Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Deceptive-Looking Vortex Line in Superfluid Led to Twice-Mistaken Identity

30.09.2014

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.


Peter Scherpelz

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.

Ensuing saga

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.’”

Symmetry problems

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.

Contact Information

Steve Koppes
Associate News Director
skoppes@uchicago.edu
Phone: 773-702-8366

Steve Koppes | newswise

More articles from Physics and Astronomy:

nachricht Basque researchers turn light upside down
23.02.2018 | Elhuyar Fundazioa

nachricht Attoseconds break into atomic interior
23.02.2018 | Max-Planck-Institut für Quantenoptik

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Attoseconds break into atomic interior

A newly developed laser technology has enabled physicists in the Laboratory for Attosecond Physics (jointly run by LMU Munich and the Max Planck Institute of Quantum Optics) to generate attosecond bursts of high-energy photons of unprecedented intensity. This has made it possible to observe the interaction of multiple photons in a single such pulse with electrons in the inner orbital shell of an atom.

In order to observe the ultrafast electron motion in the inner shells of atoms with short light pulses, the pulses must not only be ultrashort, but very...

Im Focus: Good vibrations feel the force

A group of researchers led by Andrea Cavalleri at the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg has demonstrated a new method enabling precise measurements of the interatomic forces that hold crystalline solids together. The paper Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics, published online in Nature, explains how a terahertz-frequency laser pulse can drive very large deformations of the crystal.

By measuring the highly unusual atomic trajectories under extreme electromagnetic transients, the MPSD group could reconstruct how rigid the atomic bonds are...

Im Focus: Developing reliable quantum computers

International research team makes important step on the path to solving certification problems

Quantum computers may one day solve algorithmic problems which even the biggest supercomputers today can’t manage. But how do you test a quantum computer to...

Im Focus: In best circles: First integrated circuit from self-assembled polymer

For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.

In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

 
Latest News

Basque researchers turn light upside down

23.02.2018 | Physics and Astronomy

Finnish research group discovers a new immune system regulator

23.02.2018 | Health and Medicine

Attoseconds break into atomic interior

23.02.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>