Tag Archives: #superconductor

“Magic-angle” Trilayer Graphene May Be A Rare, Magnet-proof Superconductor (Physics)

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 today 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.

Jeong Min Park stands next to a dilution refrigerator apparatus, the type of apparatus that measures at very cold temperatures, just a tenth of a degree above absolute zero temperature.
Credits:Image: Courtesy of the researchers

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.”

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.

Featured image: MIT physicists have observed signs of a rare type of superconductivity in a material called “magic-angle” twisted trilayer graphene.Credits:Image: Courtesy of the researchers

Reference: Cao, Y., Park, J.M., Watanabe, K. et al. Pauli-limit violation and re-entrant superconductivity in moiré graphene. Nature (2021). https://doi.org/10.1038/s41586-021-03685-y

Provided by MIT

Physicists Uncover Secrets of World’s Thinnest Superconductor (Physics)

First experimental evidence of spin excitations in an atomically thin material helps answer 30-year-old questions, could lead to better medical diagnostics and more.

Physicists from across three continents report the first experimental evidence to explain the unusual electronic behavior behind the world’s thinnest superconductor, a material with myriad applications because it conducts electricity extremely efficiently. In this case, the superconductor is only an atomic layer thick.

The work, led by an MIT professor and a physicist at Brookhaven National Laboratory, was possible thanks to new instrumentation available at only a few facilities in the world. The resulting data could help guide the development of better superconductors. These in turn could transform the fields of medical diagnostics, quantum computing, and energy transport, which all use superconductors.

The subject of the work belongs to an exciting class of superconductors that become superconducting at temperatures an order of magnitude higher than their conventional counterparts, making them easier to use in applications. Conventional superconductors only work at temperatures around 10 kelvins, or -442 degrees Fahrenheit.

These so-called high-temperature superconductors, however, are still not fully understood. “Their microscopic excitations and dynamics are essential to understanding superconductivity, yet after 30 years of research, many questions are still very much open,” says Riccardo Comin, the Class of 1947 Career Development Assistant Professor of Physics at MIT. The new work, which was reported recently in Nature Communications, helps answer those questions.

Comin’s colleagues on the work include Jonathan Pelliciari, a former MIT postdoc who is now an assistant physicist at Brookhaven National Laboratory and lead author of this study. Other authors are Seher Karakuzu and Thomas A. Maier of Oak Ridge National Laboratory; Qi Song, Tianlun Yu, Xiaoyang Chen, Rui Peng, Qisi Wang, Jun Zhao, and Donglai Feng of Fudan University; Riccardo Arpaia, Matteo Rossi, and Giacomo Ghiringhelli of Politecnico di Milano (Arpaia is also affiliated with Chalmers University of Technology); Abhishek Nag, Jiemin Li, Mirian García-Fernández, Andrew C. Walters, and Ke-Jin Zhou of Diamond Light Source in the United Kingdom; and Steven Johnston of the University of Tennessee at Knoxville.

World’s thinnest superconductor

In 2015 scientists discovered a new kind of high-temperature superconductor: a sheet of iron selenide only one atomic layer thick capable of superconducting at 65 K. In contrast, bulk samples of the same material superconduct at a much lower temperature (8 K). The discovery “sparked an investigative flurry to decode the secrets of the world’s thinnest superconductor,” says Comin, who is also affiliated with MIT’s Materials Research Laboratory.

In a regular metal, electrons behave much like individual people dancing in a room. In a superconducting metal, the electrons move in pairs, like couples at a dance. “And all these pairs are moving in unison, as if they were part of a quantum choreography, ultimately leading to a kind of electronic superfluid,” says Comin.

But what is the interaction, or “glue,” that holds these pairs of electrons together? Scientists have known for a long time that in conventional superconductors, that glue is derived from the motion of atoms within a material. “If you look at a solid sitting on a table, it doesn’t appear to be doing anything,” Comin says. However, “a lot is happening at the nanoscale. Inside that material, electrons are flying by in all possible directions and the atoms are rattling; they’re vibrating.” In conventional superconductors, the electrons use the energy stored in that atomic motion to pair up.

