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After 85-year search, massless particle with promise for next-generation electronics found
16-Jul-2015
An international team led by Princeton University scientists has discovered Weyl fermions, an elusive massless particle theorized 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 July 16 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 antimonopole-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 in June 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 realized 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 on July 16. The work was supported by the Gordon and Betty Moore Foundations Emergent Phenomena in Quantum Systems (EPiQS) Initiative (grant no. GBMF4547); the Singapore National Research Foundation (grant no. NRF-NRFF2013-03); the National Basic Research Program of China (grant nos. 2013CB921901 and 2014CB239302); the U.S. Department of Energy (grant no. DE-FG-02-05ER462000); and the Taiwan Ministry of Science and Technology (project no. 102-2119-M- 002-004).
http://www.eurekalert.org/pub_releases/2015-07/pu-a8s071015.php
Weyl points: Wanted for 86 years
16-Jul-2015
Weyl points, the 3D analogues of the structures that make graphene exceptional, were theoretically predicted in 1929. Today, an international team of Physicists from MIT and Zhejiang University, found them in photonic crystals, opening a new dimension in photonics.
In 1928 the English physicist Paul Dirac discovered a crucial equation in particle physics and quantum mechanics, now known as Dirac equation, which describes relativistic wave-particles. Very fast electrons were solutions to the Dirac equation. Moreover, the equation predicted the existence of anti-electrons, or positrons: particles with the same mass as electrons but having opposite charge. True to Dirac's prediction, positrons were discovered four years later, in 1932, by the American physicist Carl Anderson. In 1929 Hermann Weyl, a German-born mathematician, found another solution to the Dirac equation, this time massless [1]. A year later, the Austrian-born theoretical physicist Wolfgang Pauli postulated the existence of the neutrino, which was then thought to be massless, and it was assumed to be the sought-after solution to the Dirac equation found by Weyl. Neutrinos had not been detected yet in nature, but the case seemed to be closed. It would be decades before American physicists Frederick Reines and Clyde Cowan finally discovered neutrinos in 1957, and numerous experiments shortly thereafter indicated that neutrinos could have mass. In 1998, the Super-Kamiokande (a neutrino observatory located in Japan) Collaboration announced what had now been speculated for years: neutrinos have non-zero mass. This discovery opened a new question: what then was the zero-mass solution found by Weyl?
Dr. Ling Lu (MIT), Dr. Zhiyu Wang (Zhejiang University, China), Dr. Dexin Ye (Zhejiang University), Prof. Lixin Ran (Zhejiang University), Prof. Liang Fu (MIT), Prof. John D. Joannopoulos (MIT), and Prof. Marin Soljači? (MIT) found the answer.
Ling Lu, first author of the paper published in Science, is very enthusiastic: "Weyl points do actually exist in nature! We built a double-gyroid photonic crystal with broken parity symmetry. The light that passes through the crystal shows the signature of Weyl points in reciprocal space: two linear dispersion bands touching at isolated points." Weyl points, the solutions to the massless Dirac equation, were not found in particle experiments. The research team had to build a tailored material to observe them. The double-gyroid photonic crystal is itself a work of art. Gyroids indeed can be found in nature, in systems as different as butterfly wings and ketchup [2,3]. However, the research group wanted a double-gyroid with a very specific broken symmetry, first proposed in a theoretical work by the same group[4]. In order to fabricate this structure, with parts that are interlocking and with ad hoc defects (such as symmetry-breaking air holes), Lu and collaborators had to drill, machine, and stack slabs of ceramic-filled plastics (Figure 1).
Once the sample was ready, it was time to observe if it behaved as expected, by shining light through it and analyzing the outgoing signal. Physicists analyze these experiments in what is called reciprocal space, or momentum space. The right panel in Figure 2 shows what Weyl points are supposed to look like in reciprocal space: degenerate points, points where two linear dispersion bands meet. The left panel shows an example of the measured data, the solid proof that Weyl points do indeed exist in nature.
"The discovery of Weyl points is not only the smoking gun to a scientific mystery," comments MIT Professor Marin Soljači?, "it paves the way to absolutely new photonic phenomena and applications. Think of the graphene revolution: graphene is a 2D structure, and its electronic properties are, to a substantial extent, a consequence of the existence of linear degeneracy points (known as Dirac points) in its momentum space. Materials containing Weyl points do the same in 3D. They literally add one degree of freedom, one dimension." The discovery of graphene and its unique electronic properties was lauded with the 2010 Nobel Prize in physics, yet graphene's Dirac points are not stable to perturbations. On the other hand, the structures introduced by Lu et al. are very stable to perturbations*, offering a new tool to control how light is confined, how it bounces, and how it radiates. This discovery opens a new intriguing field in basic physics. The potential applications are equally promising. Examples include the possibility to build angularly selective 3D materials and more powerful single-frequency lasers.
###
* The stability of 3D Weyl points is due to the fact that they are topological monopoles. Monopoles can occur in two varieties, e.g. positive and negative. By analogy, electric monopoles are positive and negative charges. Electric charge is conserved, therefore electric monopoles can only be created or annihilated in pairs (positive and negative neutralize). The same is true for topological monopoles: they can only appear or disappear in pairs, making them more robust to perturbations. On the contrary, graphene's Dirac points are not topological monopoles: they are neutral, meaning that they do not need a companion to appear or disappear.
