The surprising experimental results agree with independent numerical simulations produced by collaborating scientists at the University of Queensland in Australia. The UA and Australian researchers are reporting the results of the research in Oct. 16 issue of the journal Nature.
The results are of great interest because they reveal something fundamentally new about certain kinds of "phase transitions," and nature is replete with phase transitions.
Common phase transitions include liquid water freezing to ice, or liquid water boiling to steam. Another common phase transition occurs in proteins when raw eggs are cooked. More exotic examples of phase transitions include the cooling of materials until they become superconductors, and, on the scale of the universe, the phase transition that transformed the early universe from a hot, dense system born from the Big Bang into the universe with protons, electrons, structure and forces observed today.
By first creating such a cold gas in their UA campus laboratory, and then lowering the temperature of the system just a little bit more, some atoms in the gas still behave much as they do in classical physics, bouncing around at random. However, this additional cooling induces a phase transition where other atoms of the gas become a new form of matter called a Bose-Einstein condensate, a tiny droplet of superfluid which behaves according to quantum physics.
Bose-Einstein condensates, or BECs, were first produced in Nobel Prize-winning experiments in 1995. Since then, theoretical and experimental researchers have studied BECs intensely, using BECs as valuable new tools for probing a wide range of fundamental physics. The UA experimental team, and the University of Queensland theoretical team headed by physicist Matthew Davis, paired up to push the limits of what is known about how BECs actually form.
"Scientists understand a lot more about BECs after over ten years of work, but there are still some great surprises," said Anderson.
Their work lends additional support to the idea that spontaneous "topological defect" formation in phase transitions is a widespread phenomenon, even at temperatures near absolute zero. "Defect" in this sense means that a discontinuity has appeared in the background superfluid of the BEC.
"In our experiments, we found that when we cool a very cold gas through the BEC phase transition, the BEC can spontaneously begin to rotate, creating something like a microscopic quantum mechanical hurricane where atoms rotate as a fluid around a vortex core where there is no fluid," Anderson said.
"The idea of spontaneous formation of vortices in BECs had been lightly discussed as theory before, but had not been observed in experiments," he added.
Ironically, showing that BECs could be spun up into a rotating state to form vortices was a hot research topic just a few years ago. Anderson was a postdoc on the team that was the first to create a vortex in a BEC. They used creative but relatively difficult techniques. Other groups have now used a variety of techniques to successfully create BECs with many vortices.
"What was so surprising about our work is that we saw these things just appear by themselves. You just make your condensate, and they sometimes appear. You don't have to somehow manipulate your system, all you have to do is cool through the phase transition."
" I think what we've done, for the first time, is link experimental observations of defect formation in a phase transition with a theoretical model and numerical simulations that are built on some pretty rigid foundations of quantum mechanics and quantum interactions," Anderson said.
"By collaborating with our colleagues in Australia, who are doing the theoretical research, we can back out details of the physical process that causes these vortices to spontaneously form. It will help us understand more about how superfluids develop, and perhaps more about universal phase transition dynamics in general, including the kind of phase transition that occurred in the early universe."
The experimental research was supported by grants from the National Science Foundation and the Army Research Office. The theoretical work was supported by the Australian Research Council and the University of Queensland.
The UA and University of Queensland science results agree with an important theoretical model called the "Kibble-Zurek mechanism" that concerns how defects can form in a phase transition. The model says that the faster a system undergoes a phase transition, the more defects -- in this case, the vortices -- naturally and spontaneously form. Conversely, the slower the system is cooled, the smoother the phase transition into a new state will be and the fewer defects will appear.
In the not-so-near future, Anderson said, BECs may become useful in devices in ways similar to laser light. Rotation sensors, accelerometers or interferometers based on the coherence properties of Bose-Einstein condensates are among the envisioned possible applications, he said.
But for now, perhaps most exciting use for BECs is as a tool for exploring the fundamental ideas of physics in ways that couldn't be explored before.SCIENCE CONTACT:
New research calculates capacity of North American forests to sequester carbon
16.07.2018 | University of California - Santa Cruz
Scientists discover Earth's youngest banded iron formation in western China
12.07.2018 | University of Alberta
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
18.07.2018 | Life Sciences
18.07.2018 | Materials Sciences
18.07.2018 | Health and Medicine