The glue behind electrons’ pairing in high-temperature superconductors is different. Scientists have hypothesized that this glue is related to a property of electrons called spin (another, more familiar property of electrons is their charge). The spin can be thought of as an elementary magnet, says Pelliciari. The idea is that in a high-temperature superconductor, electrons can pick up some of the energy from these spins, known as spin excitations. And that energy is the glue they use to pair up.

Until now, most physicists thought that it would be impossible to detect or measure spin excitations in a material only an atomic layer thick. That is the remarkable achievement of the work reported in Nature Communications. Not only did the physicists detect spin excitations, but, among other things, they also showed that the spin dynamics in the ultra-thin sample were dramatically different from those in the bulk sample. Specifically, the energy of the fluctuating spins in the ultra-thin sample was much higher — by a factor of four or five — than the energy of the spins in the bulk sample.

“This is the first experimental evidence of the presence of spin excitations in an atomically thin material,” says Pelliciari.

State-of-the art equipment

Historically, neutron scattering has been used to study magnetism. Since spin is the fundamental property of magnetism, neutron scattering would appear to be a good experimental probe. “The problem is that neutron scattering doesn’t work on a material that is only one atomic layer thick,” says Pelliciari.

Enter resonant inelastic X-ray scattering (RIXS), a new experimental technique that Pelliciari helped pioneer.

He and Comin discussed the potential for using RIXS to study the spin dynamics of the new ultra-thin superconductor, but Comin was initially skeptical. “I thought, ‘Yes, it would be great if we could do this, but experimentally it’s going to be next-to-impossible,’” Comin remembers. “I thought it was a true moonshot.” As a result, “when Johnny collected the very first results, it was mind-blowing for me. I’d kept my expectations low, so when I saw the data, I jumped on my chair.”

Only a few facilities in the world have advanced RIXS instruments. One, located at Diamond Light Source (UK) and led by Zhou, is where the team conducted their experiment. Another one, which was still being built at the time of the experiment, is at Brookhaven National Laboratory. Pelliciari is now part of the team running the RIXS facility, known as the Beamline SIX, at the National Synchrotron Light Source II located at Brookhaven Lab.

“The impact of this work is two-fold,” says Thorsten Schmitt, head of the Spectroscopy of Novel Materials Group at the Paul Scherrer Institut in Switzerland, who was not involved in the work. “On the experimental side, it is an impressive demonstration of the sensitivity of RIXS to the spin excitations in a superconducting material only an atomic layer thick. Furthermore, the [resulting data] are expected to contribute to the understanding of the enhancement of the superconducting transition temperature in such thin superconductors.” In other words, the work could lead to even better superconductors.

Valentina Bisogni, lead scientist for the Beamline SIX who was not involved in this study, says, “the understanding of unconventional superconductivity is one of the main challenges faced by scientists today. The recent discovery of high-temperature superconductivity in a monolayer-thin film of iron selenide renewed the interest into the iron selenide system, as it provides a new route to investigate the mechanisms enabling high-temperature superconductivity.

“In this context, the work of Pelliciari et al. presents an enlightening, comparative study of bulk iron selenide and monolayer-thin iron seleniderevealing a dramatic reconfiguration of the spin excitations,” Bisogni says.

This research was supported by the U.S. Air Force Office of Scientific Research, the MIT-POLIMI Program (Progetto Rocca), the Swiss National Science Foundation, the U.S. Department of Energy (DOE), the U.S. Office of Naval Research, the Fondazione CARIPLO and Regione Lombardia, the Swedish Research Council, the Alfred P. Sloan Foundation, and the National Natural Science Foundation of China.

This research used resources of the National Synchrotron Light Source II, a DOE Office of Science user facility located at DOE’s Brookhaven Lab.

Featured image: Former MIT postdoc Jonathan Pelliciari, now an assistant physicist at Brookhaven National Laboratory, holds onto part of the resonant inelastic X-ray scattering (RIXS) instrument at BNL. Pelliciari is lead author of a study that used RIXS to uncover secrets of the world’s thinnest superconductor. Credits: Photo courtesy of Brookhaven National Laboratory.

