- Korean team claims to have created the first room-temperature, ambient-pressure superconductor Phys.org
- First Room-Temperature Ambient-Pressure Superconductor Achieved, Claim Scientists IFLScience
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Overcoming the “Impossible” With DNA to Building Superconductor That Could Transform Technology
In DNA, scientists find a solution to building a superconductor that could transform technology.
Could let computers work at warp speed, save energy, and even make trains fly.
Scientists have used
One such superconductor was first proposed by Stanford physicist William A. Little more than 50 years ago. Scientists have spent decades trying to make it work. However, even after validating the feasibility of his idea, they were left with a challenge that appeared impossible to overcome. Until now.
Edward H. Egelman, PhD, of the University of Virginia School of Medicine’s Department of Biochemistry and Molecular Genetics, has been a leader in the field of cryo-electron microscopy (cryo-EM), and he and his colleagues used cryo-EM imaging for this seemingly impossible project. “It demonstrates,” he said, “that the cryo-EM technique has great potential in materials research.” Credit: Dan Addison, UVA Communications
Edward H. Egelman, PhD, of UVA’s Department of Biochemistry and Molecular Genetics, has been a leader in the field of cryo-electron microscopy (cryo-EM), and he and Leticia Beltran, a graduate student in his lab, used cryo-EM imaging for this seemingly impossible project. “It demonstrates,” he said, “that the cryo-EM technique has great potential in materials research.”
Engineering at the Atomic Level
One possible way to realize Little’s idea for a superconductor is to modify lattices of carbon nanotubes. These are hollow cylinders of carbon so tiny they must be measured in nanometers – billionths of a meter. However, there was a huge challenge: controlling chemical reactions along the nanotubes so that the lattice could be assembled as precisely as needed and function as intended.
Egelman and his colleagues found an answer in the very building blocks of life. They took DNA, the genetic material that tells living cells how to operate, and used it to guide a chemical reaction that would overcome the great barrier to Little’s superconductor. In short, they used chemistry to perform astonishingly precise structural engineering – construction at the level of individual molecules. The result was a lattice of carbon nanotubes assembled specifically as needed for Little’s room-temperature superconductor.
“This work demonstrates that ordered carbon nanotube modification can be achieved by taking advantage of DNA-sequence control over the spacing between adjacent reaction sites,” Egelman said.
For now, the lattice they built has not been tested for superconductivity. However, it offers proof of principle and has great potential for the future, the researchers say. “While cryo-EM has emerged as the main technique in biology for determining the atomic structures of protein assemblies, it has had much less impact thus far in materials science,” said Egelman, whose prior work led to his induction in the National Academy of Sciences, one of the highest honors a scientist can receive.
Egelman and his collaborators say their DNA-guided approach to lattice construction could have a wide variety of useful research applications, especially in physics. But it also validates the possibility of building Little’s room-temperature superconductor. The scientists’ work, combined with other breakthroughs in superconductors in recent years, could ultimately transform technology as we know it and lead to a much more “Star Trek” future.
“While we often think of biology using tools and techniques from physics, our work shows that the approaches being developed in biology can actually be applied to problems in physics and engineering,” Egelman said. “This is what is so exciting about science: not being able to predict where our work will lead.”
Findings Published
The researchers have published their findings in the journal Science. The team consisted of Zhiwei Lin, Leticia Beltran, Zeus A. De los Santos, Yinong Li, Tehseen Adel, Jeffrey A Fagan, Angela Hight Walker, Egelman and Ming Zheng.
Reference: “DNA-guided lattice remodeling of carbon nanotubes” by Zhiwei Lin, Leticia C. Beltran, Zeus A. De los Santos, Yinong Li, Tehseen Adel, Jeffrey A Fagan, Angela R. Hight Walker, Edward H. Egelman and Ming Zheng, 28 July 2022, Science.
DOI: 10.1126/science.abo4628
The work was supported by the Department of Commerce’s National Institute of Standards and Technology and by National Institutes of Health grant GM122510, as well as by an NRC postdoctoral fellowship.
