Forum Schedule Spring 2019

The response of nitrogen to extreme conditions has attracted great interest since the predictions that nitrogen would transform into a nonmolecular phase at pressures less than 1 Mbar. As a result, the experimental phase behavior of nitrogen has been well characterized. Of particular interest was the identification of the cubic-gauche single bonded polymeric structure at high pressures and temperatures (>150 GPa, 2000K), which was found to be metastable at room temperature to pressures near 40 GPa. Nitrogen is of particular interest due to its promising potential as a high-energy-density material and it has been suggested its energy release may be roughly three times that of a traditional energetic materials. However, considerable challenges exist before such a capability could be realized, such as addressing the significant pressure-temperature barrier to transition from the molecular to non-molecular form, the conditions of which are not amenable to larger scale production technique; additionally, methods must be developed to increase the metastability to ambient conditions. In an effort to increase the metastability of the extended solid, recent studies have focused on mixing, or doping, the nitrogen with small amounts of secondary gases, such as hydrogen or carbon monoxide. It was been postulated the secondary gas would passivate the terminal ends thus increasing the stability of the nitrogen extended solid. Our group was the first to demonstrate such an approach could be used successfully to decrease the transition pressure for the formation of the nitrogen extended solid through doping with hydrogen. Although recent studies on nitrogen/hydrogen mixtures by other research groups have also observed several non-molecular nitrogen/hydrogen structures, recovery of these materials to ambient conditions has not yet been demonstrated. In this talk, I will describe our progress in the study of the synthesis, characterization, and recovery of extended solids of nitrogen from high pressure conditions from nitrogen/carbon monoxide mixtures. I will also detail results from our closely coupled modeling and simulation efforts and discuss how these results help guide our experimental efforts. New opportunities and challenges that have arisen in the course of our studies that will be pursued in the future will also be presented.

The outskirts of clusters make the best, and most efficient locations to observe and trace the mass assembly processes of the Cosmic Web. Residing at the vertices of this Cosmic Web (Bond et al. 1996), galaxy clusters grow by steady accretion of matter from the surroundings, as well as by discrete mergers with nearby groups and clusters. Supported by simulations, this scenario predictions regarding the total mass content and distribution in filaments themselves remain largely untested. Filaments are vital elements of the cosmic census, containing up to half the baryonic mass of the Universe as a ‘warm hot intergalactic medium’ but also the majority of the dark matter.

Recently, some of the most massive and disturbed clusters have been the centre of attention thanks to the Hubble Frontier Fields (HFF) initiative, which constitutes the largest commitment ever of Hubble Space Telescope (HST) time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal target to trace the Cosmic Web assembly.

While combining strong- and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the substructures engaged in these processes. I will present the latest results we obtained on the HFF clusters, and discuss the different caveats present on both the observing and simulation sides. Finally, I will present the upcoming BUFFALO large HST programme, the ’spatial extension’ of the HFF that has just started back in July 2018.

Meteorites are time capsules of planet formation. The most abundant meteorites types originate from primitive bodies that never heated to the point of differentiation and contain chondrules, which were transiently molten silicate spherules. These primitive planetesimals formed contemporaneously with planets over the first few million years of the solar system. The physical processes that formed chondrules and assembled them into planetesimals is one of the biggest unsolved problems in planetary science. I will present a new physical model for the formation of chondrules and planetesimals that links their origin to the dynamical excitation by the giant planets. I propose that the uncertain history of our giant planets, their formation locations and migration distances, was recorded by planetesimals and preserved in the asteroid belt. Meteorites are the Rosetta stones of planet formation that can translate the history of plantesimals to the history of the giant planets.

The initial mass function of stars - the number of stars N formed per interval of mass M - has long been known to be a power law, with power index of -1.3. Many explanations of this distribution have been advanced over the years, including fractal dimensions of star-forming molecular clouds and/or self-similar grouping of clouds. I will discuss simplified numerical simulations of star formation which show that gravitational accretion onto initial seed masses generates a power law mass distribution approaching power index of -1 asymptotically, irrespective of complications of cloud structure. This mechanism can also explain the mass function of young star clusters, which also has a power law distribution with the index of -1. Finally, I will describe preliminary results concerning the origin of protostellar cloud angular momenta, with implications for the properties of protoplanetary disks that result from cloud collapse.

