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Wednesday, December 21st, 2016
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| 11:45a |
Majorana fermions predicted in a semiconducting material A low-temperature material made from the elements praseodymium, osmium, and antimony should be able to host subatomic particles known as Majorana fermions, MIT researchers have shown in a theoretical analysis.
Majorana fermions, first predicted by physicists in 1937, can be thought of as electrons split into two parts, each of which behaves as independent particles. These fermions do not exist as elementary particles in nature but can emerge in certain superconducting materials near absolute zero temperature. In superconducting materials, electrons flow without resistance generating little or no heat.
The new analysis by graduate student Vladyslav Kozii, postdoc Jörn Venderbos, and Lawrence C. (1944) and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu predicts this special state should occur in a praseodymium, osmium and antimony compound, PrOs4Sb12, and similar materials made of heavy metals.
Physicists describe electrons by their energy, momentum, and spin. An electron can occupy a possible energy level, and an unoccupied level is called a hole. In the new analysis, Majorana fermions emerge as a quantum superposition of an electron and a hole that move freely, with each having the same direction, or spin. This Majorana fermion spin can interact with the spin of atomic nuclei in the material, so it ought to be seen using nuclear magnetic resonance techniques, they predict.
“We address a certain class of superconductors, show that they have Majorana fermions as freely propagating quasiparticles in the bulk, and then look at how they can be detected and what other properties these materials have that one could use in the future for interesting functionality,” says Venderbos. “I think it very nicely bridges the gap between experiment and theory and it can be used by experimentalists right now.” Their paper was published this month in the journal Science Advances.
A key physics concept in this work is that of time-reversal symmetry. Such symmetry means that equations of motions governing an object or particle stay the same if one could reverse the direction of time — with time flowing backward rather forward. If the equation of motion of electrons in a material is different when time flows backwards — as is true in magnets, for instance — then time-reversal symmetry is said to be broken. This gives physicists an important way to distinguish different materials. In the proposed antimony-compound based superconductor, analysis shows that the Majorana fermions can only exist when time reversal symmetry is broken. Upon reversing the motion in time, the spin of the Majorana fermions is reversed — for example, from clockwise to counterclockwise — and this implies a different equation of motion for Majorana fermions going backward in time. “Regarding the material that we proposed, actually there is one recent experiment that confirms that time-reversal symmetry is broken in the superconducting state of this material. This reinforces our conclusion that it is indeed a very promising candidate for our theory to apply,” Kozii explains.
Majorana fermions were first proposed by Italian physicist Ettore Majorana as a special mathematical solution for quantum behavior of electrons. Princeton University researchers reported detection of a zero-dimensional realization of these particles at the end of an atom chain in October 2014. The MIT theorists now show that the three-dimensional propagating Majorana fermions they predict are governed by Majorana’s original equation. “The extensive study we have performed shows that this peculiar particle may now find its realization in solid state physics in a real material,” Venderbos says.
Electrons in materials such as metals and semiconductors can fill only certain energy levels, or bands, with excluded, or forbidden, energy levels referred to as a bandgap. In a superconductor, this is also called the superconducting gap. Ordinarily, it takes outside energy in order to lift a lower energy electron to a higher energy level, especially when it has to cross a bandgap. The Fu groups’ analysis of praseodymium, osmium, and antimony reveals that there are some special points in its electronic excitation spectrum where the bandgap vanishes in its superconducting state, which means that low energy excitations are possible. “However low energy you take, there will be always excitation at this energy. These excitations are exactly these Majorana fermions we were talking about,” Kozii explains. Venderbos adds, “There are some excitations for which you don’t have to put in any energy or just an infinitesimally tiny amount and you can still create the excitation.”
Noting that Fu has made “some fantastic predictions in the past,” Princeton University professor of chemistry Robert J. Cava, who was not involved in this research, suggests: “Experimentalists should listen to what he has to say. ... I am very happy to see that he and his coworkers have presented an analysis of real materials in which their ideas might be embodied.”
Kozii, Venderbos, and Fu analyzed these unconventional superconductors for a year. For Kozii, the work will become part of his doctoral thesis.
The researchers hope their work will inspire experimentalists to look again at some previously studied materials to identify ones that host superconducting states with Majorana fermions. “I think the first step would be just to find a material in which everyone can agree that it has these Majorana fermions. That would be really exciting and constitute the discovery of a new type of superconductor in experiment,” Venderbos says. “The next step would be to think about functionalization of these materials, what could be the specific applications.” Trying to make quantum devices out of these materials is one possible direction. “We hope this research ultimately brings closer efforts from the quantum material and quantum device community in finding out the many facets of Majorana fermions,” Fu adds.