Reference: Pelliciari, J., Karakuzu, S., Song, Q. et al. Evolution of spin excitations from bulk to monolayer FeSe. Nat Commun 12, 3122 (2021). https://doi.org/10.1038/s41467-021-23317-3

Provided by MIT

Researchers Uncover Unique Properties Of A Promising New Superconductor (Material Science)

An international team of physicists led by the University of Minnesota has discovered that a unique superconducting metal is more resilient when used as a very thin layer. The research is the first step toward a larger goal of understanding unconventional superconducting states in materials, which could possibly be used in quantum computing in the future. 

The collaboration includes four faculty members in the University of Minnesota’s School of Physics and Astronomy—Associate Professor Vlad Pribiag, Professor Rafael Fernandes, and Assistant Professors Fiona Burnell and Ke Wang—along with physicists at Cornell University and several other institutions. The study is published in Nature Physics, a monthly, peer-reviewed scientific journal published by the Nature Research.

Niobium diselenide (NbSe2) is a superconducting metal, meaning that it can conduct electricity, or transport electrons from one atom to another, with no resistance. It is not uncommon for materials to behave differently when they are at a very small size, but NbSe2 has potentially beneficial properties. The researchers found that the material in 2D form (a very thin substrate only a few atomic layers thick) is a more resilient superconductor because it has a two-fold symmetry, which is very different from thicker samples of the same material.

Motivated by Fernandes and Burnell’s theoretical prediction of exotic superconductivity in this 2D material, Pribiag and Wang started to investigate atomically-thin 2D superconducting devices. 

“We expected it to have a six-fold rotational pattern, like a snowflake.” Wang said. “Despite the six-fold structure, it only showed two-fold behavior in the experiment.” 

“This was one of the first times [this phenomenon] was seen in a real material,” Pribiag said.

The researchers attributed the newly-discovered two-fold rotational symmetry of the superconducting state in NbSe2 to the mixing between two closely competing types of superconductivity, namely the conventional s-wave type—typical of bulk NbSe2—and an unconventional d- or p-type mechanism that emerges in few-layer NbSe2. The two types of superconductivity have very similar energies in this system. Because of this, they interact and compete with each other.

Pribiag and Wang said they later became aware that physicists at Cornell University were reviewing the same physics using a different experimental technique, namely quantum tunneling measurements. They decided to combine their results with the Cornell research and publish a comprehensive study.

Burnell, Pribiag, and Wang plan to build on these initial results to further investigate the properties of atomically thin NbSe2 in combination with other exotic 2D materials, which could ultimately lead to the use of unconventional superconducting states, such as topological superconductivity, to build quantum computers.

“What we want is a completely flat interface on the atomic scale,” Pribiag said. “We believe this system will be able to give us a better platform to study materials to use them for quantum computing applications.”

In addition to Pribiag, Fernandes, Burnell, Wang, the collaboration included University of Minnesota physics graduate students Alex Hamill, Brett Heischmidt, Daniel Shaffer, Kan-Ting Tsai, and Xi Zhang; Cornell University faculty members Jie Shan and Kin Fai Mak and graduate student Egon Sohn; Helmuth Berger and László Forró, researchers at Ecole Polytechnique Fédérale de Lausanne in Switzerland; Alexey Suslov, a researcher at the National High Magnetic Field Laboratory in Tallahassee, Fla.; and Xiaoxiang Xi, a professor at Nanjing University in China. 

The University of Minnesota research was supported primarily by the National Science Foundation (NSF) through the University of Minnesota Materials Research Science and Engineering Center (MRSEC). The research at Cornell was supported by the Office of Naval Research (ONR) and NSF. The work in Switzerland was supported by the Swiss National Science Foundation.

Featured image: A team of physicists led by the University of Minnesota has discovered that the unique superconducting metal Niobium diselenide (NbSe2) is more resilient when used as a very thin layer. The above diagram depicts the different s-, p-, and d-wave superconducting states in the metal. Photo credit: Alex Hamill and Brett Heischmidt, University of Minnesota

Reference: Hamill, A., Heischmidt, B., Sohn, E. et al. Two-fold symmetric superconductivity in few-layer NbSe2. Nat. Phys. (2021). https://doi.org/10.1038/s41567-021-01219-x

Provided by University of Minnesota

Why Deep Freezing Iron-based Materials Makes Them Both Magnetic & Superconducting (Physics)

Physicists at Bath have uncovered a new mechanism for enabling magnetism and superconductivity to co-exist in the same material.

