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News Article | December 20, 2016
Site: www.eurekalert.org

UPTON, NY-Scientists studying high temperature superconductors-materials that carry electric current with no energy loss when cooled below a certain temperature-have been searching for ways to study in detail the electron interactions thought to drive this promising property. One big challenge is disentangling the many different types of interactions-for example, separating the effects of electrons interacting with one another from those caused by their interactions with the atoms of the material. Now a group of scientists including physicists at the U.S. Department of Energy's Brookhaven National Laboratory has demonstrated a new laser-driven "stop-action" technique for studying complex electron interactions under dynamic conditions. As described in a paper just published in Nature Communications, they use one very fast, intense "pump" laser to give electrons a blast of energy, and a second "probe" laser to measure the electrons' energy level and direction of movement as they relax back to their normal state. "By varying the time between the 'pump' and 'probe' laser pulses we can build up a stroboscopic record of what happens-a movie of what this material looks like from rest through the violent interaction to how it settles back down," said Brookhaven physicist Jonathan Rameau, one of the lead authors on the paper. "It's like dropping a bowling ball in a bucket of water to cause a big disruption, and then taking pictures at various times afterward," he explained. The technique, known as time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES), combined with complex theoretical simulations and analysis, allowed the team to tease out the sequence and energy "signatures" of different types of electron interactions. They were able to pick out distinct signals of interactions among excited electrons (which happen quickly but don't dissipate much energy), as well as later-stage random interactions between electrons and the atoms that make up the crystal lattice (which generate friction and lead to gradual energy loss in the form of heat). But they also discovered another, unexpected signal-which they say represents a distinct form of extremely efficient energy loss at a particular energy level and timescale between the other two. "We see a very strong and peculiar interaction between the excited electrons and the lattice where the electrons are losing most of their energy very rapidly in a coherent, non-random way," Rameau said. At this special energy level, he explained, the electrons appear to be interacting with lattice atoms all vibrating at a particular frequency-like a tuning fork emitting a single note. When all of the electrons that have the energy required for this unique interaction have given up most of their energy, they start to cool down more slowly by hitting atoms more randomly without striking the "resonant" frequency, he said. The frequency of the special lattice interaction "note" is particularly noteworthy, the scientists say, because its energy level corresponds with a "kink" in the energy signature of the same material in its superconducting state, which was first identified by Brookhaven scientists using a static form of ARPES [see: https:/ ]. Following that discovery, many scientists suggested that the kink might have something to do with the material's ability to become a superconductor, because it is not readily observed above the superconducting temperature. But the new time-resolved experiments, which were done on the material well above its superconducting temperature, were able to tease out the subtle signal. These new findings indicate that this special condition exists even when the material is not a superconductor. "We know now that this interaction doesn't just switch on when the material becomes a superconductor; it's actually always there," Rameau said. The scientists still believe there is something special about the energy level of the unique tuning-fork-like interaction. Other intriguing phenomena have been observed at this same energy level, which Rameau says has been studied in excruciating detail. It's possible, he says, that the one-note lattice interaction plays a role in superconductivity, but requires some still-to-be-determined additional factor to turn the superconductivity on. "There is clearly something special about this one note," Rameau said. Work at Brookhaven National Laboratory was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center headquartered at Brookhaven National Laboratory and funded by the DOE Office of Science. Additional funding was provided by the National Science Foundation, the Aspen Center for Physics, the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory, and by the McDevitt bequest at Georgetown University. Computational resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility headquartered at Lawrence Berkeley National Laboratory. Additional support came from Deutsche Forschungsgemeinschaft, the Mercator Research Center Ruhr, and from the European Union within the seventh Framework Program. Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov. One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization. Scientific paper: "Energy Dissipation from a Correlated System Driven Out of Equilibrium" An electronic version of this news release with related graphics is available online.