In DNA, scientists find solution to building superconductor that could transform technology
Scientists at the University of Virginia School of Medicine and their collaborators have used DNA to overcome a nearly insurmountable obstacle to engineer materials that would revolutionize electronics.
One possible outcome of such engineered materials could be superconductors, which have zero electrical resistance, allowing electrons to flow unimpeded. That means that they don’t lose energy and don’t create heat, unlike current means of electrical transmission. Development of a superconductor that could be used widely at room temperature—instead of at extremely high or low temperatures, as is now possible—could lead to hyper-fast computers, shrink the size of electronic devices, allow high-speed trains to float on magnets and slash energy use, among other benefits.
One such superconductor was first proposed more than 50 years ago by Stanford physicist William A. Little. Scientists have spent decades trying to make it work, but even after validating the feasibility of his idea, they were left with a challenge that appeared impossible to overcome. Until now.
Edward H. Egelman, Ph.D., of UVA’s Department of Biochemistry and Molecular Genetics, has been a leader in the field of cryo-electron microscopy (cryo-EM), and he and Leticia Beltran, a graduate student in his lab, used cryo-EM imaging for this seemingly impossible project. “It demonstrates,” he said, “that the cryo-EM technique has great potential in materials research.”
Engineering at the atomic level
One possible way to realize Little’s idea for a superconductor is to modify lattices of carbon nanotubes, hollow cylinders of carbon so tiny they must be measured in nanometers—billionths of a meter. But there was a huge challenge: controlling chemical reactions along the nanotubes so that the lattice could be assembled as precisely as needed and function as intended.
Egelman and his collaborators found an answer in the very building blocks of life. They took DNA, the genetic material that tells living cells how to operate, and used it to guide a chemical reaction that would overcome the great barrier to Little’s superconductor. In short, they used chemistry to perform astonishingly precise structural engineering—construction at the level of individual molecules. The result was a lattice of carbon nanotubes assembled as needed for Little’s room-temperature superconductor.
“This work demonstrates that ordered carbon nanotube modification can be achieved by taking advantage of DNA-sequence control over the spacing between adjacent reaction sites,” Egelman said.
The lattice they built has not been tested for superconductivity, for now, but it offers proof of principle and has great potential for the future, the researchers say. “While cryo-EM has emerged as the main technique in biology for determining the atomic structures of protein assemblies, it has had much less impact thus far in materials science,” said Egelman, whose prior work led to his induction in the National Academy of Sciences, one of the highest honors a scientist can receive.
Egelman and his colleagues say their DNA-guided approach to lattice construction could have a wide variety of useful research applications, especially in physics. But it also validates the possibility of building Little’s room-temperature superconductor. The scientists’ work, combined with other breakthroughs in superconductors in recent years, could ultimately transform technology as we know it and lead to a much more “Star Trek” future.
“While we often think of biology using tools and techniques from physics, our work shows that the approaches being developed in biology can actually be applied to problems in physics and engineering,” Egelman said. “This is what is so exciting about science: not being able to predict where our work will lead.”
The researchers have published their findings in the journal Science.
Atomic-scale window into superconductivity paves the way for new quantum materials
More information:
Zhiwei Lin et al, DNA-guided lattice remodeling of carbon nanotubes, Science (2022). DOI: 10.1126/science.abo4628
Zhiwei Lin et al, DNA-guided lattice remodeling of carbon nanotubes, Science (2022). DOI: 10.1126/science.abo4628
Provided by
University of Virginia
University of Virginia
Citation:
In DNA, scientists find solution to building superconductor that could transform technology (2022, August 2)
retrieved 2 August 2022
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Breakthrough Discovery of the One-Way Superconductor – Thought To Be Impossible
Artist Impression of a superconducting chip. Credit: TU Delft
Associate professor Mazhar Ali and his research group at Delft University of Technology (TU Delft) have discovered one-way superconductivity without magnetic fields, something that was thought to be impossible ever since its discovery in 1911 – until now. The discovery, published in the journal Nature, makes use of 2D quantum materials and paves the way toward superconducting computing. Superconductors can make electronics hundreds of times faster, all with zero energy loss.