Ultra-high energy cosmic rays represent the most energetic particles in Nature. Their origin is extragalactic, but remains mysterious. I will describe two experiments dedicated to resolving this mystery: the Telescope Array project in Utah and the Auger observatory in Argentina. Our current understanding of these particles will be reviewed as well as future efforts to deepen our understanding.

The growing evidence for energy-conserving outflows in powerful and luminous AGN supports the idea that high-velocity winds launched from the accretion disc evolve after undergoing a shock with the ambient medium, and that they are capable to expel enough mass and energy so as to impact the host galaxy and produce the so-called AGN feedback often invoked in galaxy formation and evolution.

This talk will give an overview of AGN Ultra Fast Outflows with focus on recent results obtained from grating X-ray spectra of bright sources, that are unveiling a more complex structure of these winds than previously thought. I will review how UFO work, their observational properties and their relation with AGN outflows in other bands, what is their impact on the host galaxies and their role in feedback processes.

In particular, I will present the case for a multi-phase outflow in the Narrow Line Seyfert 1 Galaxy IRAS17020+4544 spanning from accretion disk to galaxy-scale, which has been targeted by an unprecedented multi-wavelength campaign including XMM-Newton, Chandra/LETG,VLBI, LMT, NOEMA, HST/COS.

Unlike a localized state induced by defect in a crystalline solid, the so-called flat band is truly a Bloch state but yet without band dispersion. It arises from destructive interference of Bloch wave functions, independent of the single-particle crystalline Hamiltonian (i.e., the strength of lattice hopping). Flat band can host a range of exotic quantum phases, such as ferromagnetism, Wigner crystallization, superconductivity and high-temperature fractional quantum Hall effect. In this talk, I will first briefly review lattice models which give rise to 2D/3D flat bands including a new Coloring-Triangle lattice we found recently. I will then discuss real flat-band materials including our recent work on pyrochlore Sn₂Nb₂O₇ which hosts 3D flat bands. A novel mechanism for the formation of Weyl points enabled by doping of doubly degenerated 3D flat bands, instead of symmetry breaking of a Dirac point, will also be discussed.

Angular momentum transport by magnetic fields is the key physical process in star and disk formation processes. Non-ideal magnetohydrodynamic effects such as Ohmic dissipation and ambipolar diffusion extract magnetic flux and suppress angular momentum transport. Using MHD simulations we found that these effects can resolve the so-called magnetic braking catastrophe in the early phase of star formation. We also investigated the long-term evolution of the disk using a sink particle technique. As the accretion continues, the disk acquires larger angular momentum and grows in the size and mass. We compared our simulations with recent ALMA observations by synthetic observations, and found that our models are in good agreement with some objects.

Connecting planetary systems at different stages of stellar evolution helps us understand their formation, evolution, and fate, and provides us with exclusive and crucial insights about their dynamics and chemistry. Post-main-sequence white dwarf and giant branch stars host planetary systems which include a variety of observed objects and phenomena, such as planetary debris discs, disintegrating and embedded asteroids, and photospheric metal pollution. Here, I provide a review of both our current knowledge of these systems and models which have been used to explain them. I also highlight the transformative advances expected in upcoming years with the current and next generation of ground-based and space-based initiatives. Looming orders-of-magnitude increases in available data must be accompanied by novel theories and simulations in order to understand the results from this interdisciplinary and expanding research field.