Arnett W.D.,University of Arizona | Arnett W.D.,Aspen Center for Physics | Meakin C.,University of Arizona | Meakin C.,Los Alamos National Laboratory | And 5 more authors.
Astrophysical Journal | Year: 2015

We examine the physical basis for algorithms to replace mixing-length theory (MLT) in stellar evolutionary computations. Our 321D procedure is based on numerical solutions of the Navier-Stokes equations. These implicit large eddy simulations (ILES) are three-dimensional (3D), time-dependent, and turbulent, including the Kolmogorov cascade. We use the Reynolds-averaged Navier-Stokes (RANS) formulation to make concise the 3D simulation data, and use the 3D simulations to give closure for the RANS equations. We further analyze this data set with a simple analytical model, which is non-local and time-dependent, and which contains both MLT and the Lorenz convective roll as particular subsets of solutions. A characteristic length (the damping length) again emerges in the simulations; it is determined by an observed balance between (1) the large-scale driving, and (2) small-scale damping. The nature of mixing and convective boundaries is analyzed, including dynamic, thermal and compositional effects, and compared to a simple model. We find that (1) braking regions (boundary layers in which mixing occurs) automatically appear beyond the edges of convection as defined by the Schwarzschild criterion, (2) dynamic (non-local) terms imply a non-zero turbulent kinetic energy flux (unlike MLT), (3) the effects of composition gradients on flow can be comparable to thermal effects, and (4) convective boundaries in neutrino-cooled stages differ in nature from those in photon-cooled stages (different Péclet numbers). The algorithms are based upon ILES solutions to the Navier-Stokes equations, so that, unlike MLT, they do not require any calibration to astronomical systems in order to predict stellar properties. Implications for solar abundances, helioseismology, asteroseismology, nucleosynthesis yields, supernova progenitors and core collapse are indicated. © 2015. The American Astronomical Society. All rights reserved..


Smith N.,University of Arizona | Arnett W.D.,University of Arizona | Arnett W.D.,University of California at Santa Barbara | Arnett W.D.,Aspen Center for Physics
Astrophysical Journal | Year: 2014

Both observations and numerical simulations are discordant with predictions of conventional stellar evolution codes for the latest stages of a massive star's life before core collapse. The most dramatic example of this disconnect is in the eruptive mass loss occurring in the decade preceding Type IIn supernovae. We outline the key empirical evidence that indicates severe pre-supernova instability in massive stars, and we suggest that the chief reason that these outbursts are absent in stellar evolution models may lie in the treatment of turbulent convection in these codes. The mixing length theory that is used ignores (1) finite amplitude fluctuations in velocity and temperature and (2) their nonlinear interaction with nuclear burning. Including these fluctuations is likely to give rise to hydrodynamic instabilities in the latest burning sequences, which prompts us to discuss a number of far-reaching implications for the fates of massive stars. In particular, we explore connections to enhanced pre-supernova mass loss, unsteady nuclear burning and consequent eruptions, swelling of the stellar radius that may trigger violent interactions with a companion star, and potential modifications to the core structure that could dramatically alter calculations of the core-collapse explosion mechanism itself. These modifications may also impact detailed nucleosynthesis and measured isotopic anomalies in meteorites, as well as the interpretation of young core-collapse supernova remnants. Understanding these critical instabilities in the final stages of evolution may make possible the development of an early warning system for impending core collapse, if we can identify their asteroseismological or eruptive signatures. © 2014. The American Astronomical Society. All rights reserved..


Couch S.M.,California Institute of Technology | Couch S.M.,Michigan State University | Chatzopoulos E.,University of Chicago | Arnett W.D.,University of Arizona | And 3 more authors.
Astrophysical Journal Letters | Year: 2015

We present the first three-dimensional (3D) simulation of the final minutes of iron core growth in a massive star, up to and including the point of core gravitational instability and collapse. We capture the development of strong convection driven by violent Si burning in the shell surrounding the iron core. This convective burning builds the iron core to its critical mass and collapse ensues, driven by electron capture and photodisintegration. The non-spherical structure and motion generated by 3D convection is substantial at the point of collapse, with convective speeds of several hundreds of km s-1. We examine the impact of such physically realistic 3D initial conditions on the core-collapse supernova mechanism using 3D simulations including multispecies neutrino leakage and find that the enhanced post-shock turbulence resulting from 3D progenitor structure aids successful explosions. We conclude that non-spherical progenitor structure should not be ignored, and should have a significant and favorable impact on the likelihood for neutrino-driven explosions. In order to make simulating the 3D collapse of an iron core feasible, we were forced to make approximations to the nuclear network making this effort only a first step toward accurate, self-consistent 3D stellar evolution models of the end states of massive stars. © 2015. The American Astronomical Society. All rights reserved.


