Eric Sembrat's Test Bonanza

Somewhere out in the cosmos an ordinary galaxy spins, seemingly at slumber. Then all of a sudden, WHAM! A flash of light explodes from the galaxy's center. A star orbiting too close to the event horizon of the galaxy's central supermassive black hole has been torn apart by the force of gravity, heating up its gas and sending out a beacon to the far reaches of the universe.

In a universe with tens of billions of galaxies, how would we see it? What would such a beacon look like? And how would we distinguish it from other bright, monumental intergalactic events, such as supernovas?

"Black holes by themselves do not emit light," said Tamara Bogdanovic, an assistant professor of physics at the Georgia Institute of Technology. "Our best chance to discover them in distant galaxies is if they interact with the stars and gas that are around them."

In recent decades, with improved telescopes and observational techniques designed to repeatedly survey the vast numbers of galaxies in the sky, scientists noticed that some galaxies that previously looked inactive would suddenly light up at their very center.

"This flare of light was found to have a characteristic behavior as a function of time. It starts very bright and its luminosity then decreases in time in a particular way," she explained. "Astronomers have identified those as galaxies where a central black hole just disrupted and 'ate' a star. It's like a black hole putting up a sign that says 'Here I am.'"

Using a mix of theoretical and computer-based approaches, Bogdanovic tries to predict the dynamics of events such as the black-hole-devouring-star scenario described above, also known as a "tidal disruption." Such events would have a distinct signature to someone analyzing data from a ground-based or space-based observatory.

Using National Science Foundation-funded supercomputers at the Texas Advanced Computing Center (Stampede) and the National Institute for Computational Sciences (Kraken), Bogdanovic and her collaborators recently simulated the dynamics of these super powerful forces and charted their behavior using numerical models.

Tidal disruptions are relatively rare cosmic occurrences. Astrophysicists have calculated that a Milky Way-like galaxy stages the disruption of a star only once in about 10,000 years. The luminous flare of light, on the other hand, can fade away in only a few years. Because it is such a challenge to pinpoint tidal disruptions in the sky, astronomical surveys that monitor vast numbers of galaxies simultaneously are crucial.

Huge difference

So far, only a few dozen of these characteristic flare signatures have been observed and deemed "candidates" for tidal disruptions. But with data from PanSTARRS, Galex, the Palomar Transient Factory and other upcoming astronomical surveys becoming available to scientists, Bogdanovic believes this situation will change dramatically.

"As opposed to a few dozen that have been found over the past 10 years, now imagine hundreds per year--that's a huge difference!" she said. "It means that we will be able to build a varied sample of stars of different types being disrupted by supermassive black holes."

With hundreds of such events to explore, astrophysicists' understanding of black holes and the stars around them would advance by leaps and bounds, helping determine some key aspects of galactic physics.

"A diversity in the type of disrupted stars tells us something about the makeup of the star clusters in the centers of galaxies," Bodganovic said. "It may give us an idea about how many main sequence stars, how many red giants, or white dwarf stars are there on average."

Tidal disruptions also tell us something about the population and properties of supermassive black holes that are doing the disrupting.

"We use these observations as a window of opportunity to learn important things about the black holes and their host galaxies," she continued. "Once the tidal disruption flare dims below some threshold luminosity that can be seen in observations, the window closes for that particular galaxy."

Role of supercomputer

In a recent paper submitted to the Astrophysical Journal, Bogdanovic, working with Roseanne Cheng (Center for Relativistic Astrophysics at Georgia Tech) and Pau Amaro-Seoane (Albert Einstein Institute in Potsdam, Germany), considered the tidal disruption of a red giant star by a supermassive black hole using computer modeling.

The paper comes on the heels of the discovery of a tidal disruption event in which a black hole disrupted a helium-rich stellar core, thought to be a remnant of a red giant star, named PS1-10jh, 2.7 billion light years from Earth.

The sequence of events they described aims to explain some unusual aspects of the observational signatures associated with this event, such as the absence of the hydrogen emission lines from the spectrum of PS1-10jh.

As a follow-up to this theoretical study, the team has been running simulations on Kraken and Stampede, as well as the Georgia Tech's high performance computing clusters. The simulations reconstruct the chain of events by which a stellar core, similar to the remnant of a tidally disrupted red giant star, might evolve under the gravitational tides of a massive black hole.

"Calculating the messy interplay between hydrodynamics and gravity is feasible on a human timescale only with a supercomputer," Cheng said. "Because we have control over this virtual experiment and can repeat it, fast forward, or rewind as needed, we can examine the tidal disruption process from many perspectives. This in turn allows us to determine and quantify the most important physical processes at play."

The research shows how supercomputer simulations complement and constrain theory and observation.

"There are many situations in astrophysics where we cannot get insight into a sequence of events that played out without simulations. We cannot stand next to the black hole and look at how it accretes gas. So we use simulations to learn about these distant and extreme environments," Bogdanovic said.

One of Bogdanovic's goals is to use the knowledge gained from simulations to decode the signatures of observed tidal disruption events.

"The most recent data on tidal disruption events is already outpacing theoretical understanding and calling for the development of a new generation of models," she explained. "The new, better quality data indicates that there is a great diversity among the tidal disruption candidates. This is contrary to our perception, based on earlier epochs of observation, that they are a relatively uniform class of events. We have yet to understand what causes these differences in observational appearance, and computer simulations are guaranteed to be an important part of this journey."