Physicists at the University of Bath, in collaboration with researchers from the USA, have uncovered a new mechanism for enabling magnetism and superconductivity to co-exist in the same material. Until now, scientists could only guess how this unusual coexistence might be possible. The discovery could lead to applications in green energy technologies and in the development of superconducting devices, such as next-generation computer hardware.

As a rule, superconductivity (the ability of a material to pass an electrical current with perfect efficiency) and magnetism (seen at work in fridge magnets) make poor bedfellows because the alignment of the tiny electronic magnetic particles in ferromagnets generally leads to the destruction of the electron pairs responsible for superconductivity. Despite this, the Bath researchers have found that the iron-based superconductor RbEuFe4As4, which is superconducting below -236°C, exhibits both superconductivity and magnetism below -258°C.

Physics postgraduate research student David Collomb, who led the research, explained: “There’s a state in some materials where, if you get them really cold – significantly colder than the Antarctic – they become superconducting. But for this superconductivity to be taken to next-level applications, the material needs to show co-existence with magnetic properties. This would allow us to develop devices operating on a magnetic principle, such as magnetic memory and computation using magnetic materials, to also enjoy the benefits of superconductivity.

“The problem is that superconductivity is usually lost when magnetism is turned on. For many decades, scientists have tried to explore a host of materials that have both properties in a single material, and material scientists have recently had some success fabricating a handful of such materials. However, so long as we don’t understand why the coexistence is possible, the hunt for these materials can’t be done with as fine a comb.

“This new research gives us a material that has a wide temperature range where these phenomena co-exist, and this will allow us to study the interaction between magnetism and superconductivity more closely and in great detail. Hopefully, this will result in us being able to identify the mechanism through which this co-existence can occur.”

In a study published in Physical Review Letters, the team investigated the unusual behaviour of RbEuFe4As4 by creating magnetic field maps of a superconducting material as the temperature was dropped. To their surprise, they found the vortices (the points in the superconducting material where the magnetic field penetrates) showed a pronounced broadening near the temperature of -258°C, indicating a strong suppression of superconductivity as the magnetism turned on.

crystals coasted in gold
On the left: a crystal coated in gold – the gold coating allows the magnetic imaging tool to get within nanometers of the material’s surface. On the right: a magnetic picture of a segment of the crystal showing the vortices (dark holes) that were studied. © University of Bath

These observations agree with a theoretical model recently proposed by Dr Alexei Koshelev at Argonne National Laboratory in the USA. This theory describes the suppression of superconductivity by magnetic fluctuations due to the Europium (Eu) atoms in the crystals. Here, the magnetic direction of each Eu atom begins to fluctuate and align with the others, as the material drops below a certain temperature. This causes the material to become magnetic. The Bath researchers conclude that while superconductivity is considerably weakened by the magnetic effect, it is not fully destroyed.

“This suggests that in our material, the magnetism and superconductivity are held apart from each other in their own sub-lattices, which only minimally interact,” said Mr Collomb.

“This work significantly advances our understanding of these rare coexisting phenomena and could lead to possible applications in the superconducting devices of the future. It will spawn a deeper hunt into materials that display both superconductivity and magnetism. We hope it will also encourage researchers in more applied fields to take some of these materials and make the next-generation computing devices out of them.

“Hopefully, the scientific community will gradually enter an era where we move from blue-sky research to making devices from these materials. In a decade or so, we could be seeing prototype devices using this technology that do a real job.”

The American collaborators for this project were the Argonne National Laboratory, Hofstra University and Northwestern University.

Featured image: Magnetism can be generated simply by passing a current through a wire, but how it interacts with other physical phenomena (such as superconductivity) is shrouded in mystery. © University of Bath

Reference: D. Collomb, S. J. Bending, A. E. Koshelev, M. P. Smylie, L. Farrar, J.-K. Bao, D. Y. Chung, M. G. Kanatzidis, W.-K. Kwok, and U. Welp, “Observing the Suppression of Superconductivity in RbEuFe4As4 by Correlated Magnetic Fluctuations”, Phys. Rev. Lett. 126, 157001 – Published 14 April 2021. DOI: https://doi.org/10.1103/PhysRevLett.126.157001

Provided by University of Bath

Brand New Physics of Superconducting Metals Refuted by Lancaster Physicists (Physics)

Lancaster scientists have demonstrated that other physicists’ recent “discovery” of the field effect in superconductors is nothing but hot electrons after all.