Ali: “If the 20th century was the century of semiconductors, the 21st can become the century of the superconductor.”
Throughout the twentieth century, many scientists, including Nobel laureates, struggled over the nature of superconductivity, which was discovered in 1911 by Dutch physicist Kamerlingh Onnes. In superconductors, a current flows across a wire with no resistance, which means inhibiting this current or even blocking it is hardly possible – let alone getting the current to flow only one way and not the other. The fact that Ali’s group was able to make superconducting one-directional – which is required for computing – is remarkable: it’s like inventing a special type of ice that has zero friction one way but insurmountable friction the other.
Superconductor: super-fast, super-green
The advantages of applying superconductors to electronics are twofold. Superconductors can make electronics hundreds of times faster, and incorporating superconductors into our daily lives would make IT much more eco-friendly: if you spun a superconducting wire from here to the moon, it would transport the energy without any loss. For instance, the use of superconductors instead of regular semiconductors might save up to 10% of all western energy reserves according to NWO.
According to the Dutch Research Council (NWO), using superconductors instead of conventional semiconductors might save up to 10% of all Western energy reserves.
The (im)possibility of applying superconducting
In the 20th century and beyond, no one could tackle the barrier of making superconducting electrons go in just one-direction, which is a fundamental property needed for computing and other modern electronics (consider for example diodes that go one way as well). In normal conduction the electrons fly around as separate particles; in superconductors they move in pairs of twos, without any loss of electrical energy. In the 70s, scientists at IBM tried out the idea of superconducting computing but had to stop their efforts: in their papers on the subject, IBM mentions that without non-reciprocal superconductivity, a computer running on superconductors is impossible.
Superconductivity is a set of physical properties seen in some materials in which electrical resistance disappears and magnetic flux fields are expelled. A superconductor is any substance that possesses these qualities.
Interview with corresponding author Mazhar Ali
Q: Why, when one-way direction works with normal semi-conduction, has one-way superconductivity never worked before?
Mazhar Ali: “Electrical conduction in semiconductors, like Si, can be one-way because of a fixed internal electric dipole, so a net built in potential they can have. The textbook example is the famous “pn junction”; where we slap together two semiconductors: one has extra electrons (-) and the other has extra holes (+). The separation of charge makes a net built-in potential that an electron flying through the system will feel. This breaks symmetry and can result in “one-way” properties because forward vs backward, for example, are no longer the same. There is a difference in going in the same direction as the dipole vs going against it; similar to if you were swimming with the river or swimming up the river.”
“Superconductors never had an analog of this one-directional idea without magnetic field; since they are more related to metals (i.e. conductors, as the name says) than semiconductors, which always conduct in both directions and don’t have any built-in potential. Similarly, Josephson Junctions (JJs), which are sandwiches of two superconductors with non-superconducting, classical barrier materials in-between the superconductors, also haven’t had any particular symmetry-breaking mechanism that resulted in a difference between “forward” and “backward.”
Q: How did you manage to do what first seemed impossible?
Ali: “It was really the result of one of my group’s fundamental research directions. In what we call “Quantum Material Josephson Junctions” (QMJJs), we replace the classical barrier material in JJs with a quantum material barrier, where the quantum material’s intrinsic properties can modulate the coupling between the two superconductors in novel ways. The Josephson Diode was an example of this: we used the quantum material Nb3Br8, which is a 2D material like
Read original article here
A kagome lattice superconductor reveals a ‘cascade’ of quantum electron states
Researchers have discovered a complex landscape of electronic states that can co-exist on a kagome lattice, resembling those in high-temperature superconductors, a team of Boston College physicists reports in an advance electronic publication of the journal Nature.
The focus of the study was a bulk single crystal of a topological kagome metal, known as CsV3Sb5—a metal that becomes superconducting below 2.5 degrees Kelvin, or minus 455 degrees Fahrenheit. The exotic material is built from atomic planes composed of Vanadium atoms arranged on a so-called kagome lattice—described as a pattern of interlaced triangles and hexagons—stacked on top of one another, with Cesium and Antimony spacer layers between the kagome planes.