The Department of Energy Office of Basic Energy Sciences, Division of Materials Sciences and Engineering supported Fu’s and Kozii’s work. The Netherlands Organization for Scientific Research supported Venderbos through a Rubicon grant. | | 11:45a |
Majorana fermions predicted in a superconducting material A low-temperature material made from the elements praseodymium, osmium, and antimony should be able to host subatomic particles known as Majorana fermions, MIT researchers have shown in a theoretical analysis.
Majorana fermions, first predicted by physicists in 1937, can be thought of as electrons split into two parts, each of which behaves as independent particles. These fermions do not exist as elementary particles in nature but can emerge in certain superconducting materials near absolute zero temperature. In superconducting materials, electrons flow without resistance generating little or no heat.
The new analysis by graduate student Vladyslav Kozii, postdoc Jörn Venderbos, and Lawrence C. (1944) and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu predicts this special state should occur in a praseodymium, osmium and antimony compound, PrOs4Sb12, and similar materials made of heavy metals.
Physicists describe electrons by their energy, momentum, and spin. An electron can occupy a possible energy level, and an unoccupied level is called a hole. In the new analysis, Majorana fermions emerge as a quantum superposition of an electron and a hole that move freely, with each having the same direction, or spin. This Majorana fermion spin can interact with the spin of atomic nuclei in the material, so it ought to be seen using nuclear magnetic resonance techniques, they predict.
“We address a certain class of superconductors, show that they have Majorana fermions as freely propagating quasiparticles in the bulk, and then look at how they can be detected and what other properties these materials have that one could use in the future for interesting functionality,” says Venderbos. “I think it very nicely bridges the gap between experiment and theory and it can be used by experimentalists right now.” Their paper was published this month in the journal Science Advances.
A key physics concept in this work is that of time-reversal symmetry. Such symmetry means that equations of motions governing an object or particle stay the same if one could reverse the direction of time — with time flowing backward rather forward. If the equation of motion of electrons in a material is different when time flows backwards — as is true in magnets, for instance — then time-reversal symmetry is said to be broken. This gives physicists an important way to distinguish different materials. In the proposed antimony-compound based superconductor, analysis shows that the Majorana fermions can only exist when time reversal symmetry is broken. Upon reversing the motion in time, the spin of the Majorana fermions is reversed — for example, from clockwise to counterclockwise — and this implies a different equation of motion for Majorana fermions going backward in time. “Regarding the material that we proposed, actually there is one recent experiment that confirms that time-reversal symmetry is broken in the superconducting state of this material. This reinforces our conclusion that it is indeed a very promising candidate for our theory to apply,” Kozii explains.
Majorana fermions were first proposed by Italian physicist Ettore Majorana as a special mathematical solution for quantum behavior of electrons. Princeton University researchers reported detection of a zero-dimensional realization of these particles at the end of an atom chain in October 2014. The MIT theorists now show that the three-dimensional propagating Majorana fermions they predict are governed by Majorana’s original equation. “The extensive study we have performed shows that this peculiar particle may now find its realization in solid state physics in a real material,” Venderbos says.
Electrons in materials such as metals and semiconductors can fill only certain energy levels, or bands, with excluded, or forbidden, energy levels referred to as a bandgap. In a superconductor, this is also called the superconducting gap. Ordinarily, it takes outside energy in order to lift a lower energy electron to a higher energy level, especially when it has to cross a bandgap. The Fu groups’ analysis of praseodymium, osmium, and antimony reveals that there are some special points in its electronic excitation spectrum where the bandgap vanishes in its superconducting state, which means that low energy excitations are possible. “However low energy you take, there will be always excitation at this energy. These excitations are exactly these Majorana fermions we were talking about,” Kozii explains. Venderbos adds, “There are some excitations for which you don’t have to put in any energy or just an infinitesimally tiny amount and you can still create the excitation.”
Noting that Fu has made “some fantastic predictions in the past,” Princeton University professor of chemistry Robert J. Cava, who was not involved in this research, suggests: “Experimentalists should listen to what he has to say. ... I am very happy to see that he and his coworkers have presented an analysis of real materials in which their ideas might be embodied.”
Kozii, Venderbos, and Fu analyzed these unconventional superconductors for a year. For Kozii, the work will become part of his doctoral thesis.
The researchers hope their work will inspire experimentalists to look again at some previously studied materials to identify ones that host superconducting states with Majorana fermions. “I think the first step would be just to find a material in which everyone can agree that it has these Majorana fermions. That would be really exciting and constitute the discovery of a new type of superconductor in experiment,” Venderbos says. “The next step would be to think about functionalization of these materials, what could be the specific applications.” Trying to make quantum devices out of these materials is one possible direction. “We hope this research ultimately brings closer efforts from the quantum material and quantum device community in finding out the many facets of Majorana fermions,” Fu adds.