A team of scientists in the Lancaster Physics Department have found new and compelling evidence that the observation of the field effect in superconducting metals by another group can be explained by a simple mechanism involving the injection of the electrons, without the need for novel physics.

Dr Sergey Kafanov, who initiated this experiment, said: “Our results unambiguously refute the claim of the electrostatic field effect claimed by the other group. This gets us back on the ground and helps maintain the health of the discipline.”

The experimental team also includes Ilia Golokolenov, Andrew Guthrie, Yuri Pashkin and Viktor Tsepelin.

Their work is published in the latest issue of Nature Communications.

When certain metals are cooled to a few degrees above absolute zero, their electrical resistance vanishes – a striking physical phenomenon known as superconductivity. Many metals, including vanadium, which was used in the experiment, are known to exhibit superconductivity at sufficiently low temperatures.

For decades it was thought that the exceptionally low electrical resistance of superconductors should make them practically impervious to static electric fields, owing to the way the charge carriers can easily arrange themselves to compensate for any external field.

It therefore came as a shock to the physics community when a number of recent publications claimed that sufficiently strong electrostatic fields could affect superconductors in nanoscale structures – and attempted to explain this new effect with corresponding new physics. A related effect is well known in semiconductors and underpins the entire semiconductor industry.

The Lancaster team embedded a similar nanoscale device into a microwave cavity, allowing them to study the alleged electrostatic phenomenon at much shorter timescales than previously investigated. At short timescales, the team could see a clear increase in the noise and energy loss in the cavity – the properties strongly associated with the device temperature. They propose that at intense electric fields, high-energy electrons can “jump” into the superconductor, raising the temperature and therefore increasing the dissipation.

This simple phenomenon can concisely explain the origin of the “electrostatic field effect” in nanoscale structures, without any new physics.

Featured image: Superconducting circuits find applications in sensing and information processing © Lancaster University

Provided by Lancaster University

Mapping the Electronic States in an Exotic Superconductor (Material Science)

Scientists characterized how these states depend on local chemical composition, narrowing the search for where to look compositionally to enable quantum computing

Scientists characterized how the electronic states in a compound containing iron, tellurium, and selenium depend on local chemical concentrations. They discovered that superconductivity (conducting electricity without resistance), along with distinct magnetic correlations, appears when the local concentration of iron is sufficiently low; a coexisting electronic state existing only at the surface (topological surface state) arises when the concentration of tellurium is sufficiently high. Reported in Nature Materials, their findings point to the composition range necessary for topological superconductivity. Topological superconductivity could enable more robust quantum computing, which promises to deliver exponential increases in processing power.

“Quantum computing is still in its infancy, and one of the key challenges is reducing the error rate of the computations,” said first author Yangmu Li, a postdoc in the Neutron Scattering Group of the Condensed Matter Physics and Materials Science (CMPMS) Division at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. “Errors arise as qubits, or quantum information bits, interact with their environment. However, unlike trapped ions or solid-state qubits such as point defects in diamond, topological superconducting qubits are intrinsically protected from part of the noise. Therefore, they could support computation less prone to errors. The question is, where can we find topological superconductivity?

Photo of Yangmu Li
Yangmu Li © BNL

In this study, the scientists narrowed the search in one compound known to host topological surface states and part of the family of iron-based superconductors. In this compound, topological and superconducting states are not distributed uniformly across the surface. Understanding what’s behind these variations in electronic states and how to control them is key to enabling practical applications like topologically protected quantum computing.

From previous research, the team knew modifying the amount of iron could switch the material from a superconducting to nonsuperconducting state. For this study, physicist Gendu Gu of the CMPMS Division grew two types of large single crystals, one with slightly more iron relative to the other. The sample with the higher iron content is nonsuperconducting; the other sample is superconducting.

To understand whether the arrangement of electrons in the bulk of the material varied between the superconducting and nonsuperconducting samples, the team turned to spin-polarized neutron scattering. The Spallation Neutron Source (SNS), located at DOE’s Oak Ridge National Laboratory, is home to a one-of-a-kind instrument for performing this technique.  