Bernstein J.,Aspen Center for Physics
Physics in Perspective | Year: 2010

I discuss the origin of the idea of making a fusion (hydrogen) bomb and the physics involved in it, and then turn to the design proposed for one by the unlikely collaborators John von Neumann and Klaus Fuchs in a patent application they filed at Los Alamos in May 1946, which Fuchs passed on to the Russians in March 1948, and which with substantial modifications was tested on the island of Eberiru on the Eniwetok atoll in the South Pacific on May 8, 1951. This test showed that the fusion of deuterium and tritium nuclei could be ignited, but that the ignition would not propagate because the heat produced was rapidly radiated away. Meanwhile, Stanislaw Ulam and C.J. Everett had shown that Edward Teller's Classical Super could not work, and at the end of December 1950, Ulam had conceived the idea of super compression, using the energy of a fission bomb to compress the fusion fuel to such a high density that it would be opaque to the radiation produced. Once Teller understood this, he invented a greatly improved, new method of compression using radiation, which then became the heart of the Ulam-Teller bomb design, which was tested, also in the South Pacific, on November 1, 1952. The Russians have freely acknowledged that Fuchs gave them the fission bomb, but they have insisted that no one gave them the fusion bomb, which grew out of design involving a fission bomb surrounded by alternating layers of fusion and fission fuels, and which they tested on November 22, 1955. Part of the irony of this story is that neither the American nor the Russian hydrogen-bomb programs made any use of the brilliant design that von Neumann and Fuchs had conceived as early as 1946, which could have changed the entire course of development of both programs. © 2009 Birkhäuser Verlag, Basel/Switzerland.


Chatzopoulos E.,University of Chicago | Couch S.M.,Michigan State University | Arnett W.D.,University of Arizona | Arnett W.D.,Aspen Center for Physics | Timmes F.X.,Arizona State University
Astrophysical Journal | Year: 2016

We explore the effects of rotation on convective carbon, oxygen, and silicon shell burning during the late stages of evolution in a 20 M o star. Using the Modules for Experiments in Stellar Astrophysics we construct one-dimensional (1D) stellar models both with no rotation and with an initial rigid rotation of 50% of critical. At different points during the evolution, we map the 1D models into 2D and follow the multidimensional evolution using the FLASH compressible hydrodynamics code for many convective turnover times until a quasi-steady state is reached. We characterize the strength and scale of convective motions via decomposition of the momentum density into vector spherical harmonics. We find that rotation influences the total power in solenoidal modes, with a slightly larger impact for carbon and oxygen shell burning than for silicon shell burning. Including rotation in 1D stellar evolution models alters the structure of the star in a manner that has a significant impact on the character of multidimensional convection. Adding modest amounts of rotation to a stellar model that ignores rotation during the evolutionary stage, however, has little impact on the character of the resulting convection. Since the spatial scale and strength of convection present at the point of core collapse directly influence the supernova mechanism, our results suggest that rotation could play an important role in setting the stage for massive stellar explosions. © 2016. The American Astronomical Society. All rights reserved.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: Elem. Particle Physics/Theory | Award Amount: 475.00K | Year: 2016