The Department of Energy Office of Basic Energy Sciences, Division of Materials Sciences and Engineering supported Fu’s and Kozii’s work. The Netherlands Organization for Scientific Research supported Venderbos through a Rubicon grant. | | 12:00p |
Distinctive brain pattern may underlie dyslexia A distinctive neural signature found in the brains of people with dyslexia may explain why these individuals have difficulty learning to read, according to a new study from MIT neuroscientists.
The researchers discovered that in people with dyslexia, the brain has a diminished ability to acclimate to a repeated input — a trait known as neural adaptation. For example, when dyslexic students see the same word repeatedly, brain regions involved in reading do not show the same adaptation seen in typical readers.
This suggests that the brain’s plasticity, which underpins its ability to learn new things, is reduced, says John Gabrieli, the Grover M. Hermann Professor in Health Sciences and Technology, a professor of brain and cognitive sciences, and a member of MIT’s McGovern Institute for Brain Research.
“It’s a difference in the brain that’s not about reading per se, but it’s a difference in perceptual learning that’s pretty broad,” says Gabrieli, who is the study’s senior author. “This is a path by which a brain difference could influence learning to read, which involves so many demands on plasticity.”
Former MIT graduate student Tyler Perrachione, who is now an assistant professor at Boston University, is the lead author of the study, which appears in the Dec. 21 issue of Neuron.
Reduced plasticity
The MIT team used magnetic resonance imaging (MRI) to scan the brains of young adults with and without reading difficulties as they performed a variety of tasks. In the first experiment, the subjects listened to a series of words read by either four different speakers or a single speaker.
The MRI scans revealed distinctive patterns of activity in each group of subjects. In nondyslexic people, areas of the brain that are involved in language showed neural adaption after hearing words said by the same speaker, but not when different speakers said the words. However, the dyslexic subjects showed much less adaptation to hearing words said by a single speaker.
Neurons that respond to a particular sensory input usually react strongly at first, but their response becomes muted as the input continues. This neural adaptation reflects chemical changes in neurons that make it easier for them to respond to a familiar stimulus, Gabrieli says. This phenomenon, known as plasticity, is key to learning new skills.
“You learn something upon the initial presentation that makes you better able to do it the second time, and the ease is marked by reduced neural activity,” Gabrieli says. “Because you’ve done something before, it’s easier to do it again.”
The researchers then ran a series of experiments to test how broad this effect might be. They asked subjects to look at series of the same word or different words; pictures of the same object or different objects; and pictures of the same face or different faces. In each case, they found that in people with dyslexia, brain regions devoted to interpreting words, objects, and faces, respectively, did not show neural adaptation when the same stimuli were repeated multiple times.
“The brain location changed depending on the nature of the content that was being perceived, but the reduced adaptation was consistent across very different domains,” Gabrieli says.
He was surprised to see that this effect was so widespread, appearing even during tasks that have nothing to do with reading; people with dyslexia have no documented difficulties in recognizing objects or faces.
He hypothesizes that the impairment shows up primarily in reading because deciphering letters and mapping them to sounds is such a demanding cognitive task. “There are probably few tasks people undertake that require as much plasticity as reading,” Gabrieli says.
Early appearance
In their final experiment, the researchers tested first and second graders with and without reading difficulties, and they found the same disparity in neural adaptation.
“We got almost the identical reduction in plasticity, which suggests that this is occurring quite early in learning to read,” Gabrieli says. “It’s not a consequence of a different learning experience over the years in struggling to read.”
Guinevere Eden, a professor of pediatrics and director of the Center for the Study of Learning at Georgetown University Medical Center, described the study as “groundbreaking.”
“For children with dyslexia, we know that the brain looks different in terms of anatomy and function, but we have not been able to establish why,” says Eden, who was not involved in the research. “This study makes an important step in that direction: It gets to the true characteristics of the properties of the neurons in these brain regions, not just their outward appearance.”
Gabrieli’s lab now plans to study younger children to see if these differences might be apparent even before children begin to learn to read. They also hope to use other types of brain measurements such as magnetoencephalography (MEG) to follow the time course of the neural adaptation more closely.
The research was funded by the Ellison Medical Foundation, the National Institutes of Health, and a National Science Foundation Graduate Research Fellowship. | | 3:55p |
New approach calculates benefits of building hazard-resistant structures Hazard-induced maintenance costs can be significant over the lifetime of a building. Researchers at the MIT Concrete Sustainability Hub (CSHub) are developing new methods to calculate the benefits of investing in more hazard-resistant structures. Jeremy Gregory, executive director of the CSHub recently presented one metric, the CSHub’s Break-Even Hazard Mitigation Percentage (BEMP), to officials in Florida and Georgia — states that can see millions in property damage due to hurricanes.