“Neutron scattering can tell us the magnetic moments, or spins, of electrons and the atomic structure of a material,” explained corresponding author, Igor Zaliznyak, a physicist in the CMPMS Division Neutron Scattering Group who led the Brookhaven team that helped design and install the instrument with collaborators at Oak Ridge. “In order to single out the magnetic properties of electrons, we polarize the neutrons using a mirror that reflects only one specific spin direction.”

To their surprise, the scientists observed drastically different patterns of electron magnetic moments in the two samples. Therefore, the slight alteration in the amount of iron caused a change in electronic state.

Photo of Igor Zaliznyak
Igor Zaliznyak © BNL

“After seeing this dramatic change, we figured we should look at the distribution of electronic states as a function of local chemical composition,” said Zaliznyak.

At Brookhaven’s Center for Functional Nanomaterials (CFN), Li, with support from CFN staff members Fernando Camino and Gwen Wright, determined the chemical composition across representative smaller pieces of both sample types through energy-dispersive x-ray spectroscopy. In this technique, a sample is bombarded with electrons, and the emitted x-rays characteristic of different elements are detected. They also measured the local electrical resistance—which indicates how coherently electrons can transport charge—with microscale electrical probes. For each crystal, Li defined a small square grid (100 by 100 microns). In total, the team mapped the local composition and resistance at more than 2,000 different locations.

“Through the experiments at the CFN, we characterized the chemistry and overall conduction properties of the electrons,” said Zaliznyak. “But we also need to characterize the microscopic electronic properties, or how electrons propagate in the material, whether in the bulk or on the surface. Superconductivity induced in electrons propagating on the surface can host topological objects called Majorana modes, which are in theory one of the best ways to perform quantum computations. Information on bulk and surface electronic properties can be obtained through photoemission spectroscopy.”

For the photoemission spectroscopy experiments, Zaliznyak and Li reached out to Peter Johnson, leader of the CMPMS Division Electron Spectroscopy Group, and Nader Zaki, a scientific associate in Johnson’s group. By measuring the energy and momentum of electrons ejected from the samples (using the same spatial grid) in response to light, they quantified the strengths of the electronic states propagating on the surface, in the bulk, and forming the superconducting state. They quantitatively fit the photoemission spectra to a model that characterizes the strengths of these states.

Then, the team mapped the electronic state strengths as a function of local composition, essentially building a phase diagram.

“This phase diagram includes the superconducting and topological phase transitions and points to where we could find a useful chemical composition for quantum computation materials,” said Li. “For certain compositions, no coherent electronic states exist to develop topological superconductivity. In previous studies, people thought instrument failure or measurement error were why they weren’t seeing features of topological superconductivity. Here we show that it’s due to the electronic states themselves.”

“When the material is close to the transition between the topological and nontopological state, you can expect fluctuations,” added Zaliznyak. “For topology to arise, the electronic states need to be well-developed and coherent. So, from a technological perspective, we need to synthesize materials away from the transition line.”

Next, the scientists will expand the phase diagram to explore the compositional range in the topological direction, focusing on samples with less selenium and more tellurium. They are also considering applying neutron scattering to understand an unexpected energy gap (an energy range where no electrons are allowed) opening in the topological surface state of the same compound. Johnson’s group recently discovered this gap and hypothesized it was caused by surface magnetism.

This work was supported by the DOE Office of Science. The CFN and SNS are both DOE Office of Science User Facilities.

Featured image: (Left) Through neutron scattering experiments, scientists observed distinct patterns of magnetic correlations in superconducting (“single-stripe magnetism”) and nonsuperconducting (“double-stripe magnetism”) samples of a compound containing iron (Fe), tellurium (Te), and selenium (Se). (Right) A material phase diagram showing where the superconducting state (SC), nonsuperconducting state (NSC), and topological superconducting state (SC + TSS) appear as a function of Fe and Te concentrations. The starred A refers to the nonsuperconducting sample and the starred B to the superconducting sample. Overlaid on the phase diagram are photoemission spectra showing the emergence (left) and absence (right) of the topological state. Topological superconductivity is an electronic state that could be harnessed for more robust quantum computing. © BNL

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New 2D Superconductor Forms at Higher Temperatures Than Ever Before (Material Science)

New interfacial superconductor has novel properties that raise new fundamental questions and might be useful for quantum information processing or quantum sensing.

