Sprinkling Spin Physics onto a Superconductor

Details

Published: Wednesday, August 27 2014 01:00

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.

Read More

Creating Optical Cables Out of Thin Air

Details

Published: Thursday, July 31 2014 01:00

Click image for information.

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.

Read More

Researchers See Kelvin Wave on Quantum “Tornado” for First Time

Details

Published: Thursday, March 27 2014 09:30

Click image for information.

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.

Researchers Demonstrate Long-Lived High-Power Optical Waveguides in Air

Details

Published: Monday, March 10 2014 21:29

The Era of Neutrino Astronomy Has Begun

Details

Published: Friday, November 22 2013 09:27

Astrophysicists using a telescope embedded in Antarctic ice have succeeded in a quest to detect and record the mysterious phenomena known as cosmic neutrinos – nearly massless particles that stream to Earth at the speed of light from outside our solar system, striking the surface in a burst of energy that can be as powerful as a baseball pitcher’s fastball. Next they hope to build on the early success of the IceCube Neutrino Observatory to detect the source of these high-energy particles, said Physics Professor Gregory Sullivan, who led the University of Maryland’s 12-person team of contributors to the IceCube Collaboration.

“The era of neutrino astronomy has begun,” Sullivan said as the IceCube Collaboration announced the observation of 28 very high-energy particle events that constitute the first solid evidence for astrophysical neutrinos from cosmic sources.

By studying the neutrinos that IceCube detects, scientists can learn about the nature of astrophysical phenomena occurring millions, or even billions of light years from Earth, Sullivan said. “The sources of neutrinos, and the question of what could accelerate these particles, has been a mystery for more than 100 years. Now we have an instrument that can detect astrophysical neutrinos. It’s working beautifully, and we expect it to run for another 20 years.”

The collaboration’s report on the first cosmic neutrino records from the IceCube Neutrino Observatory, collected from instruments embedded in one cubic kilometer of ice at the South Pole, was published Nov. 22 on the cover of Science.

“This is the first indication of very high-energy neutrinos coming from outside our solar system,” said University of Wisconsin-Madison Physics Professor Francis Halzen, principal investigator of IceCube. “It is gratifying to finally see what we have been looking for. This is the dawn of a new age of astronomy.”

“Neutrinos are one of the basic building blocks of our universe,” said UMD Physics Associate Professor and Director Kara Hoffman, an IceCube team member. Billions of them pass through our bodies unnoticed every second. These extremely high-energy particles maintain their speed and direction unaffected by magnetic fields. The vast majority of neutrinos originate either in the sun or in Earth’s own atmosphere. Far more rare are astrophysical neutrinos, which come from the outer reaches of our galaxy or beyond.

The origin and cause of astrophysical neutrinos are unknown, though gamma ray bursts, active galactic nuclei and black holes are potential sources. Better understanding of these neutrinos is critically important in particle physics, astrophysics and astronomy, and scientists have worked for more than 50 years to design and build a high-energy neutrino detector of this type.

IceCube was designed to accomplish two major scientific goals: measure the flux, or rate, of high-energy neutrinos and try to identify some of their sources. The neutrino observatory was built and is operated by an international collaboration of more than 250 physicists and engineers. UMD physicists have been key collaborators on IceCube since 2002, when its unique design was devised and construction began.

IceCube is made up of 5,160 digital optical modules suspended along 86 strings embedded in ice beneath the South Pole. The National Science Foundation-supported observatory detects neutrinos through the tiny flashes of blue light, called Cherenkov light, produced when neutrinos interact in the ice. Computers at the IceCube laboratory collect near-real-time data from the optical sensors and send information about interesting events north via satellite. The UMD team designed the data collection system and much of IceCube's analytic software. Construction took nearly a decade, and the completed detector began gathering data in May 2011.

“IceCube is a wonderful and unique astrophysical telescope – it is deployed deep in the Antarctic ice but looks over the entire Universe, detecting neutrinos coming through the Earth from the northern skies, as well as from around the southern skies,” said Vladimir Papitashvili of the National Science Foundation (NSF) Division of Polar Programs.

In April 2012 IceCube detected two high-energy events above 1 PeV, nicknamed Bert and Ernie, the first astrophysical neutrinos definitively recorded by a terrestrial detector. After Bert and Ernie were discovered, the IceCube team searched their records from May 2010 to May 2012 of events that fell slightly below the energy level of their original search. They discovered 26 more high-energy events, all at levels of 30 teraelectronvolts (TeV) or higher, indicative of astrophysical neutrinos. Preliminary results of this analysis were presented May 15 at the IceCube Particle Astrophysics Symposium at UW–Madison. The analysis presented in Science reveals a highly statistically significant signal (more than 4 sigma), providing solid evidence that IceCube has successfully detected high-energy extraterrestrial neutrinos, said UMD’s Sullivan.

Since astrophysical neutrinos move in straight lines unimpeded by outside forces, they can act as pointers to the place in the galaxy where they originated. The 28 events recorded so far are too few to point to any one location, Sullivan said. Over the coming years, the IceCube team will watch, “like waiting for a long exposure photograph,” as more measurements fill in a picture that may reveal the point of origin of these intriguing phenomena.

New detection systems for astrophysical neutrinos are also in the works. Hoffman is leading the development of the Askaryan Radio Array, a neutrino telescope that uses radio frequency, which transmits best through very cold ice, to detect the particles. Plans are underway for 37 subsurface clusters of radio antennae

The IceCube Neutrino Observatory was built under a NSF Major Research Equipment and Facilities Construction grant, with assistance from partner funding agencies around the world. The NSF's Division of Polar Programs and Physics Division continue to support the project with a Maintenance and Operations grant, along with international support from participating institutes and their funding agencies.

UMD contributors to the IceCube collaboration include Sullivan and Hoffman; faculty and staff members Erik Blaufuss, John Felde, Jordan Goodman, Henrike Wissing, Alex Olivas, Donald La Dieu, and Torsten Schmidt; and graduate students Elim Cheung, Robert Hellauer, Ryan Maunu, and Michael Richman.