Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J13: Focus Session: Simulations of Matter at Extreme Conditions IV: Crystalline Solids, Liquids, and Methods |
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Sponsoring Units: DCOMP GSCCM Chair: Ann Mattsson, Sandia National Laboratories Room: Morial Convention Center 204 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J13.00001: Structure and dynamics of supercooled liquid silicon under pressure: A first-principles molecular-dynamics study. Tetsuya Morishita Recent investigations have suggested that silicon (Si) may exhibit liquid-liquid transitions under pressure and/or supercooling [1]. Here, we report first-principles molecular-dynamics simulations of supercooled liquid Si focusing on the pressure dependence of structure and dynamics [2]. The pair correlation function of deeply supercooled liquid Si (1100 K) for pressures 0 - 18 GPa shows considerable structural changes resulting from the collapse of tetrahedral configurations. The power spectrum of the velocity auto-correlation function also confirms the reduction of the tetrahedral order by pressurization. The self-diffusion coefficient as a function of pressure shows a broad maximum around 10 GPa. However, at a higher temperature (1500 K), the diffusion coefficient simply decreases with increasing pressure, indicating conspicuous dependence of the dynamics and relevant structure upon temperature. [1] T. Morishita, Phys. Rev. Lett. 93, 055503 (2004); ibid. 97, 165502 (2006). [2] T. Morishita, Phys. Rev. E 72, 021201 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J13.00002: Molecular dynamics simulation of the shock-induced wurtzite-to-rocksalt transition in CdSe and CdS Aidan Thompson, Marcus Knudson The shock-induced wurtzite-to-rocksalt structural transformation is studied using large-scale molecular dynamics simulation. The primary goal is to understand the atomistic mechanisms underlying the interesting transformation kinetics observed in the case of cadmium sulfide [M. D. Knudson and Y. M. Gupta, J. Appl. Phys, v. 91, p. 9561, 2002]. Since the mechanical and structural properties of CdS are very similar to those of CdSe, we have performed multi-million atom MD simulations of the shock-induced phase transformation in CdSe single-crystals using the well-established interatomic potential of Rabani, which has been shown to correctly describe the wurtzite and rocksalt phases and the transformation pressure. In MD simulations of shock along the wurtzite $c$-axis, the elastically-compressed wurtzite transforms directly to grains of rocksalt. Along the $a$-axis, a three-wave structure is observed; the wurtzite first transforms to a tetragonal crystal phase, which in turn transforms to rocksalt grains. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J13.00003: Large-scale molecular dynamics modeling of shock wave propagation in silicon Xiang Gu, Mikalai Budzevich, Ivan Oleynik, Sergey Zybin, Carter White We performed molecular dynamics simulations of shock wave propagation in silicon. The different regimes of materials response were studied as a function of shock wave intensity and crystalline orientation of shock wave propagation. The shock Hugoniots are predicted in a wide range of piston velocities (0-12 km/s), and for several crystallographic orientations $<$100$>$, $<$110$>$, and $<$111$>$. Shock Hugoniots were used for a detailed analysis of a material's response to complex, split shock-wave structures. The special regime of an anomalous response of the material which is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities was investigated. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J13.00004: Shock front broadening in polycrystalline materials John Barber, Kai Kadau We analyze a model for the evolution of weak shock fronts (or elastic precursor waves) in polycrystalline materials. This model is based on the idea of Meyers and Carvalho [Mater. Sci. Eng. \textbf{24}, 131 (1976)] that the shock velocity anisotropy within the polycrystal is the most important factor in shock front broadening. Our analysis predicts that the shock front width increases as the 1/2 power of the front penetration distance into the crystal. Our theoretical prediction is in plausible agreement with previous experimental results for the elastic precursor rise time, and it should therefore provide a useful shock width estimate. Furthermore, our theoretical framework is also applicable to other problems involving front propagation in heterogeneous media. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J13.00005: Large Scale Molecular Dynamics Simulations of Dense Plasmas. Jerome Daligault, Guy Dimonte Plasmas are generally created and probed by depositing energy into matter, driving it far from equilibrium. Knowledge over a wide range of physical conditions of the rate at which the electronic and ionic subsystems come into thermal equilibrium is important for explicit practical purposes. The microscopic mechanisms vary with the strength of the coupling among particles and the degree of degeneracy of the electrons. Though a variety of models for the electron-ion energy equilibration rate were proposed, these models apply to specific regimes, their range of validity and the transition from one regime to another remains unclear. Molecular dynamics (MD) simulations provide a powerful tool to investigate the validity of the various models. In order to study the temperature relaxation rates over a wide range of plasma coupling, from very weakly coupled to strongly coupled, multi-million particles simulations are necessary. To this end, we have developed a parallel MD code that employs the particle-particle particle-mesh algorithm and allow the simulation of very large, complex Coulomb systems and over long time scales. We have performed detailed, multi-million particle MD simulations to investigate and shed some new light on the electron-ion energy relaxation in hot, dense plasmas. In this talk, we will describe the MD code and discuss the original results obtained for the temperature relaxation rates. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J13.00006: Effect of Crystalline Anisotropy on Shock Propagation in Sapphire. W.J. Nellis, G.I. Kanel, S.V. Razorenov, A.S. Savinykh, A.M. Rajendran The major impediment to measuring reshock temperatures is opacity induced in anvil/windows by shock. We report measured shock profiles of c-, d-, and r-cut single crystals and comparison of these mechanical responses with optical snapshots measured by Hare et al. Profiles were measured at three peak stresses and two sample thicknesses. Particle velocity histories were recorded for sapphire/LiF interfaces. VISAR waveforms are noisy as a result of heterogeneous inelastic deformation and noise depends on crystal orientation and stress amplitude. Heterogeneity is least for r-cut and most for c-cut, which correlates with observed optical heterogeneity. At 2.4 mm thickness r-cut has a three-wave structure that might indicate several elastic-wave speeds off an axis of symmetry. The small signal of the third wave might also indicate a phase transition in the small volume of the sample at higher temperatures. The Hugoniot elastic limit of c-sapphire scatters from shot to shot; scatter in the HEL of r- and d-cut are smaller. Radial pre-stressing of c-sapphire resulted in some increase of the rise time of the second wave; no significant effect of pre-stressing was observed for d- and r- samples. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J13.00007: Shock Pulse Effects in PTFE Shocked Through the Crystalline Phase II--III Transition Eric N. Brown, George T. Gray III, Philip J. Rae, Neil K. Bourne We present an experimental study of crystalline structure evolution of polytetrafluoroethylene (PTFE) due to pressure-induced phase transitions in a semi-crystalline polymer using soft-recovery, shock-loading techniques coupled with mechanical and chemical post-shock analysis. Gas-launched, plate impact experiments have been performed on pedigreed PTFE 7C, mounted in momentum-trapped, shock assemblies, with impact pressures above and below the phase II to phase III crystalline transition. Below the phase transition only subtle changes were observed in the crystallinity, microstructure, and mechanical response of PTFE. Shock loading of PTFE~7C above the phase II--III transition was seen to cause both an increase in crystallinity from 38{\%} to $\sim $53{\%} (by Differential Scanning Calorimetry, DSC) and a finer crystalline microstructure, and changed the yield and flow stress behavior. We particularly focus on the effect of pulse duration on the microstructure evolution. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J13.00008: Electrical Breakdown and Lock-On in Photoconductive Semiconductor Switch (PCSS) Devices Harold Hjalmarson, Kenneth Kambour, Fred Zutavern, Charley Myles Optically-triggered, high-power photoconductive semiconductor switches (PCSS's) using semi-insulating GaAs are being developed at Sandia Labs. These switches carry current in high carrier-density filaments. The properties of these filaments can be explained by redistribution of carrier energy caused by carrier-carrier scattering within the filament. This process enhances the impact ionization rate thus allowing these filaments to be sustained by relatively low fields, a process called lock-on. For GaAs, the sustaining field is approximately 4.5 kV/cm. For this talk, the physics mechanisms for lock-on and high-field electrical breakdown are described. Also, a continuum implementation of these physics mechanisms is used to compute the properties of these filaments. These continuum calculations are based on previous calculations in which the filament properties are computed using a Monte Carlo method to solve the steady-state Boltzmann equation. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J13.00009: Ideal Strength of Hexagonal Diamond and Related B-C-N Compounds Changfeng Chen, Yi Zhang, Zicheng Pan, Hong Sun We have performed first principles calculations on the ideal strength of h-diamond, w-BN, and $\Phi$- BC$_2$N. We have considered structural deformation under pure tensile, pure shear or biaxial stress fields. The calculated results reveal new atomistic fracture mechanism for these materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J13.00010: Equation of state of crystalline FeO from diffusion Monte Carlo simulations Jindrich Kolorenc, Lubos Mitas We investigate equation of state of stoichiometric FeO (at $T=0$ K) by means of the diffusion quantum Monte Carlo method (DMC). We find a pressure induced transition from the B1 (rocksalt) structure, which represents the ambient pressure ground state, to the inverse B8 (NiAs) lattice. Experimental evidence for such a transition is still rather controversial, being detected in some measurements and not seen in others. Our DMC estimate for transition pressure, $P=65\pm 5$ GPa, is compared to outcome of other computational approaches, such as the density functional theory combined with hybrid exchange-correlation functionals. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J13.00011: An efficient method for calculating high PT elastic constants Zhongqing Wu First-principles quasi-harmonic calculations play a very important role in mineral physics because they can accurately predict the structure and thermodynamic properties of materials at pressure and temperature conditions that are still challenging for experiments. It also enables calculations of thermoelastic properties by obtaining the second-order derivatives of the free energies with respect to strain. However, these are exceedingly demanding computations requiring thousands of large jobs running on 10$^{1}$ processors each. Here we introduce a simpler approach that requires only calculations of static elastic constants and phonon density of states for unstrained configurations. This approach decreases the computational time by more than one order of magnitude. We show results on MgO and forsterite that are in very good agreement with previous first-principles results and experimental data. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J13.00012: Radiation in Particle Simulations of Hot Dense Matter Frank Graziani, Richard More The variety of complex processes that take place in hot dense radiative plasmas where temperatures are in excess of several keV and densities are higher than metals, has forced computational physicists in ICF and astrophysics to make a number of assumptions regarding how to model non-equilibrium plasmas undergoing thermal relaxation.~ In order to make the simulations feasible, variations on the Landau-Spitzer model are frequently invoked. There has been recent work on the theoretical properties of thermal relaxation in such plasmas, but there is controversy due to the various approximations needed to make the calculations tractable. Experimental validations in the regimes of interest are prohibitive. Direct Numerical Simulation (DNS) of the many-body interactions of plasmas is a promising approach to model validation but unfortunately, previous work either relies on the collisionless approximation or radiation is entirely absent. We present a new numerical simulation capability that will address a currently unsolved problem: the extension of molecular dynamics to collisional plasmas where Brehmstrahlung and Compton scattering are present. This new tool provides a method for assessing the accuracy of energy and momentum exchange models in hot dense plasmas. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J13.00013: Trajectories and escape rates from a collapsing basin: dependence on the rate of collapse of the basin Antonio Cadilhe, A.F. Voter It has been shown that trajectories with different initial conditions synchronize in harmonic basins during Langevin-thermostatted molecular dynamics under the same time-noise sequence. Synchronization leads to exponential trajectory coalescence onto a single master trajectory [1]. We present our preliminary results extending previous research to include time dependent harmonic potentials, of the form $V (x, t)= A(1-e^{\alpha t})V_o$, where $A$, $\alpha$, and $V_o$ are constants, which are relevant for understanding the dynamics of driven systems. We present analytical results for the synchronization behavior in the various regimes arising in this context. We also complement the study with numerical estimates of the escape rates for basins with escape paths and compare them to static, conventional rates of escape. [1] Blas P. Uberuaga, Marian Anghel, and Arthur F. Voter, J. Chem. Phys. 14, 6363 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J13.00014: Momentum Transfer in Soft X-ray - Induced Shock Loading of Meteorite and Planetary Materials John Remo, Michael Furnish The response of meteorite and planetary materials to high- intensity $<$1 keV x-rays from Z-pinch sources is described. These materials include iron and stony meteorites, magnesium rich olivine (dunite), and Al and Fe calibration samples. Input stresses varied from 6.1 to 12.4 GPa, attenuating to $\sim$1.4 to 2.5 GPa for the iron meteorites, $\sim$0.3 to 1.9 GPa for the stony meteorites, and 1.64 to 1.91 GPa for dunite. The calibration (pure) metals showed less attenuation than the highly inhomogeneous natural materials: 9.5 to $\sim$5 GPa for Fe and 12.4 to 10.6 GP for Al. Methods for deducing momentum and energy coupling into these materials from the radiation are discussed. These data are useful for planetary and astrophysical modeling and for near-Earth object mitigation studies requiring momentum coupling, and momentum enhancement coefficients. [Preview Abstract] |
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