saturates at \textasciitilde 0.5. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V21.00006: High pressure hydrogen stabilised by quantum nuclear motion Richard Needs, Bartomeu Monserrat, Chris Pickard Hydrogen under extreme pressures is of fundamental interest, as it might exhibit exotic physical phenomena, and of practical interest, as it is a major component of many astrophysical objects. Structure searches have been successful at identifying promising candidates for the known phases of high pressure hydrogen. However, these searches have so far been restricted to the location of minima of the potential energy landscape. In this talk, we will describe a new structure searching method, ``saddle-point ab initio random structure searching'' (sp-AIRSS), that allows us to identify structures associated with saddle points of the potential energy landscape. Using sp-AIRSS, we find two new high-pressure hydrogen structures that exhibit a harmonic dynamical instability, but quantum and thermal anharmonic motion render them dynamically stable. These structures are formed by mixed layers of strongly and softly bound hydrogen molecules, and become thermodynamically competitive at the highest pressures reached in experiment. The experimental implications of these new structures will also be discussed. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V21.00007: Warm dense iron equation of state from quantum molecular dynamics Travis Sjostrom, Scott Crockett Through quantum molecular dynamics (QMD), utilizing both Kohn-Sham (orbital-based) and orbital-free density functional theory, we calculate the equation of state of warm dense iron in the density range 7-30 g/cm$^3$ and temperatures from 1 to 100 eV. A critical examination of the iron pseudopotential is made, from which we find the previous QMD calculations of Wang \textit{et al.} [Phys. Rev. E 89, 023101 (2014)] to be in error. Our results also significantly extend the ranges of density and temperature which are attempted in that prior work. We calculate the shock Hugoniot and find very good agreement with experimental results to pressures over 20 TPa. Additionally we have utilized the QMD results to generate a new SESAME tabular equation of state for fluid iron, accurate in the warm dense matter region, and also extending to much broader regions of density and temperature than can be accessed by the QMD alone. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V21.00008: Overview of Warm Dense Matter Experiments at LCLS Eric Galtier, Anna Levy, Gareth Williams, Luke Fletcher, Fabien Dorchies, Jérôme Gaudin, Philipp Sperling Warm Dense Matter (WDM) is found in numerous astrophysical systems, from giant planets to brown dwarves or cool dense stars. Being this intermediate regime where condensed matter or plasma theories do not apply, it can be produced in all laser-induced plasma experiments on Earth. As a consequence, understanding its properties is fundamental and the whole community is investigating this extreme state of matter. With the advent of the 4th generation of light sources, namely the Free Electron Lasers (FELs), a new way of producing and diagnosing WDM becomes available. In 2009, the Linac Coherent Light Source (LCLS) at SLAC was the first FEL to produce X-ray photons to be used by the user community. Since then, various experiments took place at LCLS to produce and measure specific physical properties of WDM. In this talk, we will present an overview of key experiments performed at LCLS to study WDM. The LCLS has been used in a variety of configuration: as the main heating mechanism, as a probe or both at the same time. When used as a probe, high power lasers have been used to shock matter and excite it into the WDM regime. Finally, we will describe exciting perspectives on the WDM research, as the LCLS-II will become available in about 5 years. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V21.00009: First-principles equation of state and electronic properties of warm dense oxygen Shuai Zhang, Kevin Driver, François Soubiran, Burkhard Militzer We perform all-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) calculations to explore warm dense matter states of oxygen. Our simulations cover a wide density-temperature range of 1-100 g cm\textasciicircum \textbraceleft $-$3\textbraceright and 10\textasciicircum 4--10\textasciicircum 9 K. By combining results from PIMC and DFT-MD, we are able to compute pressures and internal energies from first-principles at all temperatures and provide a coherent equation of state. We compare our first-principles calculations with analytic equations of state, which tend to agree for temperatures above 8 x 10\textasciicircum 6 K. Pair-correlation functions and the electronic density of states reveal an evolving plasma structure and ionization process that is driven by temperature and density. As we increase the density at constant temperature, we find that the ionization fraction of the 1s state decreases while the other electronic states move towards the continuum. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized. This work is funded by the DOE (DE-SC0010517). [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V21.00010: Nuclear quantum effects in high-pressure ice Yael Bronstein, Philippe Depondt, Fabio Finocchi Because of their mass, hydrogen nuclei are subjected to nuclear quantum effects (NQE), mainly tunneling and zero-point energy. They can be crucial to describe correctly the properties of H-containing systems, even at room temperature. A prototypical example of the importance of NQE is the transition from asymmetric H-bonds in phase VII to symmetric bonds in phase X of high-pressure ice, in which NQE drastically reduce the transition pressure \footnote{Benoit et al, Nature 392, 258 (1999); Bronstein et al, Phys. Rev. B 89, 214101 (2014)}. However, natural ice is rarely pure and even small concentrations of salt (LiCl or NaCl) in ice have a strong effect on the phase diagram: the VII to X transition is shifted to higher pressures, questioning the resilience of NQE in the presence of ionic impurities \footnote{Bove et al, PNAS 112, 8216 (2015)}. We investigate these questions using the Quantum Thermal Bath \footnote{Dammak et al, Phys. Rev. Lett. 103, 190601 (2009)}, a semi-classical Langevin dynamics, taking into account both NQE and thermal effects in pure and salty ices. We show why NQE can be sensitive to the presence of impurities and that non-trivial phenomena could result, such as the spectacular upshift of the transition pressure and the peculiar motion of ions. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V21.00011: Influence of exchange-correlation temperature effects on electric conductivity of aluminum in WDM regime Valentin Karasiev, L\'azaro Calder\'in, Sam Trickey Calculation of transport properties in the warm dense matter (WDM) regime and comparison with experiment is an important development challenge. Computationally affordable, reliable theoretical methods are required. Current best practice is Kohn-Sham molecular dynamics (KS-MD) to sample ionic configurations and Kubo-Greenwood (KG) conductivity calculations at selected configurations. Relevant aspects are (i) the very high computational cost and unfavorable cost-scaling of the KS-MD at WDM temperatures, and (ii) neglect of explicit temperature effects in the ground state exchange-correlation (XC) functionals often used to approximate the XC free energy. We address both issues. We sample configurations of aluminum ions in the WDM regime with drastically lowered MD cost via finite-temperature orbital-free MD, including explicitly T-dependent XC [1]. Then we delineate the XC T-effects by comparing KG conductivities calculated with and without explicit XC T-dependence. The result is that explicitly T-dependent XC gives an unequivocal improvement with respect to experiment for aluminum at low material density and elevated temperatures. [1] V.V. Karasiev, T. Sjostrom, J. Dufty, and S.B. Trickey, Phys. Rev. Lett. 112, 076403 (2014) [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V21.00012: Orbital-free Molecular Dynamics Simulations to Characterize the Liquid-vapor Critical Point of Aluminum Debajit Chakraborty, Valentin Karasiev, Samuel Trickey Aluminum is frequently used in warm-dense matter (WDM) experiments. However, experimental diagnostic limitations make computational exploration of the Al liquid-vapor transition important[1]. The elevated temperaure and low-density make ab initio molecular dynamics (AIMD) with Kohn-Sham (KS) density functional theory (DFT) searches for the divergent compressibility extremely time consuming. Orbital free DFT (OFDFT) in principle is a cost-effective alternative. Here we report on calculations utilizing the PROFESS@QuantumEspresso interface [2] to explore suitable pseudo-potentials [3], the limitations of our wholly constraint-based VT84F [4] non-ineracting free-energy functional as exposed in the low-density regime, and possible extensions or extrapolations via tunable non-interacting free energy functionals [5]. [1] Atom.\ Proc.\ Plasmas \textbf{CP-1161} K.\ B.\ Fournier\ ed.\ (2009); [2] Comput.\ Phys.\ Commun.\ \textbf{185}, 3240 (2014); [3] J.\ Phys.:\ Condens.\ Matter \textbf{2}, 351 (1990); [4] Phys.\ Rev.\ B \textbf{88}, 161108(R) (2013); [5] ``Tunable non-interacting free-energy functionals", V.V.\ Karasiev \{unpublished\} [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V21.00013: Accurate exchange-correlation energies for the warm dense electron gas Fionn Malone, Nicholas Blunt, James Shepherd, Derek Lee, James Spencer, Matthew Foulkes The accurate treatment of matter at high temperatures and densities is of increasing importance to many fields in physics and chemistry, with applications ranging from planetary physics to inertial confinement fusion and plasmonic catalysis. Faithfully including the effects of temperature in density functional theory simulations of warm dense matter requires accurate results for the uniform electron gas (UEG) across the whole temperature-density plane. While accurate ground state quantum Monte Carlo data have existed for over 30 years \footnote{Phys. Rev. Lett. 45, 566 (1980)}, there remains significant disagreement between results obtained using different path integral Monte Carlo methods at finite temperature \footnote{Phys. Rev. Lett. 110, 146405 (2013)}\footnote{Phys. Rev. Lett. 115, 130402 (2015)}. To resolve this disagreement, we use the systematically improvable density matrix quantum Monte Carlo method \footnote{Phys. Rev. B 89, 245124 (2014)}\footnote{J. Chem. Phys. 143, 044116 (2015)} to calculate the exchange-correlation energy of the UEG. We also demonstrate how the evaluation of free energies emerges naturally from our method. [Preview Abstract] |
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