Zvyagin S.A.,Helmholtz Center Dresden |
Aimar E.,Pj Afarik University |
Ozerov M.,Helmholtz Center Dresden |
Wosnitza J.,Helmholtz Center Dresden |
And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011
Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron-spin-resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field Hsat=48.5 T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the density matrix renormalization group calculations for a spin-1/2 Heisenberg chain with a staggered transverse field. © 2011 American Physical Society. Source
Flaherty K.M.,Wesleyan University |
Hughes A.M.,Wesleyan University |
Rosenfeld K.A.,Harvard - Smithsonian Center for Astrophysics |
Andrews S.M.,Harvard - Smithsonian Center for Astrophysics |
And 5 more authors.
Astrophysical Journal | Year: 2015
Turbulence can transport angular momentum in protoplanetary disks and influence the growth and evolution of planets. With spatially and spectrally resolved molecular emission line measurements provided by (sub)millimeter interferometric observations, it is possible to directly measure non-thermal motions in the disk gas that can be attributed to this turbulence. We report a new constraint on the turbulence in the disk around HD 163296, a nearby young A star, determined from Atacama Large Millimeter/submillimeter Array Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions). The different optical depths for these lines permit probes of non-thermal line-widths at a range of physical conditions (temperature and density) and depths into the disk interior. We derive stringent limits on the non-thermal motions in the upper layers of the outer disk such that any contribution to the line-widths from turbulence is <3% of the local sound speed. These limits are approximately an order of magnitude lower than theoretical predictions for full-blown magnetohydrodynamic turbulence driven by the magnetorotational instability, potentially suggesting that this mechanism is less efficient in the outer (R 30 AU) disk than has been previously considered. © 2015. The American Astronomical Society. All rights reserved. Source
Naugolnykh K.,University of Colorado and
39th International Congress on Noise Control Engineering 2010, INTER-NOISE 2010 | Year: 2010
A sound wave in unstable convective atmospheric layer can stimulate perturbations development. High-intensity atmospheric perturbations such as cyclones and thunderstorms generate infrasound, which is detectable at large distances from the source. The waveconvection instability can produce variations in the level of infrasound radiation by the developing cyclone, and this can serve as a precursor of intense atmospheric events. Source
News Article | September 22, 2015
Greenhouse gases released as global warming thaws permafrost in Arctic regions could increase the economic toll of climate change by trillions of dollars, scientists say. The release of the carbon dioxide and methane normally trapped in the frozen ground could increase the economic damage from climate change by as much as $43 trillion by the end of the next century, a study by U.S. and British researchers predicts. The researchers at the University of Colorado and the University of Cambridge have created the first models of the economic impact of melting Arctic permafrost and added that to the $326 trillion impact already predicted by other climate and economic modeling. The researchers combined two models, one estimating total likely emissions of greenhouse gases from melting permafrost, the other calculating the temperature increases that would result in climate-related impacts worldwide, to create their prediction for 2200. "There is almost nothing in the literature on this," says Chris Hope, a policy modeling expert at Cambridge. "We are the first to combine a physical model and an integrated assessment model in this way," says Hope, lead author of the study appearing in the journal Nature Climate Change. The researchers based their estimates on the assumption that anthropogenic greenhouse gas emissions resulting from human activities such as fossil fuel burning would continue until at least 2100. By that year, release of methane and carbon dioxide from melting permafrost could push global temperatures up by between 0.11 and 0.25 degrees Celsius over what would already be present due to human-caused emissions, they predict. By 2100, those human activities by themselves are expected to boost carbon dioxide levels by 75 percent over what they are today, they say. The Arctic — where around 1,700 gigatons of carbon are trapped in permafrost — is warming at twice the rate of the global average, the researchers note, and if that rate continues, hundreds of billions of tons of greenhouse gases will be released as the permafrost thaws. "These results show just how much we need urgent action to slow the melting of the permafrost in order to minimize the scale of the release of greenhouse gases," Hope says. The researchers acknowledge that their models have a large uncertainty factor due to something known as the "transient climate response," a difficulty in gauging just how sensitive global temperatures are to atmospheric carbon dioxide concentrations. "If you assume a bigger transient climate response, the model climate is warmer for the same amount of carbon dioxide input," says study senior author Kevin Schaefer at the National Snow and Ice Data Center at the University of Colorado. "If it's a smaller value, then the warming is less." Still, says Hope, the study findings point to a clear need to reduce human-caused emission levels as much as possible and as soon as possible. "There's only one way to stop the thawing of permafrost, and that's to stop climate change," he says.
All living things require nitrogen for survival, but the world depends on only two known processes to break nitrogen’s ultra-strong bonds and allow conversion to a form humans, animals and plants can consume. One is a natural, bacterial process on which farmers have relied since the dawn of agriculture. The other is the century-old Haber-Bösch process, which revolutionized fertilizer production and spurred unprecedented growth of the global food supply. Credit: Al Hicks/National Renewable Energy Laboratory. All living things require nitrogen for survival, but the world depends on only two known processes to break nitrogen's ultra-strong bonds and allow conversion to a form humans, animals and plants can consume. One is a natural, bacterial process on which farmers have relied since the dawn of agriculture. The other is the century-old Haber-Bösch process, which revolutionized fertilizer production and spurred unprecedented growth of the global food supply. "We live in a sea of nitrogen, yet our bodies can't access it from the air," says Utah State University biochemist Lance Seefeldt. "Instead, we get this life-sustaining compound from protein in our food." Now, Seefeldt and colleagues announce a light-driven process that could, once again, revolutionize agriculture, while reducing the world food supply's dependence on fossil fuels and relieving Haber-Bösch's heavy carbon footprint. The research team, which includes USU's Seefeldt, Derek Harris, Andrew Rasmussen and Nimesh Khadka; Katherine A. Brown and Paul W. King of Colorado's National Renewable Energy Laboratory; Molly Wilker, Hayden Hamby and Gordana Dukovic of the University of Colorado and Stephen Keable and John Peters of Montana State University, publishes findings in the April 22, 2016 issue of the journal Science. "Our research demonstrates photochemical energy can replace adenosine triphosphate, which is typically used to convert dinitrogen, the form of nitrogen found in the air, to ammonia, a main ingredient of commercially produced fertilizers," says Seefeldt, professor in USU's Department of Chemistry and Biochemistry and an American Association for the Advancement of Science Fellow. Any way you slice it, he says, nitrogen fixation is an energy-intensive process. "The Haber-Bösch process currently consumes about two percent of the world's fossil fuel supply," Seefeldt says. "So, the new process, which uses nanomaterials to capture light energy, could be a game-changer." "Using light directly to create a catalyst is much more energy efficient, says Brown, NREL research scientist. "This new ammonia-producing process is the first example of how light energy can be directly coupled to dinitrogen reduction, meaning sunlight or artificial light can power the reaction." Energy-efficient production of ammonia holds promise not only for food production, but also for development of technologies that enable use of environmentally cleaner alternative fuels, including improved fuel cells to store solar energy. In addition to its practical applications, the research sheds light on fundamental aspects of how bacterial enzymes known as nitrogenases function; an area of chemistry Seefeldt has studied for nearly two decades. "Our current findings are the result of interdisciplinary collaboration," he says. "Each institution brought unique expertise to the project. We couldn't have succeeded without each partner's contributions to the collaboration." Explore further: Biochemists reveal new twist on old fuel source