The material offers a window into how the physical properties of quantum solids—such as light transmission, electrical conduction, or response to a magnetic field—relate to the underlying geometry of the atomic lattice structure. Because its geometry causes destructive interference and “frustrates” the kinetic motion of traversing electrons, kagome lattice materials are prized for offering the unique and fertile ground for the study of quantum electronic states described as frustrated, correlated and topological.
The majority of experimental efforts thus far have focused on kagome magnets. The material the team examined is not magnetic, which opens the door to investigate how electrons in kagome systems behave in the absence of magnetism. The electronic structure of these crystals can be classified as “topological”, while high electrical conductivity makes it a “metal”.
“This topological metal becomes superconducting at low temperature, which is a very rare occurrence of superconductivity in a kagome material,” said Boston College Associate Professor of Physics Ilija Zeljkovic, a lead co-author of the report, titled “Cascade of correlated electron states in a kagome superconductor CsV3Sb5.”
In a metal, electrons in the crystal form a liquid state. Electrical conduction happens when the charged liquid flows under a bias voltage. The team used scanning tunneling spectroscopy to probe the quantum interference effects of the electron liquid, said Zeljkovic, who conducted the research with Boston College colleagues Professor of Physics Ziqiang Wang, graduate student Hong Li, and He Zhao, who earned his doctorate in Physics at BC in 2020, as well as colleagues from the University of California, Santa Barbara.
The experiments revealed a “cascade” of symmetry-broken phases of the electron liquid driven by the correlation between the electrons in the material, the team reported.
Occurring consecutively as the temperature of the material was lowered, ripples, or standing waves, emerge first in the electron liquid, known as charge density waves, with periodicity different from the underlying atomic lattice. At a lower temperature, a new standing wave component nucleates only along one direction of the crystal axes, such that electrical conduction along this direction is different than in any other direction.
These phases develop in the normal state—or the non-superconducting metallic state—and persist below the superconducting transition, Wang said. The experiments demonstrate that superconductivity in CsV3Sb5 emerges from, and coexists with, a correlated quantum electronic state that breaks spatial symmetries of the crystal.
The findings could have strong implications for how the electrons form “Cooper” pairs and turn into a charged superfluid at an even lower temperature, or a superconductor capable of electrical conduction without resistance. In this family of kagome superconductors, other research has already suggested the possibility of unconventional electron pairing, said Zeljkovic.
Researchers in the field have noted a phenomenon called time-reversal symmetry breaking in CsV3Sb5. This symmetry rule—which holds that actions would be performed in reverse if time were to run backwards—is typically broken in magnetic materials, but the kagome metal shows no substantial magnetic moments. Zeljkovic said next steps in this research are to understand this apparent contradiction and how the electronic states revealed in this recent work are related to time-reversal symmetry breaking.
The level of significance and research into these recently-discovered kagome lattice superconductors is reflected in an associated Nature article published in the same advance electronic edition. Also co-authored by BC’s Ziqiang Wang, the paper, titled “Roton pair density wave in a strong-coupling kagome superconductor,” reports the observation of novel standing waves formed by Cooper pairs with yet another periodicity in the same kagome superconductor, CsV3Sb5.
“The publishing of these two reports side-by-side not only reveals new and broad insights into kagome lattice superconductors, but also signals the high level of interest and excitement surrounding these materials and their unique properties and phenomena, which researchers at Boston College and institutions around the world are discovering with increasing frequency,” Wang said.
Fully-gapped pairing in the new vanadium-based Kagome superconductors
More information:
He Zhao et al, Cascade of correlated electron states in a kagome superconductor CsV3Sb5, Nature (2021). DOI: 10.1038/s41586-021-03946-w
He Zhao et al, Cascade of correlated electron states in a kagome superconductor CsV3Sb5, Nature (2021). DOI: 10.1038/s41586-021-03946-w
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Boston College
Boston College
Citation:
A kagome lattice superconductor reveals a ‘cascade’ of quantum electron states (2021, September 30)
retrieved 1 October 2021
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What is a superconductor? | Live Science
A superconductor is a material that achieves superconductivity, which is a state of matter that has no electrical resistance and does not allow magnetic fields to penetrate. An electric current in a superconductor can persist indefinitely.