“The BEMP evaluates the cost-effectiveness of mitigation features for a building in a particular location by factoring in the expected damage a conventional building designed to code would endure over its lifetime, and comparing it to a more resilient, enhanced building design,” says Gregory. “In areas prone to natural disasters, more spending on mitigation is justified — the BEMP helps to identify how much extra spending is recommended.”
The southeastern United States was hit hard by weather patterns resulting from Hurricane Matthew in October. Georgia has sustained some $90 million in insured losses to date, and total claims are expected to rise. Florida was spared Matthew’s worst effects, but the state is regularly witness to the destructive power of such storms and there’s a lot at stake: The insured value of residential and commercial properties in Florida’s coastal counties now exceeds $13 trillion.
Gregory spoke to officials and members of the building community in Atlanta, Georgia, and Tallahassee, Florida, this month during roundtable discussions about building resilience, the BEMP, and hazard mitigation. He also presented the topic to journalists and industry professionals during a recent webinar.
“Structures in coastal areas states like Florida and Georgia are prone to damage from high winds and hurricanes,” says Gregory. “Through previous case studies we’ve demonstrated that investing in more hazard-resistant residential construction in some locations can be very cost-effective, especially in costal states where the impact of hurricanes can have devastating economic effects.”
One case study showed a BEMP of 3.4 percent for in the coastal city Galveston, Texas, meaning for a $10 million midrise apartment building, $340,000 could be spent on mitigation, and costs would break even over the building life. The highest BEMP calculations are in cities in southeastern Florida, where the values are approximately 8 percent.
Too often, building developers make decisions about materials or building techniques to keep initial costs down. Although the resulting structures are built to code, those codes often fail to factor in the long-term costs or impacts on future owners and communities. One of the goals of this research is widespread adoption of codes and standards that incorporate hazard mitigation into building design.
“Hazard mitigation efforts offer benefits to society at large,” says Gregory. “Builders or short-term owners might have to invest more up front, but — by decreasing recovery costs and lessening the impact on lives — insurance agencies, taxpayers, and future occupants benefit in the long run. Because of these long-term benefits, this is a concept that it makes sense for state officials to get behind.” | | 3:55p |
New approach calculates benefits of building hazard-resistant structures Hazard-induced maintenance costs can be significant over the lifetime of a building. Researchers at the MIT Concrete Sustainability Hub (CSHub) are developing new methods to calculate the benefits of investing in more hazard-resistant structures. Jeremy Gregory, executive director of the CSHub recently presented one metric, the CSHub’s Break-Even Hazard Mitigation Percentage (BEMP), to officials in Florida and Georgia — states that can see millions in property damage due to hurricanes.
“The BEMP evaluates the cost-effectiveness of mitigation features for a building in a particular location by factoring in the expected damage a conventional building designed to code would endure over its lifetime, and comparing it to a more resilient, enhanced building design,” says Gregory. “In areas prone to natural disasters, more spending on mitigation is justified — the BEMP helps to identify how much extra spending is recommended.”
The southeastern United States was hit hard by weather patterns resulting from Hurricane Matthew in October. Georgia has sustained some $90 million in insured losses to date, and total claims are expected to rise. Florida was spared Matthew’s worst effects, but the state is regularly witness to the destructive power of such storms and there’s a lot at stake: The insured value of residential and commercial properties in Florida’s coastal counties now exceeds $13 trillion.
Gregory spoke to officials and members of the building community in Atlanta, Georgia, and Tallahassee, Florida, this month during roundtable discussions about building resilience, the BEMP, and hazard mitigation. He also presented the topic to journalists and industry professionals during a recent webinar.
“Structures in coastal areas states like Florida and Georgia are prone to damage from high winds and hurricanes,” says Gregory. “Through previous case studies we’ve demonstrated that investing in more hazard-resistant residential construction in some locations can be very cost-effective, especially in costal states where the impact of hurricanes can have devastating economic effects.”
One case study showed a BEMP of 3.4 percent for in the coastal city Galveston, Texas, meaning for a $10 million midrise apartment building, $340,000 could be spent on mitigation, and costs would break even over the building life. The highest BEMP calculations are in cities in southeastern Florida, where the values are approximately 8 percent.
Too often, building developers make decisions about materials or building techniques to keep initial costs down. Although the resulting structures are built to code, those codes often fail to factor in the long-term costs or impacts on future owners and communities. One of the goals of this research is widespread adoption of codes and standards that incorporate hazard mitigation into building design.
“Hazard mitigation efforts offer benefits to society at large,” says Gregory. “Builders or short-term owners might have to invest more up front, but — by decreasing recovery costs and lessening the impact on lives — insurance agencies, taxpayers, and future occupants benefit in the long run. Because of these long-term benefits, this is a concept that it makes sense for state officials to get behind.” |
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