"We weren't surprised to see the Kelvin waves on the quantum vortex, but we were excited to see them because they had never been seen before," said Daniel Lathrop, a UMD physics professor. "Seeing the Kelvin waves provided the first experimental evidence that previous theories predicting they would be launched from vortex reconnection were correct."
Understanding turbulence in quantum fluids, such as ultracold liquid helium, may offer clues to neutron stars, trapped atom systems and superconductors. Superconductors, which are materials that conduct electricity without resistance below certain temperatures, develop quantized vortices. Understanding the behavior of the vortices may help researchers develop superconductors that remain superconducting at higher current densities.
Physicists Richard Feynman and Lars Onsager predicted the existence of quantum vortices more than a half-century ago. However, no one had seen quantum vortices until 2006. In Lathrop's laboratory at UMD, researchers prepared a cylinder of supercold helium—at 2 degrees Celsius above absolute zero—injected with frozen tracer particles made from atmospheric air and helium gases. When they shined a laser into the cylinder, the researchers saw the particles trapped on the vortices like dew drops on a spider web.
"Kelvin waves on quantized vortices had been predicted, but the experiments were challenging because we had to conduct them at lower temperatures than our previous experiments," explained Lathrop.
This video describes the experiments that were conducted when researchers in Daniel Lathrop's laboratory at the University of Maryland confirmed visually, for the first time, that the reconnection of quantum vortexes launches Kelvin waves.
(Photo Credit: Enrico Fonda)
Since 2006, the researchers have used the same technique to further examine quantum vortexes. During an experiment in February 2012, they witnessed a unique reconnection event. One vortex reconnected with another and a wave propagated down the vortex. To quantitatively study the wave's motion, the researchers tracked the position of the particles on the vortex. The resulting waveforms agreed generally with theories of Kelvin waves propagating from quantum vortexes.
"These first observations of Kelvin waves will surely lead to exciting new experiments that push the limits of our knowledge of these exotic quantum motions," added Lathrop.
In the future, Lathrop plans to use florescent nanoparticles to investigate what happens near the transition to the superfluid state.
Lathrop conducted the current study with David Meichle, a UMD physics graduate student;Enrico Fonda, who was a research scholar at UMD and graduate student at the University of Trieste when the study was performed and is now a postdoctoral researcher at New York University; Nicholas Ouellette, who was a visiting assistant professor at UMD when the study was performed and is now an associate professor in mechanical engineering & materials science at Yale University; and Sahand Hormoz, a postdoctoral researcher at the University of California, Santa Barbara's Kavli Institute for Theoretical Physics.
Illustration of Kelvin waves on retracting quantized vortices after they met, crossed and exchanged tails -- a process called reconnection. A new study provides visual evidence that after the vortexes snap away from each other, they develop ripples called "Kelvin waves" to quickly get rid of the energy caused by the connection and relax the system.
(Photo Credit: Enrico Fonda)
Schematic diagram of two quantum vortices meeting, crossing and exchanging tails -- a process called reconnection. The red and blue arrows represent the direction of vorticity. A new study provides visual evidence that after the vortexes exchange tails and snap away from each other, they develop ripples called "Kelvin waves" to quickly get rid of the energy caused by the connection and relax the system.
(Photo Credit: Daniel Lathrop)
Source: University of Maryland