Superconductivity can only typically be achieved at very cold temperatures. Superconductors have a wide variety of everyday applications, from MRI machines to super-fast maglev trains that use magnets to levitate the trains off the track to reduce friction. Researchers are now trying to find and develop superconductors that work at higher temperatures, which would revolutionize energy transport and storage.
Who discovered superconductivity?
The credit for the discovery of superconductivity goes to Dutch physicist Heike Kamerlingh Onnes. In 1911, Onnes was studying the electrical properties of mercury in his laboratory at Leiden University in The Netherlands when he found that the electrical resistance in the mercury completely vanished when he dropped the temperature to below 4.2 Kelvin — that’s just 4.2 degrees Celsius (7.56 degrees Fahrenheit) above absolute zero.
To confirm this result, Onnes applied an electric current to a sample of supercooled mercury, then disconnected the battery. He found that the electric current persisted in the mercury without decreasing, confirming the lack of electrical resistance and opening the door to future applications of superconductivity.
History of superconductivity
Physicists spent decades trying to understand the nature of superconductivity and what caused it. They found that many elements and materials, but not all, become superconducting when cooled below a certain critical temperature.
In 1933, physicists Walther Meissner and Robert Ochsenfeld discovered that superconductors “expel” any nearby magnetic fields, meaning weak magnetic fields can’t penetrate far inside a superconductor, according to Hyper Physics, an educational site from the Georgia State University department of physics and astronomy. This phenomenon is called the Meissner effect.
It wasn’t until 1950 that theoretical physicists Lev Landau and Vitaly Ginzburg published a theory of how superconductors work, according to Ginzburg’s biography on The Nobel Prize website. While successful in predicting the properties of superconductors, their theory was “macroscopic,” meaning it focused on the large-scale behaviors of superconductors while remaining ignorant of what was going on at a microscopic level.
Finally, in 1957, physicists John Bardeen, Leon N. Cooper and Robert Schrieffer developed a complete, microscopic theory of superconductivity. To create electrical resistance, the electrons in a metal need to be free to bounce around. But when the electrons inside a metal become incredibly cold, they can pair up, preventing them from bouncing around. These electron pairs, called Cooper pairs, are very stable at low temperatures, and with no electrons “free” to bounce around, the electrical resistance disappears. Bardeen, Cooper and Schrieffer put these pieces together to form their theory, known as BCS theory, which they published in the journal Physical Review Letters.
How do superconductors work?
When a metal drops below a critical temperature, the electrons in the metal form bonds called Cooper pairs. Locked up like this, the electrons can’t provide any electrical resistance, and electricity can flow through the metal perfectly, according to the University of Cambridge.
However, this only works at low temperatures. When the metal gets too warm, the electrons have enough energy to break the bonds of the Cooper pairs and go back to offering resistance. That is why Onnes, in his original experiments, found that mercury behaved as a superconductor at 4.19 K, but not 4.2 K.
What are superconductors used for?
It’s very likely that you’ve encountered a superconductor without realizing it. In order to generate the strong magnetic fields used in magnetic resonance imaging (MRI) and nuclear magnetic resonance imaging (NMRI), the machines use powerful electromagnets, as described by the Mayo Clinic. These powerful electromagnets would melt normal metals due to the heat of even a little bit of resistance. However, because superconductors have no electrical resistance, no heat is generated, and the electromagnets can generate the necessary magnetic fields.
Similar superconducting electromagnets are also used in maglev trains, experimental nuclear fusion reactors and high-energy particle accelerator laboratories.Superconductors are also used to power railguns and coilguns, cell phone base stations, fast digital circuits and particle detectors.
Essentially, any time you need a really strong magnetic field or electric current and don’t want your equipment to melt the moment you turn it on, you need a superconductor.
“One of the most interesting applications of superconductors is for quantum computers,” said Alexey Bezryadin, a condensed matter physicist at the University of Illinois at Urbana-Champaign. Because of the unique properties of electrical currents in superconductors, they can be used to construct quantum computers.
