Weyl Hunt

A Weyl Of A Time.

Part of a 1929 prediction by physicist Hermann Weyl, of a kind of massless particle that features a singular point in its energy spectrum called the “Weyl point”, has finally been confirmed by direct observation for the first time, says an international team of physicists led by researchers at MIT. The finding could lead to new kinds of high-power single-mode lasers and other optical devices, the team says.

For decades, physicists thought that the subatomic particles called neutrinos, were in fact the massless particles that Weyl had predicted, a possibility that was ultimately eliminated by the 1998 discovery that neutrinos do have a small mass. While thousands of scientific papers have been written about the theoretical particles, until this year there had seemed little hope of actually confirming their existence.

“Every single paper written about Weyl points was theoretical until now,” says Marin Soljačić, a professor of physics at MIT and the senior author of a paper published this week in the journal Science confirming the detection. (Another team of researchers at Princeton University and elsewhere independently made a different detection of Weyl particles; their paper appears in the same issue of Science).

Ling Lu, a research scientist at MIT and lead author of that team’s paper, says the elusive points can be thought of as equivalent to theoretical entities known as magnetic monopoles. These do not exist in the real world: They would be the equivalent of cutting a bar magnet in half and ending up with separate north and south magnets, whereas what really happens is you end up with two shorter magnets, each with two poles. But physicists often carry out their calculations in terms of momentum space (also called reciprocal space) rather than ordinary three-dimensional space, Lu explains and in that framework magnetic monopoles can exist and their properties match those of Weyl points.

The achievement was made possible by a novel use of a material called a photonic crystal. In this case, Lu was able to calculate precise measurements for the construction of a photonic crystal predicted to produce the manifestation of Weyl points, with dimensions and precise angles between arrays of holes drilled through the material, a configuration known as a gyroid structure. This prediction was then proved correct by a variety of sophisticated measurements that exactly matched the characteristics expected for such points.

Some kinds of gyroid structures exist in nature Lu points out, such as in certain butterfly wings. In such natural occurrences, gyroids are self-assembled, and their structure was already known and understood.

Two years ago, researchers had predicted that by breaking the symmetries in a kind of mathematical surfaces called “gyroids” in a certain way, it might be possible to generate Weyl points, but realising that prediction required the team to calculate and build their own materials. In order to make these easier to work with, the crystal was designed to operate at microwave frequencies, but the same principles could be used to make a device that would work with visible light, Lu says. “We know a few groups that are trying to do that,” he says.

A number of applications could take advantage of these new findings, Soljačić says. For example, photonic crystals based on this design could be used to make large-volume single-mode laser devices. Usually, Soljačić says, when you scale up a laser, there are many more modes for the light to follow, making it increasingly difficult to isolate the single desired mode for the laser beam and drastically limiting the quality of the laser beam that can be delivered.

But with the new system “No matter how much you scale it up, there are very few possible modes,” he says. “You can scale it up as large as you want, in three dimensions, unlike other optical systems.”

That issue of scalability in optical systems is “quite fundamental,” Lu says; this new approach offers a way to circumvent it. “We have other applications in mind” he says, to take advantage of the device’s “optical selectivity in a 3-D bulk object.” For example, a block of material could allow only one precise angle and colour of light to pass through, while all others would be blocked.

“This is an interesting development, not just because Weyl points have been experimentally observed, but also because they endow the photonics crystals which realise them with unique optical properties,” says Ashvin Vishwanath, a professor of physics at the University of California at Berkeley who was not involved in this research. “Professor Soljačić’s group has a track record of rapidly converting new science into creative devices with industry applications and I am looking forward to seeing how Weyl photonics crystals evolve.”

Besides Lu and Soljačić, the team included Zhiyu Wang, Dexin Ye, and Lixin Ran of Zhejiang University in China and, at MIT, assistant professor of physics Liang Fu and John Joannopoulos, the Francis Wright Davis Professor of Physics and director of the Institute for Soldier Nanotechnologies (ISN). The work was supported by the U.S. Army through the ISN, the Department of Energy, the National Science Foundation, and the Chinese National Science Foundation.

An international team led by Princeton University scientists has discovered Weyl fermions, an elusive massless particle theorised 85 years ago. The particle could give rise to faster and more efficient electronics because of its unusual ability to behave as matter and antimatter inside a crystal, according to new research.

The researchers report in the journal Science, the first observation of Weyl fermions, which if applied to next-generation electronics, could allow for a nearly free and efficient flow of electricity in electronics and thus greater power, especially for computers the researchers suggest.

Proposed by the mathematician and physicist Hermann Weyl in 1929, Weyl fermions have been long sought by scientists because they have been regarded as possible building blocks of other subatomic particles and are even more basic than the ubiquitous, negative-charge carrying electron (when electrons are moving inside a crystal). Their basic nature means that Weyl fermions could provide a much more stable and efficient transport of particles than electrons, which are the principle particle behind modern electronics. Unlike electrons, Weyl fermions are massless and possess a high degree of mobility; the particle’s spin is both in the same direction as its motion, which is known as being right-handed and in the opposite direction in which it moves, or left-handed.

“The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we’re just not capable of imagining now,” said corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team.

