Mysterious effect found in superfluids were pedestrian whirlpool-like structures, not exotic solitons.
So long, solitons: University of Chicago physicists have shown that a group of scientists were incorrect when they concluded that a mysterious effect found in superfluids indicated the presence of solitons—exotic, solitary waves. Instead, they explain, the result was due to more pedestrian, whirlpool-like structures in the fluid. They published their explanation in the Sept. 19 issue of Physical Review Letters.
Researchers produced this image in a computer simulation of an unexpected phenomenon found in an experiment involving ultracold superfluids. This image shows a three-dimensional view of a vortex line (red) as it forms from a decaying vortex ring in a superfluid.
The debate began in July 2013, when a group of scientists from the Massachusetts Institute of Technology published results in Nature showing a long-lived structure in a superfluid — a liquid cooled until it flows without friction.
The researchers created the structure in a superfluid made of ultra-cold lithium atoms, by hitting half of the fluid with a laser, so that the lithium particles would be in different quantum-mechanical configurations in the two halves.
When they imaged the result, the researchers observed a dark line cutting across the cigar-shaped volume of superfluid, indicating a region where the density of particles in the fluid was lower. This, they concluded, was a soliton, which behaves like a sparsely populated wall between two halves of the fluid, separating the particles found in the two different states. This wall persisted for a long time, and oscillated back and forth across the fluid.
The appearance of the soliton wall was a surprising conclusion, because it didn’t fit in with the accepted theories about the behavior of such systems.
“If it were a wall, that would mean that there’s some very unusual physics that theorists did not know about going on, so it of course attracted a huge amount of attention,” said Peter Scherpelz, a postdoctoral scientist in physics and lead author of the paper.
A scientific saga ensued, in which multiple groups from different institutions attempted to understand the result. But the UChicago group—led by Kathryn Levin, professor in physics—was the first to present the correct explanation.
Levin’s group tried to reproduce the puzzling result with a computer simulation of a superfluid. The group had developed the simulation thanks to a collaboration with Argonne National Laboratory. Meanwhile, other groups tried their hands at simulations as well. Some concluded that the region of lower density in the fluid was the result not of a soliton but of a vortex ring — a swirling, donut-shaped structure, around which particles circulate. A smoke ring is a well-known example of a vortex ring.
But Levin’s group couldn’t reproduce these results in their simulation. Instead, they found that a vortex ring was briefly established, but quickly decayed to a simple vortex line, akin to a tornado or whirlpool stretching across the fluid.
Shortly after Levin’s group posted their results on the preprint server arXiv, the MIT researchers released their new results in a preprint, explaining that what they had seen were simple vortices—validating the UChicago theory.
“We swam upstream in a way,” said Levin. “Not too often theory anticipates experiment, and not too often theory’s bold enough to say, ‘Wait a minute. We don’t agree with what the going story is. We think it had to be something else.’”
The problems with the earlier simulations came down to symmetry. Much like a cigar looks the same if you rotate it around its long axis, other teams had assumed in their simulations that the behavior in the fluid was symmetric—an approximation that made it easier for structures like rings to persist, but which didn’t account for imperfections that are inevitable in real-world experiments.
The original MIT experiment had also assumed an incorrect symmetry to come to their original conclusion. They measured only a two-dimensional projection of their experiment, meaning that they couldn’t distinguish between the three possible structures, because a ring or a wall viewed from the side looks just like a line. The MIT group had incorrectly assumed that the feature was symmetric, and that it sliced all the way through the cigar to form a soliton wall.
Physicists are intrigued by the physics of superfluids in part because they are related to superconductors, which have a multitude of technological applications due to their ability to conduct electricity without any resistance. Superfluids, however, often are an easier system to study. The materials are so similar that the simulation code used by the group was originally developed for superconductors, and modified for superfluids.
Another reason physicists want to understand this system is to study physics out of equilibrium, in which the material hasn’t reached a balanced, comfortable state. After the superfluid is hit with the laser, half of the atoms are in a different state than the other half, and they want to return to the same state. Vortices form as the superfluid moves toward equilibrium.
“Everything we know about physics is sort of confined to equilibrium and we’re trying really hard to test ourselves and learn what goes on out of equilibrium, because that’s a lot of the real world,” Levin said. —Emily Conover
Funding: National Science Foundation, U.S. Department of Energy, and the Hertz Foundation.
Citation: “Phase Imprinting in Equilibrating Fermi Gases: The Transience of Vortex Rings and Other Defects,” by Peter Scherpelz, Karmela Padavić, Adam Rançon, Andreas Glatz, Igor S. Aranson, and K. Levin, Physical Review Letters, Vol. 113, Issue 12, Sept. 19, 2014. DOI: 10.1103/PhysRevLett.113.125301.
Associate News Director
Steve Koppes | newswise
From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison
Prediction: More gas-giants will be found orbiting Sun-like stars
22.02.2017 | Carnegie Institution for Science
Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...
The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.
The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...
Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...
Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".
Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...
13.02.2017 | Event News
10.02.2017 | Event News
09.02.2017 | Event News
24.02.2017 | Earth Sciences
24.02.2017 | Agricultural and Forestry Science
24.02.2017 | Life Sciences