“Such computers are composed of quantum bits or qubits. Qubits, unlike classical bits of information, can exist in quantum superposition states of being ‘0’ and ‘1’ at the same time. Superconducting devices can mimic this,” Bezryadin told Live Science. “For example, the current in a superconducting loop can flow clockwise and counterclockwise at the same time. Such a state constitutes an example of a superconducting qubit.”
What’s the latest in superconductor research?
The first challenge for today’s researchers is “to develop materials that are superconductors at ambient conditions, because currently superconductivity only exists either at very low temperatures or at very high pressures,” said Mehmet Dogan, a postdoctoral researcher at the University of California, Berkeley. The next challenge is to develop a theory that explains how the novel superconductors work and predict the properties of those materials, Dogan told Live Science in an email.
Superconductors are separated into two main categories: low-temperature superconductors (LTS), also known as conventional superconductors, and high-temperature superconductors (HTS), or unconventional superconductors. LTS can be described by the BCS theory to explain how the electrons form Cooper pairs, while HTS use other microscopic methods to achieve zero resistance. The origins of HTS are one of the major unsolved problems of modern-day physics.
Most of the historical research on superconductivity has been in the direction of LTS, because those superconductors are much easier to discover and study, and almost all applications of superconductivity involve LTS.
HTS, in contrast, are an active and exciting area of modern-day research. Anything that works as a superconductor above 70 K is generally considered an HTS. Even though that’s still pretty cold, that temperature is desirable because it can be reached by cooling with liquid nitrogen, which is far more common and readily available than the liquid helium needed to cool to the even lower temperatures that are needed for LTS.
The future of superconductors
The “holy grail” of superconductor research is to find a material that can act as a superconductor at room temperatures. To date, the highest superconducting temperature was reached with extremely pressurized carbonaceous sulfur hydride, which reached superconductivity at 59 F (15 C, or about 288 K), but required 267 gigapascals of pressure to do it. That pressure is equivalent to the interior of giant planets like Jupiter, which makes it impractical for everyday applications.
Room-temperature superconductors would allow for the electrical transmission of energy with no losses or waste, more efficient maglev trains, and cheaper and more ubiquitous use of MRI technology. The practical applications of room-temperature superconductors are limitless — physicists just need to figure out how superconductors work at room temperatures and what the “Goldilocks” material to allow for superconductivity might be.
Additional resources
MIT Physicists Discover “Magic-Angle” Trilayer Graphene May Be a Rare, Magnet-Proof Superconductor
MIT physicists have observed signs of a rare type of superconductivity in a material called “magic-angle” twisted trilayer graphene. Credit: Courtesy of Pablo Jarillo-Herrero, Yuan Cao, Jeong Min Park, et al
New findings might help inform the design of more powerful MRI machines or robust quantum computers.
MIT physicists have observed signs of a rare type of superconductivity in a material called magic-angle twisted trilayer graphene. In a study appearing in Nature, the researchers report that the material exhibits superconductivity at surprisingly high magnetic fields of up to 10 Tesla, which is three times higher than what the material is predicted to endure if it were a conventional superconductor.
The results strongly imply that magic-angle trilayer graphene, which was initially discovered by the same group, is a very rare type of superconductor, known as a “spin-triplet,” that is impervious to high magnetic fields. Such exotic superconductors could vastly improve technologies such as magnetic resonance imaging, which uses superconducting wires under a magnetic field to resonate with and image biological tissue. MRI machines are currently limited to magnet fields of 1 to 3 Tesla. If they could be built with spin-triplet superconductors, MRI could operate under higher magnetic fields to produce sharper, deeper images of the human body.
The new evidence of spin-triplet superconductivity in trilayer graphene could also help scientists design stronger superconductors for practical quantum computing.
“The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity,” says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT.
His co-authors on the paper include postdoc Yuan Cao and graduate student Jeong Min Park at MIT, and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.
Strange shift
Superconducting materials are defined by their super-efficient ability to conduct electricity without losing energy. When exposed to an electric current, electrons in a superconductor couple up in “Cooper pairs” that then travel through the material without resistance, like passengers on an express train.