This award funds the scientific activities at the Aspen Center for Physics (ACP), one of the premier theoretical physics research institutions in the world. The ACP is unique in its combination of programs of high scientific quality coupled with an extraordinarily fertile collaborative research environment. Each year, from mid-May to mid-September, the ACP attracts over 500 of the worlds leading physicists to Aspen where, participating in multiple-week workshops, working groups, or individual research projects, they interact, discuss, collaborate, and challenge each other while sharing ideas at the forefront of their disciplines. The ACP also organizes between five and eight one-week Winter Conferences, bringing together as many as 100 researchers each week to respond to rapidly breaking developments in areas of current interest in physics and interdisciplinary research. As a result of this intense activity, each year over 500 journal articles, book chapters and scientific manuscripts explicitly acknowledge the ACP for the birth of new ideas, for the formation and nurturing of collaborations, for the opportunity to concentrate on advancing or completing work, and for essential ingredients contributed by fellow researchers to overcome roadblocks to progress. Thus funding for the ACP advances the national interest by promoting the progress of fundamental science across a variety of subdisciplines of physics.

In both summer and winter, participating physicists also contribute to a rich and visible science outreach program. Each winter conference features a free public lecture with a typical attendance of 200, preceded by a Physics Café in which two to three conference participants engage with an audience of 50 to 100. Each summer, ten to twelve free public talks are held on the ACP campus, with typical attendance close to 100. All lectures and talks are recorded, shown on local television, and made available on the ACP web site. Weekly Physics is for Kids summer barbeques, co-hosted with the Aspen Science Center, feature hands-on activities and an interactive demonstration/talk by an ACP participant. Also in the summer, 14 top physics students from local high schools work at the ACP for a week each and meet one-on-one with physicists to discuss STEM interests and careers.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: Molecular Biophysics | Award Amount: 2.20M | Year: 2011

This award funds the scientific activities at the Aspen Center for Physics (ACP). The ACP is one of the premier theoretical physics research institutions in the world and fulfills its mission of fostering physics research primarily through its summer program. From mid-May to mid-September the Center brings over 500 of the worlds leading physicists each year to Aspen where, within the broad framework of workshops lasting several weeks, they interact, discuss, collaborate, and challenge each other while sharing ideas at the forefront of their disciplines. The ACP is unique in the combination of programs of high scientific quality coupled with an extraordinarily fertile collaborative research environment. The ACP also organizes between five and seven one-week Winter Conferences, bringing together as many as 100 researchers each week to respond rapidly to breaking developments in areas of current interest in physics and interdisciplinary research.

The physicists participating during the summer and winter also contribute to a rich and visible public outreach program. This includes public lectures, held off-site in larger auditoria, together with more informal `dialogues with physicists held at the ACP in the summer (with recordings for all lectures and dialogues available over the internet) and Physics Cafe discussions held in downtown Aspen prior to the Winter public lectures. These are complemented by more `kid-friendly events such as visits to local schools and, in collaboration with the Aspen Science Center, Physics is for Kids weekly summer picnics (on the ACP campus) with talks by participating physicists and physics-related activities. All public events are free and well-attended.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: | Award Amount: 7.00K | Year: 2011

ID: MPS/DMR/BMAT(7623) 1156065 PI: Marchetti, Christina ORG: Aspen Center for Physics

Title: 2012 Aspen Winter Conference Growth and Form: Pattern Formation in Biology

INTELLECTUAL MERIT: The quantitative understanding of the origin of the complex variety of patterns found in nature has challenged scientist for centuries. Such patterns range from the morphology of embryonic and adult organisms to vein structures in plants, to the organization of birds in large flocks that exhibit coordinated behavior. Thanks to advances in imaging techniques on various scale and computer modeling, we are now making progress in connecting the cellular or single individual scale to the organism or large group scale. The goal of the conference is to bring together physicists, chemists, mathematicians, engineers, and biologists who are using physical principles, statistical mechanics, and modeling to approach problems such as developmental biology, biomineralization, and flocking.

BROADER IMPACTS: The organizers of this Aspen Winter Conference have made a special effort to increase the diversity of the group of invited speakers. Currently nine of the twenty invited speakers are women and six are junior scientists. The organizers will also work to attract a large number of junior participants. Several will be selected to give contributed talks. The conference will host an Aspen Winter Public Lecture on some aspect of pattern formation in biology. The lecture will be held on Wednesday evening at the Wheeler Opera House, in downtown Aspen and preceded by a ?Meet the Physicist? session where members of the Aspen community will meet informally with two of the conference participants for a question and answer session. The Winter Public Lecture will also be broadcast on local television GrassRoots TV 12 and streamed over the internet. Finally, to ensure broad dissemination of conference material slides and other content of the conference will be posted on the conference website.


