The report describing a three-fold enhancement of the superconducting transition temperature is featured as an Editor's Suggestion in Physical Review B - Rapid Communications.
A University of Maryland-led research team, which includes Physics Professor Wolfgang Losert, has been awarded a three-year $1.7 million grant from the National Institutes of Health (NIH) to develop new imaging technologies and data analysis techniques that will further our understanding of how large networks of neurons in the brain interact to process sensory information. This knowledge will help researchers identify the precise interactions between millions of nerve cells that drive behavior, like decision-making and speaking, and alterations in these interactions that may be responsible for disorders such as schizophrenia, autism and epilepsy.
Antiferromagnetic spin-spin interactions are mediated and enhanced by electrons in a superconductor. Graphic credit, S. Kelley/E.Edwards
Jay Sau, in collaboration with physicists from Harvard and Yale, has been studying the effects of embedding magnetic spins onto the surface of a superconductor. They recently report in paper that was chosen as an "Editor's Suggestion" in Physical Review Letters, that the spins can interact differently than previously thought. This hybrid platform could be useful for quantum simulations of complex spin systems, having the special feature that the interactions may be controllable, something quite unusual for most condensed matter systems.
The textbook quantum system known as a spin can be realized in different physical platforms. Due to advances in fabrication and imaging, magnetic impurities embedded onto a substrate have emerged as an exciting prospect for studying spin physics. Quantum ‘spin’ is related to a particle’s intrinsic angular momentum. What’s neat is that while the concept is fairly abstract, numerous effects in nature, such as magnetism, map onto mathematical spin models.
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Imagine being able to instantaneously run an optical cable or fiber to any point on earth, or even into space. That’s what Howard Milchberg, professor of physics and electrical and computer engineering at the University of Maryland, wants to do.
In a paper published in the July 2014 issue of the journal Optica, Milchberg and his lab report using an “air waveguide” to enhance light signals collected from distant sources. These air waveguides could have many applications, including long-range laser communications, detecting pollution in the atmosphere, making high-resolution topographic maps and laser weapons.
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Draining the water from a bathtub causes a spinning tornado to appear. The downward flow of water into the drain causes the water to rotate, and as the rotation speeds up, a vortex forms that obeys the laws of classical mechanics. However, if the water is extremely cold liquid helium, the fluid will swirl around an invisible line to form a vortex that obeys the laws of quantum mechanics. Sometimes, two of these quantum tornadoes flex into curved lines, cross over one another to form a letter X shape, swap ends, and then violently retract from one another—a process called reconnection.
Computer simulations have suggested 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. However, the existence of these waves had never been experimentally proven.
Now, for the first time, researchers provide visual evidence confirming that the reconnection of quantum vortexes launches Kelvin waves. The study, which was conducted at the University of Maryland, will be published the week of March 24, 2014 in the online early edition of the journal Proceedings of the National Academy of Sciences. The research was supported by the National Science Foundation.
“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.
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.
This research was supported by the National Science Foundation (NSF) under Award No. DMR-0906109. The content of this article does not necessarily reflect the views of the NSF.
The research paper, “Direct observation of Kelvin waves excited by quantized vortex reconnections,” Enrico Fonda, David P. Meichle, Nicholas T. Ouellette, Sahand Hormoz, and Daniel P. Lathrop, published the week of March 24, 2014 in the online early edition of the journal Proceedings of the National Academy of Sciences.