RESEARCH PAPER: "Experimental observation of Weyl points", Ling Lu, Zhiyu Wang, Dexin Ye, Lixin Ran, Liang Fu, John D. Joannopoulos, and Marin Soljači?, Science URL: http://www.sciencemag.org/lookup/doi/10.1126/science.aaa9273
http://www.eurekalert.org/pub_releases/2015-07/miot-wpw071515.php
16-Jul-2015
An international team led by Princeton University scientists has discovered Weyl fermions, an elusive massless particle theorized 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 July 16 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 antimonopole-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 in June 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 realized 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 on July 16. The work was supported by the Gordon and Betty Moore Foundations Emergent Phenomena in Quantum Systems (EPiQS) Initiative (grant no. GBMF4547); the Singapore National Research Foundation (grant no. NRF-NRFF2013-03); the National Basic Research Program of China (grant nos. 2013CB921901 and 2014CB239302); the U.S. Department of Energy (grant no. DE-FG-02-05ER462000); and the Taiwan Ministry of Science and Technology (project no. 102-2119-M- 002-004).
http://www.eurekalert.org/pub_releases/2015-07/pu-a8s071015.php
Weyl points: Wanted for 86 years
16-Jul-2015
Weyl points, the 3D analogues of the structures that make graphene exceptional, were theoretically predicted in 1929. Today, an international team of Physicists from MIT and Zhejiang University, found them in photonic crystals, opening a new dimension in photonics.
In 1928 the English physicist Paul Dirac discovered a crucial equation in particle physics and quantum mechanics, now known as Dirac equation, which describes relativistic wave-particles. Very fast electrons were solutions to the Dirac equation. Moreover, the equation predicted the existence of anti-electrons, or positrons: particles with the same mass as electrons but having opposite charge. True to Dirac's prediction, positrons were discovered four years later, in 1932, by the American physicist Carl Anderson. In 1929 Hermann Weyl, a German-born mathematician, found another solution to the Dirac equation, this time massless [1]. A year later, the Austrian-born theoretical physicist Wolfgang Pauli postulated the existence of the neutrino, which was then thought to be massless, and it was assumed to be the sought-after solution to the Dirac equation found by Weyl. Neutrinos had not been detected yet in nature, but the case seemed to be closed. It would be decades before American physicists Frederick Reines and Clyde Cowan finally discovered neutrinos in 1957, and numerous experiments shortly thereafter indicated that neutrinos could have mass. In 1998, the Super-Kamiokande (a neutrino observatory located in Japan) Collaboration announced what had now been speculated for years: neutrinos have non-zero mass. This discovery opened a new question: what then was the zero-mass solution found by Weyl?
Dr. Ling Lu (MIT), Dr. Zhiyu Wang (Zhejiang University, China), Dr. Dexin Ye (Zhejiang University), Prof. Lixin Ran (Zhejiang University), Prof. Liang Fu (MIT), Prof. John D. Joannopoulos (MIT), and Prof. Marin Soljači? (MIT) found the answer.
Ling Lu, first author of the paper published in Science, is very enthusiastic: "Weyl points do actually exist in nature! We built a double-gyroid photonic crystal with broken parity symmetry. The light that passes through the crystal shows the signature of Weyl points in reciprocal space: two linear dispersion bands touching at isolated points." Weyl points, the solutions to the massless Dirac equation, were not found in particle experiments. The research team had to build a tailored material to observe them. The double-gyroid photonic crystal is itself a work of art. Gyroids indeed can be found in nature, in systems as different as butterfly wings and ketchup [2,3]. However, the research group wanted a double-gyroid with a very specific broken symmetry, first proposed in a theoretical work by the same group[4]. In order to fabricate this structure, with parts that are interlocking and with ad hoc defects (such as symmetry-breaking air holes), Lu and collaborators had to drill, machine, and stack slabs of ceramic-filled plastics (Figure 1).
Once the sample was ready, it was time to observe if it behaved as expected, by shining light through it and analyzing the outgoing signal. Physicists analyze these experiments in what is called reciprocal space, or momentum space. The right panel in Figure 2 shows what Weyl points are supposed to look like in reciprocal space: degenerate points, points where two linear dispersion bands meet. The left panel shows an example of the measured data, the solid proof that Weyl points do indeed exist in nature.
"The discovery of Weyl points is not only the smoking gun to a scientific mystery," comments MIT Professor Marin Soljači?, "it paves the way to absolutely new photonic phenomena and applications. Think of the graphene revolution: graphene is a 2D structure, and its electronic properties are, to a substantial extent, a consequence of the existence of linear degeneracy points (known as Dirac points) in its momentum space. Materials containing Weyl points do the same in 3D. They literally add one degree of freedom, one dimension." The discovery of graphene and its unique electronic properties was lauded with the 2010 Nobel Prize in physics, yet graphene's Dirac points are not stable to perturbations. On the other hand, the structures introduced by Lu et al. are very stable to perturbations*, offering a new tool to control how light is confined, how it bounces, and how it radiates. This discovery opens a new intriguing field in basic physics. The potential applications are equally promising. Examples include the possibility to build angularly selective 3D materials and more powerful single-frequency lasers.
###
* The stability of 3D Weyl points is due to the fact that they are topological monopoles. Monopoles can occur in two varieties, e.g. positive and negative. By analogy, electric monopoles are positive and negative charges. Electric charge is conserved, therefore electric monopoles can only be created or annihilated in pairs (positive and negative neutralize). The same is true for topological monopoles: they can only appear or disappear in pairs, making them more robust to perturbations. On the contrary, graphene's Dirac points are not topological monopoles: they are neutral, meaning that they do not need a companion to appear or disappear.
RESEARCH PAPER: "Experimental observation of Weyl points", Ling Lu, Zhiyu Wang, Dexin Ye, Lixin Ran, Liang Fu, John D. Joannopoulos, and Marin Soljači?, Science URL: http://www.sciencemag.org/lookup/doi/10.1126/science.aaa9273
http://www.eurekalert.org/pub_releases/2015-07/miot-wpw071515.php