The researchers’ find differs from the other particle discoveries in that the Weyl fermion can be reproduced and potentially applied, Hasan said. Typically, particles such as the famous Higgs boson are detected in the fleeting aftermath of particle collisions, he said. The Weyl fermion, however, was discovered inside a synthetic metallic crystal called tantalum arsenide that the Princeton researchers designed in collaboration with researchers at the Collaborative Innovation Center of Quantum Matter in Beijing and at National Taiwan University.

The Weyl fermion possesses two characteristics that could make its discovery a boon for future electronics, including the development of the highly prized field of efficient quantum computing, Hasan explained.

For a physicist, the Weyl fermions are most notable for behaving like a composite of monopole and anti-monopole like particles when inside a crystal, Hasan said. This means that Weyl particles that have opposite magnetic-like charges can nonetheless move independently of one another with a high degree of mobility.

The researchers also found that Weyl fermions can be used to create massless electrons that move very quickly with no backscattering, wherein electrons are lost when they collide with an obstruction. In electronics, backscattering hinders efficiency and generates heat. Weyl electrons simply move through and around roadblocks, Hasan said.

“It’s like they have their own GPS and steer themselves without scattering,” Hasan said. “They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing.”

Prior to the Science paper, Hasan and his co-authors published a report in the journal Nature Communications  that theorized that Weyl fermions could exist in a tantalum arsenide crystal. Guided by that paper, the researchers used the Princeton Institute for the Science and Technology of Materials (PRISM) and Laboratory for Topological Quantum Matter and Spectroscopy in Princeton’s Jadwin Hall to research and simulate dozens of crystal structures before seizing upon the asymmetrical tantalum arsenide crystal, which has a differently shaped top and bottom.

The crystals were then loaded into a two-story device known as a scanning tunneling spectromicroscope that is cooled to near absolute zero and suspended from the ceiling to prevent even atom-sized vibrations. The spectromicroscope determined if the crystal matched the theoretical specifications for hosting a Weyl fermion. “It told us if the crystal was the house of the particle,” Hasan said.

The Princeton team took the crystals passing the spectromicroscope test to the Lawrence Berkeley National Laboratory in California to be tested with high-energy accelerator-based photon beams. Once fired through the crystal, the beams’ shape, size and direction indicated the presence of the long-elusive Weyl fermion.

First author Su-Yang Xu, a postdoctoral research associate in Princeton’s Department of Physics, said that the work was unique for encompassing theory and experimentalism.

“The nature of this research and how it emerged is really different and more exciting than most of other work we have done before,” Xu said. “Usually, theorists tell us that some compound might show some new or interesting properties, then we as experimentalists grow that sample and perform experiments to test the prediction. In this case, we came up with the theoretical prediction ourselves and then performed the experiments. This makes the final success even more exciting and satisfying than before.”

In pursuing the elusive particle, the researchers had to pull from a number of disciplines, as well as just have faith in their quest and scientific instincts, Xu said.

“Solving this problem involved physics theory, chemistry, material science and most importantly, intuition,” he said. “This work really shows why research is so fascinating, because it involved both rational, logical thinking and also sparks and inspiration.”

Weyl, who worked at the Institute for Advanced Study, suggested his fermion as an alternative to the theory of relativity proposed by his colleague Albert Einstein. Although that application never panned out, the characteristics of his theoretical particle intrigued physicists for nearly a century, Hasan said. Actually observing the particle was a trying process; one ambitious experiment proposed colliding high-energy neutrinos to test if the Weyl fermion was produced in the aftermath, he said.

The hunt for the Weyl fermion began in the earliest days of quantum theory when physicists first realised that their equations implied the existence of antimatter counterparts to commonly known particles such as electrons, Hasan said.

“People figured that although Weyl’s theory was not applicable to relativity or neutrinos, it is the most basic form of fermion and had all other kinds of weird and beautiful properties that could be useful,” he said.

“After more than 80 years, we found that this fermion was already there, waiting. It is the most basic building block of all electrons,” he said. “It is exciting that we could finally make it come out following Weyl’s 1929 theoretical recipe.”

Ashvin Vishwanath, a professor of physics at the University of California-Berkeley who was not involved in the study, commented “Professor Hasan’s experiments report the observation of both the unusual properties in the bulk of the crystal as well as the exotic surface states that were theoretically predicted. While it is early to say what practical implications this discovery might have, it is worth noting that Weyl materials are direct 3-D electronic analogs of graphene, which is being seriously studied for potential applications.”

Other co-authors were Chenglong Zhang, Zhujun Yuan and Shuang Jia from Peking University; Raman Sankar and Fangcheng Chou from the National Taiwan University; Guoqing Chang, Chi-Cheng Lee, Shin-Ming Huang, BaoKai Wang and Hsin Lin from the National University of Singapore; Jie Ma from Oak Ridge National Laboratory; and Arun Bansil from Northeastern University. BaoKai Wang is also affiliated with Northeastern University, and Shuang Jia is affiliated with the Collaborative Innovation Center of Quantum Matter in Beijing.

The paper, “Discovery of Weyl fermions and topological Fermi arcs,” was published online by Science


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