J. H. Eggert

Ramp compression of diamond to five terapascals

The recent discovery of more than a thousand planets outside our Solar System, together with the significant push to achieve inertially confined fusion in the laboratory, has prompted a renewed interest in how dense matter behaves at millions to billions of atmospheres of pressure. The theoretical description of such electron-degenerate matter has matured since the early quantum statistical model of Thomas and Fermi, and now suggests that new complexities can emerge at pressures where core electrons (not only valence electrons) influence the structure and bonding of matter. Recent developments in shock-free dynamic (ramp) compression now allow laboratory access to this dense matter regime. Here we describe ramp-compression measurements for diamond, achieving 3.7-fold compression at a peak pressure of 5 terapascals (equivalent to 50 million atmospheres). These equation-of-state data can now be compared to first-principles density functional calculations and theories long used to describe matter present in the interiors of giant planets, in stars, and in inertial-confinement fusion experiments. Our data also provide new constraints on mass–radius relationships for carbon-rich planets.

High strain-rate plastic flow in Al and Fe

Thin Fe and Al foils were ramp-compressed over several to tens of ns timescales to study the time-dependence associated with the onset of plastic flow. Peak stress states of 15–200 GPa were achieved through laser ramp-compression where the strain rate was varied, shot-to-shot, between 106 to 108 s−1. Our data combined with data from other dynamic compression platforms reveals a strong correlation between the peak elastic precursor stress, σE, and the strain rate at the onset of plastic flow, ɛ·p. In fcc Al, phonon drag dislocation flow dominates above ɛ·p~103s-1 and σE ∼ 0.03 GPa where σE scales as ɛ·p0.43. By contrast, the Al alloy 6061-T6 exhibits a relatively weak dependency of σE with ɛ·p up to strain rates of ∼107 s−1. Our Fe data, reveals a sharp increase in σE at ɛ·p>5×106s-1. This is consistent with a transition in plastic flow to a phonon drag regime.

Time-dependence of the alpha to epsilon phase transformation in iron

Iron was ramp-compressed over timescales of 3 ≤ t(ns) ≤ 300 to study the time-dependence of the α→e (bcc→hcp) phase transformation. Onset stresses (σα→e)  for the transformation ∼14.8-38.4 GPa were determined through laser and magnetic ramp-compression techniques where the transition strain-rate was varied between 106 ≤μα→e(s−1) ≤ 5×108. We find σα→e= 10.8 + 0.55 ln(μα→e) for μα→e   106/s. This μ response is quite similar to recent results on incipient plasticity in Fe [Smith et al., J. Appl. Phys. 110, 123515 (2011)] suggesting that under high rate ramp compression the α→e phase transition and plastic deformation occur through similar mechanisms, e.g., the rate limiting step for μ > 106/s is due to phonon scattering from defects moving to relieve strain. We show that over-pressurization of equilibrium phase boundaries is a common feature exhibited under high strain-rate compression of many materials encompassing many orders of magnitude of strain-rate.

Precision equation-of-state measurements on National Ignition Facility ablator materials from 1 to 12 Mbar using laser-driven shock waves

A large uncertainty in the design of ignition capsules for use in the National Ignition Campaign (NIC) is the ablator equation of state. In this article, we report equation-of-state measurements for two candidate NIC ablator materials, glow-discharge polymer (GDP), and germanium-doped GDP. These materials were driven to pressures of 1 to 12 Mbar using laser-driven shock waves. Hugoniot measurements were obtained using the impedance matching technique with an α-quartz standard. This article presents the first kinematic measurements in the high-pressure fluid regime for these materials, which show to be in close agreement with Livermore equation-of-state model predictions.

Heterogeneous flow and brittle failure in shock-compressed silicon

We combine a recently developed high-resolution two-dimensional (2D) imaging velocimetry technique (velocity interferometer system for any reflector (VISAR)) with 1D VISAR measurements to construct a moving picture of heterogeneous deformation in shock-compressed single crystal silicon. The 2D VISAR takes an intensity snapshot of target velocity and reflectivity over a mm field-of-view while the compression history is simultaneously recorded by the 1D VISAR. Our data show particle velocity surface roughening due to the anisotropic onset of plasticity and, above ∼13 GPa, a structural phase transformation. Shock arrival at the Si free-surface is characterized by the formation of fracture networks and incipient velocity jetting.