Interfaces in solids form the basis for much of modern technology. For example, transistors found in all our electronic devices work by controlling the electrons at interfaces of semiconductors. More broadly, the interface between any two materials can have unique properties that are dramatically different from those found within either material separately, setting the stage for new discoveries.

Like semiconductors, superconducting materials have many important implications for technology, from magnets for MRIs to speeding up electrical connections or perhaps making possible quantum technology. The vast majority of superconducting materials and devices are 3D, giving them properties that are well understood by scientists.

One of the foundational questions with superconducting materials involves the transition temperature — the extremely cold temperature at which a material becomes superconducting.  All superconducting materials at regular pressures become superconducting at temperatures far below the coldest day outside.

Now, researchers at the U.S. Department of Energy’s Argonne National Laboratory have discovered a new way to generate 2D superconductivity at a material interface at a relatively high — though still cold —  transition temperature. This interfacial superconductor has novel properties that raise new fundamental questions and might be useful for quantum information processing or quantum sensing.

In the study, Argonne postdoctoral researcher Changjiang Liu and colleagues, working in a team led by Argonne materials scientist Anand Bhattacharya, have discovered that a novel 2D superconductor forms at the interface of an oxide insulator called KTaO3 (KTO). Their results were published online in the journal Science on February 12.

In 2004, scientists observed a thin sheet of conducting electrons between two other oxide insulators, LaAlO3 (LAO) and SrTiO(STO). It was later shown that that this material, called a 2D electron gas (2DEG) can even become superconducting — allowing the transport of electricity without dissipating energy. Importantly, the superconductivity could be switched on and off using electric fields, just like in a transistor.

However, to achieve such a superconducting state, the sample had to be cooled down to about 0.2 K — a temperature that is close to absolute zero (– 273.15 °C), requiring a specialized apparatus known as a dilution refrigerator. Even with such low transition temperatures (TC), the LAO/STO interface has been heavily studied in the context of superconductivity, spintronics and magnetism.

In the new research, the team discovered that in KTO, interfacial superconductivity could emerge at much higher temperatures. To obtain the superconducting interface, Liu, graduate student Xi Yan and coworkers grew thin layers of either europium oxide (EuO) or LAO on KTO using state-of-the-art thin film growth facilities at Argonne.

“This new oxide interface makes the application of 2D superconducting devices more feasible,” Liu said. ​“With its order-of-magnitude higher transition temperature of 2.2 K, this material will not need a dilution refrigerator to be superconducting. Its unique properties raise many interesting questions.”

A strange superconductor

Surprisingly, this new interfacial superconductivity shows a strong dependence on the orientation of the facet of the crystal where the electron gas is formed.

Adding to the mystery, measurements suggest the formation of stripe-like superconductivity in lower doping samples where rivulets of superconducting regions are separated by normal, nonsuperconducting regions. This kind of spontaneous stripe formation is also called nematicity, and is usually found in liquid crystal materials used for displays.

“Electronic realizations of nematicity are rare and of great fundamental interest. It turns out that EuO overlayer is magnetic, and the role of this magnetism in realizing the nematic state in KTO remains an open question,” Bhattacharya said.

In their Science paper, the authors also discuss the reasons why the electron gas forms. Using atomic resolution transmission electron microscopes, Jianguo Wen at the Center for Nanoscale Materials at Argonne, along with Professor Jian-Min Zuo’s group at the University of Illinois at Urbana-Champaign, showed that defects formed during the growth of the overlayer may play a central role.

In particular, they found evidence for oxygen vacancies and substitutional defects, where the potassium atoms are replaced by europium or lanthanum ions — all of which add electrons to the interface and turn it into a 2D conductor. Using ultrabright X-rays at the Advanced Photon Source (APS), Yan along with Argonne scientists Hua Zhou and Dillon Fong, probed the interfaces of KTO buried under the overlayer and observed spectroscopic signatures of these extra electrons near the interface.