-- Written by Aaron Dubrow of the National Science Foundation.

 Dr. Uzi Landman's project recognized in recent edition of journal, Nature Materials and others.

www.research.gatech.edu/news/self-assembled-silver-superlattices-create-molecular-machines-hydrogen-bond-%E2%80%9Chinges%E2%80%9D-and-moving

A combined computational and experimental study of self-assembled silver-based structures known as superlattices has revealed an unusual and unexpected behavior: arrays of gear-like molecular-scale machines that rotate in unison when pressure is applied to them.

Computational and experimental studies show that the superlattice structures, which are self-assembled from smaller clusters of silver nanoparticles and organic protecting molecules, form in layers with the hydrogen bonds between their components serving as “hinges” to facilitate the rotation. Movement of the “gears” is related to another unusual property of the material: increased pressure on the superlattice softens it, allowing subsequent compression to be done with significantly less force.

Materials containing the gear-like nanoparticles – each composed of nearly 500 atoms – might be useful for molecular-scale switching, sensing and even energy absorption. The complex superlattice structure is believed to be among the largest solids ever mapped in detail using a combined X-ray and computational techniques.

“As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change,” said Uzi Landman, a Regents’ and F.E. Callaway professor in the School of Physics at the Georgia Institute of Technology. “When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed.”

The gears rotate as much as 23 degrees, and return to their original position when the pressure is released. Gears in alternating layers move in opposite directions, said Landman, who is director of the Center for Computational Materials Science at Georgia Tech.

Supported by the Air Force Office of Scientific Research and the Office of Basic Energy Sciences in the Department of Energy, the research was reported April 6 in the journal Nature Materials. Researchers from Georgia Tech and the University of Toledo collaborated on the project.

The research studied superlattice structures composed of clusters with cores of 44 silver atoms each. The silver clusters are protected by 30 ligand molecules of an organic material – mercaptobenzoic acid (p-MBA) – that includes an acid group. The organic molecules are attached to the silver by sulfur atoms.

“It’s not the individual atoms that form the superlattice,” explained Landman. “You actually make the larger structure from clusters that are already crystallized. You can make an ordered array from those.”

In solution, the clusters assemble themselves into the larger superlattice, guided by the hydrogen bonds, which can only form between the p-MBA molecules at certain angles.

“The self-assembly process is guided by the desire to form hydrogen bonds,” Landman explained. “These bonds are directional and cannot vary significantly, which restricts the orientation that the molecules can have.”

The superlattice was studied first using quantum-mechanical molecular dynamics simulations conducted in Landman’s lab. The system was also studied experimentally by a research group headed by Terry Bigioni, an associate professor in the Department of Chemistry and Biochemistry at the University of Toledo.

The unusual behavior occurred as the superlattice was being compressed using hydrostatic techniques. After the structure had been compressed by about six percent of its volume, the pressure required for additional compression suddenly dropped significantly. The researchers discovered that the drop occurred when the nanocrystal components rotated, layer-by-layer, in opposite directions.

Just as the hydrogen bonds direct how the superlattice structure is formed, so also do they guide how the structure moves under pressure.

“The hydrogen bond likes to have directionality in its orientation,” Landman explained. “When you press on the superlattice, it wants to maintain the hydrogen bonds. In the process of trying to maintain the hydrogen bonds, all the organic ligands bend the silver cores in one layer one way, and those in the next layer bend and rotate the other way.”

When the nanoclusters move, the structure pivots about the hydrogen bonds, which act as “molecular hinges” to allow the rotation. The compression is possible at all, Landman noted, because the crystalline structure has about half of its space open.

The movement of the silver nanocrystallites could allow the superlattice material to serve as an energy-absorbing structure, converting force to mechanical motion. By changing the conductive properties of the silver superlattice, compressing the material could also allow it be used as molecular-scale sensors and switches.  

The combined experimental and computation study makes the silver superlattice one of the most thoroughly studied materials in the world.

“We now have complete control over a unique material that by its composition has a diversity of molecules,” Landman said. “It has metal, it has organic materials and it has a stiff metallic core surrounded by a soft material.”

For the future, the researchers plan additional experiments to learn more about the unique properties of the superlattice system. The unique system shows how unusual properties can arise when nanometer-scale systems are combined with many other small-scale units.

“We make the small particles, and they are different because small is different,” said Landman. “When you put them together, having more of them is different because that allows them to behave collectively, and that collective activity makes the difference.”

In addition to those already mentioned, Georgia Tech co-authors included research scientist Bokwon Yoon – the paper’s first author – and senior research scientists W.David Luedtke, Robert Barnett and Jianping Gao. Co-authors from the University of Toledo include Anil Desireddy and Brian E. Conn.

This research was supported by the Air Force Office of Scientific Research (AFOSR), and by the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) under Contract FG05-86ER45234. Any conclusions or opinions expressed are those of the authors and do not necessarily represent the official views of the AFOSR or the DOE.

CITATION: Bokwon Yoon, et al., “Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice.” (Nature Materials, 2014). http://dx.doi.org/ 10.1038/NMAT3923.

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