In a vast majority of superconductors, these passenger pairs have opposite spins, with one electron spinning up, and the other down — a configuration known as a “spin-singlet.” These pairs happily speed through a superconductor, except under high magnetic fields, which can shift the energy of each electron in opposite directions, pulling the pair apart. In this way, and through mechanisms, high magnetic fields can derail superconductivity in conventional spin-singlet superconductors.
“That’s the ultimate reason why in a large-enough magnetic field, superconductivity disappears,” Park says.
But there exists a handful of exotic superconductors that are impervious to magnetic fields, up to very large strengths. These materials superconduct through pairs of electrons with the same spin — a property known as “spin-triplet.” When exposed to high magnetic fields, the energy of both electrons in a Cooper pair shift in the same direction, in a way that they are not pulled apart but continue superconducting unperturbed, regardless of the magnetic field strength.
Jarillo-Herrero’s group was curious whether magic-angle trilayer graphene might harbor signs of this more unusual spin-triplet superconductivity. The team has produced pioneering work in the study of graphene moiré structures — layers of atom-thin carbon lattices that, when stacked at specific angles, can give rise to surprising electronic behaviors.
The researchers initially reported such curious properties in two angled sheets of graphene, which they dubbed magic-angle bilayer graphene. They soon followed up with tests of trilayer graphene, a sandwich configuration of three graphene sheets that turned out to be even stronger than its bilayer counterpart, retaining superconductivity at higher temperatures. When the researchers applied a modest magnetic field, they noticed that trilayer graphene was able to superconduct at field strengths that would destroy superconductivity in bilayer graphene.
“We thought, this is something very strange,” Jarillo-Herrero says.
A super comeback
In their new study, the physicists tested trilayer graphene’s superconductivity under increasingly higher magnetic fields. They fabricated the material by peeling away atom-thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle one by 1.56 degrees with respect to the outer layers. They attached an electrode to either end of the material to run a current through and measure any energy lost in the process. Then they turned on a large magnet in the lab, with a field which they oriented parallel to the material.
As they increased the magnetic field around trilayer graphene, they observed that superconductivity held strong up to a point before disappearing, but then curiously reappeared at higher field strengths — a comeback that is highly unusual and not known to occur in conventional spin-singlet superconductors.
“In spin-singlet superconductors, if you kill superconductivity, it never comes back — it’s gone for good,” Cao says. “Here, it reappeared again. So this definitely says this material is not spin-singlet.”
They also observed that after “re-entry,” superconductivity persisted up to 10 Tesla, the maximum field strength that the lab’s magnet could produce. This is about three times higher than what the superconductor should withstand if it were a conventional spin-singlet, according to Pauli’s limit, a theory that predicts the maximum magnetic field at which a material can retain superconductivity.
Trilayer graphene’s reappearance of superconductivity, paired with its persistence at higher magnetic fields than predicted, rules out the possibility that the material is a run-of-the-mill superconductor. Instead, it is likely a very rare type, possibly a spin-triplet, hosting Cooper pairs that speed through the material, impervious to high magnetic fields. The team plans to drill down on the material to confirm its exact spin state, which could help to inform the design of more powerful MRI machines, and also more robust quantum computers.
“Regular quantum computing is super fragile,” Jarillo-Herrero says. “You look at it and, poof, it disappears. About 20 years ago, theorists proposed a type of topological superconductivity that, if realized in any material, could [enable] a quantum computer where states responsible for computation are very robust. That would give infinite more power to do computing. The key ingredient to realize that would be spin-triplet superconductors, of a certain type. We have no idea if our type is of that type. But even if it’s not, this could make it easier to put trilayer graphene with other materials to engineer that kind of superconductivity. That could be a major breakthrough. But it’s still super early.”
Reference: “Pauli-limit violation and re-entrant superconductivity in moiré graphene” by Yuan Cao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi and Pablo Jarillo-Herrero, 21 July 2021, Nature.
DOI: 10.1038/s41586-021-03685-y
This research was supported by the U.S. Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, the Fundacion Ramon Areces, and the CIFAR Quantum Materials Program.