News Article | February 3, 2016
Site: www.rdmag.com

Bursts of gamma rays from the center of our galaxy are not likely to be signals of dark matter, but rather other astrophysical phenomena, such as fast-rotating stars called millisecond pulsars, according to two new studies, one from a team based at Princeton University and the Massachusetts Institute of Technology and another based in the Netherlands. Previous studies suggested that gamma rays coming from the dense region of space in the inner Milky Way galaxy could be caused when invisible dark matter particles collide. But using new statistical analysis methods, the two research teams independently found that the gamma ray signals are uncharacteristic of those expected from dark matter. Both teams reported the finding in the journal Physical Review Letters this week. "Our analysis suggests that what we are seeing is evidence for a new astrophysical source of gamma rays at the center of the galaxy," said Mariangela Lisanti, an assistant professor of physics at Princeton. "This is a very complicated region of the sky and there are other astrophysical signals that could be confused with dark matter signals." The center of the Milky Way galaxy is thought to contain dark matter because it is home to a dense concentration of mass, including dense clusters of stars and a black hole. A conclusive finding of dark matter collisions in the galactic center would be a major step forward in confirming our understanding of our universe. "Finding direct evidence for these collisions would be interesting, because it would help us understand the relationship between dark matter and ordinary matter," said Benjamin Safdi, a postdoctoral researcher at MIT who earned his Ph.D. in 2014 at Princeton. To tell whether the signals were from dark matter versus other sources, the Princeton/MIT research team turned to image-processing techniques. They looked at what the gamma rays should look like if they indeed come from the collision of hypothesized dark matter particles known as weakly interacting massive particles, or WIMPs. For the analysis, Lisanti, Safdi and Samuel Lee, a former postdoctoral research fellow at Princeton who is now at the Broad Institute, along with colleagues Wei Xue and Tracy Slatyer at MIT, studied images of gamma rays captured by NASA's Fermi Gamma-ray Space Telescope, which has been mapping the rays since 2008. Dark matter particles are thought to make up about 85 percent of the mass in the universe but have never been directly detected. The collision of two WIMPs, according to a widely accepted model of dark matter, causes them to annihilate each other to produce gamma rays, which are the highest-energy form of light in the universe. According to this model, the high-energy particles of light, or photons, should be smoothly distributed among the pixels in the images captured by the Fermi telescope. In contrast, other sources, such as rotating stars known as pulsars, release bursts of light that show up as isolated, bright pixels. The researchers applied their statistical analysis method to images collected by the Fermi telescope and found that the distribution of photons was clumpy rather than smooth, indicating that the gamma rays were unlikely to be caused by dark matter particle collisions. Exactly what these new sources are is unknown, Lisanti said, but one possibility is that they are very old, rapidly rotating stars known as millisecond pulsars. She said it would be possible to explore the source of the gamma rays using other types of sky surveys involving telescopes that detect radio frequencies. Douglas Finkbeiner, a professor of astronomy and physics at Harvard University who was not directly involved in the current study, said that, although the finding complicates the search for dark matter, it leads to other areas of discovery. "Our job as astrophysicists is to characterize what we see in the universe, not get some predetermined, wished-for outcome. Of course, it would be great to find dark matter, but just figuring out what is going on and making new discoveries is very exciting." According to Christoph Weniger from the University of Amsterdam and lead author of the Netherlands-based study, the finding is a win-win situation: "Either we find hundreds or thousands of millisecond pulsars in the upcoming decade, shedding light on the history of the Milky Way, or we find nothing. In the latter case, a dark matter explanation for the gamma ray excess will become much more obvious." Funding for the Princeton/MIT research came from the U.S. Department of Energy under grant Contract Numbers DE-SC00012567, DE-SC0013999 and DE-SC0007968, and from the National Science Foundation under grant PHY-1066293 to the Aspen Center for Physics.

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