JARVIS (Joint Automated Repository for Various Integrated Simulations) is a NIST-developed, unique integrated framework to accelerate material design using classical force-fields (FF), density functional theory (DFT), machine learning (ML) calculations and benchmark experiments (Exp). The JARVIS-DFT hosts data for more than 35000 materials, both bulk and monolayer counterparts. We identified more than 1500 2D materials using lattice parameter criteria and exfoliation energy calculations. We charted improved lattice parameters, formation energies and elastic tensors using van der Waals functional for more than 12000 materials and established relations between exfoliation energies and elastic constants. To alleviate band gap underestimation in conventional DFT and to improve frequency dependent dielectric function predictions, we re-evaluated band gaps and dielectric functions using meta-GGA based approaches for more than 10000 materials. We used a spectroscopic limited maximum efficiency (SLME) approach to identify potential photovoltaic materials. Using spin-orbit spillage criteria, we identified more than 1500 potential topological materials including topological insulators, Weyland Dirac semimetals, and topological crystalline insulators. We also used data-driven approaches to identify novel 3D and 2D thermoelectric materials. Finally, the data from the JARVIS-DFT and JARVIS-FF are subjected to classical force-field inspired descriptors (CFID) and gradient boosting decision trees (GBDT) to produce highly accurate machine learning models. The ML models can significantly alleviate the computational cost for conventional computational methods. The database and tools are publicly available at https://jarvis.nist.gov/.

That properties of pure water can best be rationalized as a mixture of distinct fluid phases is not a new idea. The motivations for this concept and the contributions of recent high-pressure work in our group are the focus in this talk.

Our measurements of thermodynamic and transport properties of water and aqueous electrolytic solutions extend to high pressures over a range of temperatures. Determinations of thermal diffusivities, viscosities, and sound speeds to 10 GPa and to 800 K are possible in diamond anvil cells. We make highly accurate (100 ppm) sound speed measurements to 0.7 GPa for temperatures from 250 K to 360 K. From such data, we construct Gibbs energy representations that can be differentiated to give accurate thermodynamic properties.

The emerging view based on the interpretation of the high-pressure data reinforces the long-standing notion that water has multiple “faces” depending on pressure, temperature, and solute concentration. At low pressures for temperatures below ambient, water favors a local four-fold coordination associated with hydrogen-bonded molecules. With increasing temperature, pressure, or solute concentration, the local coordination of water molecules increases. Above 1 GPa water behaves more as a “simple” (nearly close-packed) fluid. In the intermediate pressure regime (near or below 1 GPa), water has a strong temperature dependence of local coordination, increasing with decreasing temperature below ambient. The complex electrolytic chemistry of aqueous solutions found near ambient pressures is quenched by high pressure. Above 1 GPa, electrolytic solutions are essentially “ideal” with volumes of mixing equal to intrinsic ion sizes.

A more speculative idea follows. The presence of several amorphous ice phases in the far supercooled regime and the anomalous properties of water at low pressure and low temperature have previously provided evidence for a hypothetical lower critical point. Water might separate into two distinct fluids in a deeply metastable regime of temperature. The existence of a high pressure (very dense) amorphous ice phase and the behavior observed in our new high-pressure equation of state for water can be interpreted as requiring an additional (higher pressure) critical point.

Recent progress in global simulation of pulsar magnetospheres is changing our models of pulsar particle acceleration, cascade pair production and high-energy emission. Simulation of a force-free pulsar magnetosphere 15-20 years ago marked a major advance in understanding the current closure and the structure of the wind. The first simulations of dissipative MHD pulsar magnetospheres with finite conductivity opened the door to modeling particle acceleration and radiation on a global scale. For simulations having very high conductivity, as would be expected for young and energetic pulsars, most of the high-energy emission comes from outside the light cylinder near the current sheet. Most recently, particle-in-cell simulations that couple particle dynamics to field structure are the next step in self-consistent pulsar magnetosphere modeling. The radiation characteristics in models based on these simulations match those of gamma-ray pulsars detected by Fermi. I will also discuss whether gamma-ray millisecond pulsars can explain the excess emission at the Galactic center and a model for the multi-TeV emission recently detected from the Vela pulsar by HESS.

The Event Horizon Telescope (EHT) is a global effort to construct an Earth-sized virtual radio telescope array, with the ultimate goal to actually make pictures of some nearby supermassive black holes. The initial results of the first full EHT observing run in April 2017 were presented at six press conferences on 10 April 2019. In my talk I will overview the EHT results with focus on numerical modeling of black holes and physical interpretation of the asymmetric ring.