Spectroscopic Studies of p-H2 to Above 200 GPa

Raman and infrared vibrational spectra of H2have been measured to pressures in excess of 200 GPa and at liquid helium temperatures using new high sensitivity techniques. Detailed study of the pressure dependence of o-p conversion rate reveals an initial increase followed by a decrease above 1 GPa. The conversion rate then increases dramatically with pressure, and this continues to above 50 GPa. New sets of vibron, phonon, roton, and libron excitations in converted para samples are documented as a function of pressure through phases I, II, and III. The results provide important information on the crystal structures, molecular orientational state, and vibrational dynamics of the high-pressure phases.

Influence of order-disorder on the vibron excitations of H2 and D2 in ortho-para mixed crystals

We present a detailed experimental and theoretical study of the vibrons in ortho-para mixed crystals of solid hydrogen and deuterium at ambient and high pressure. Experimental results were obtained at ambient pressure and T=6–7 K (e.g., for hydrogen samples having ortho fractions of 19–62%) using high-resolution Fabry-Perot techniques, and at high pressure and T=77 K (e.g., hydrogen 50–50% ortho-para samples) using dispersive spectrographic techniques with diamond-anvil cells. The numerical calculations are based on the James and Van Kranendonk theory, and were performed by exactly diagonalizing the Hamiltonian for a large supercell of randomly placed molecular “species” on a crystalline lattice. Overall, excellent agreement between theory and experiment is obtained. The calculations show that disorder leads to Anderson localized vibrons for many of the pressures and concentrations studied experimentally and that a substantial portion of the Raman intensity is derived from these localized vibrons. We also calculate the species characteristics of the individual Raman peaks, the results of which suggest an explanation for the previously noted disagreement between experimental high-pressure results and the predictions of the van Kranendonk theory. Specifically, our analysis indicates that the higher frequency peak is associated with anisotropic scattering arising from partial alignment of J=1 angular momenta with respect to the crystallographic axes. Finally, our calculations show that the observed doublet structure in the lower frequency Raman peak for deuterium at low para (J=1) concentrations is well represented by added (para-molecule) diagonal terms in the van Kranendonk Hamiltonian that are plausibly associated with electric quadrupole-quadrupole interactions.

Charge transfer and electron-vibron coupling in dense solid hydrogen

Abstract We examine the role of charge transfer (CT) interactions in dense molecular hydrogen in relation to recently observed spectroscopic properties at megabar pressures. Specifically, we consider virtual H 2 + H 2 − states in which an electron is transferred to a neighbor. The Mulliken CT integral t admixes H 2 + H 2 − fluctuations of overlapping molecules. The amplitude of the charge fluctuations is the ionicity γ, which is found in H 2 dimers and lattices with nonoverlapping valence and conduction bands for t small compared to the CT excitation energy ω CT . Vibronic coupling within a dimer is found using a Herzberg-Teller expansion and gives explicit expressions for vibron shifts in Raman and IR spectra and for IR oscillator strength due to electron-vibron coupling. We estimate linear electron-vibron coupling constants and other parameters required to interpret the vibrational data at and above the 150-GPa transition of dense hydrogen within a CT model. We discuss important structural implications of equal IR and Raman shifts at the transition and relate anisotropic CT processes to the orientational state of the dense molecular solid.

Computations of Vibron Excitations and Raman Spectra of Solid Hydrogens

Numerical calculations for the vibronic states in the mixed ortho-para solid hydrogens are reviewed. They were performed within the supercell method on the basis of the Van Kranendonk theory and the Raman spectra were calculated and compared with experiment. Anderson localization is a feature of the results especially in the Raman scattering region of frequencies and especially when the ortho-para mixture is substantial. A simple approximate Hamiltonian model has been used and is surprisingly useful, leading to a suggestion that at high pressures the high frequency peak of the vibron spectrum of a mixed system is due to anisotropic Raman scattering from the ortho (para) hydrogen (deuterium) molecules, even under the assumption that the orbital angular momenta are disoriented.

Erratum: Measurement of Body-Centered-Cubic Aluminum at 475 GPa [Phys. Rev. Lett. 119 , 175702 (2017)]

This corrects the article DOI: 10.1103/PhysRevLett.119.175702.