“Interface-sensitive X-ray toolkits available at the APS empower us to reveal the structural basis for the 2DEG formation and the unusual crystal-facet dependence of the 2D superconductivity. A more detailed understanding is in progress,” Zhou said.

Beyond describing the mechanism of 2DEG formation, these results point the way to improving the quality of the interfacial electron gas by controlling synthesis conditions. Being that the superconductivity occurs for both the EuO and LAO oxide overlayers that have been tried thus far, many other possibilities remain to be explored.

The research is discussed in the paper ​“Two-dimensional superconductivity and anisotropic transport at KTaO3 (111) interfaces,” ScienceDOI: 10.1126/science.aba5511.

The authors are Changjiang Liu, Xi Yan, Dafei Jin, Yang Ma, Haw-Wen Hsiao, Yulin Lin, Terence M. Bretz-Sullivan, Xianjing Zhou, John Pearson, Brandon Fisher, J. Samuel Jiang, Wei Han, Jian-Min Zuo, Jianguo Wen, Dillon D. Fong, Jirong Sun, Hua Zhou and Anand Bhattacharya.

The work at Argonne was supported by DOE’s Office of Science (Office of Basic Energy Sciences). The Center for Nanoscale Materials and the Advanced Photon Source are both DOE Office of Science User Facilities.

Featured image: Superconducting state discovered at interfaces with (111) oriented KTaO3 surfaces, which has a buckled honeycomb lattice. Cooper pairs of electrons are shown in purple. Transport measurements suggest that the superconducting state is anisotropic. (Image by Anand Bhattacharya/Argonne National Laboratory.)

Provided by Argonne National Laboratory

In a first, Scientists Watch 2D Puddles of Electrons Spontaneously Emerge in a 3D Superconducting Material (Physics)

It’s an example of how surprising properties can spontaneously emerge in complex materials – a phenomenon scientists hope to harness for novel technologies.

Creating a two-dimensional material, just a few atoms thick, is often an arduous process requiring sophisticated equipment. So scientists were surprised to see 2D puddles emerge inside a three-dimensional superconductor – a material that allows electrons to travel with 100% efficiency and zero resistance – with no prompting.  

Within those puddles, superconducting electrons acted as if they were confined inside an incredibly thin, sheet-like plane, a situation that requires them to somehow cross over to another dimension, where different rules of quantum physics apply.

“This is a tantalizing example of emergent behavior, which is often difficult or impossible to replicate by trying to engineer it from scratch,” said Hari Manoharan, a professor at Stanford University and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at the Department of Energy’s SLAC National Accelerator Laboratory, who led the research. 

“It’s as if when given the power to superconduct,” he said, “the 3D electrons choose for themselves to live in a 2D world.”

The research team calls this new phenomenon “inter-dimensional superconductivity,” and in a report in the Proceedings of the National Academy of Sciences today, they suggest that this is how 3D superconductors reorganize themselves just before undergoing an abrupt shift into an insulating state, where electrons are confined to their home atoms and can’t move around at all.

“What we found was a system where electrons behave in unexpected ways. That’s the beauty of physics,” said Carolina Parra, a postdoctoral researcher at SLAC and Stanford at the time of the study who carried out the experiments that led to the visualization of this intriguing result. “We were very lucky to find this behavior.”

SLAC and Stanford scientists observed puddles of 2D superconducting behavior emerging from a 3D unconventional superconductor, which conducts electricity with 100% efficiency at unusually high temperatures. Their study suggests that this so-called “emergent” behavior may be how 3D superconductors reorganize themselves just before undergoing an abrupt shift into an insulating state, where electrons are confined to their home atoms and can’t move around at all. (Greg Stewart/SLAC National Accelerator Laboratory)

Electrons acting strangely

Although superconductivity was discovered more than a century ago, its usefulness was limited by the fact that materials became superconducting only at temperatures close to those of deep space.

So the announcement in 1986 that scientists had discovered a new and unexpected class of superconducting materials that operated at much higher – although still very cold – temperatures set off a tsunami of research that continues to this day, with the goal of figuring out how the new materials operate and developing versions that work at closer to room temperature for applications such as perfectly efficient power lines and maglev trains.

This study started with a high-temperature superconductor named BPBO for its four atomic ingredients – barium, lead, bismuth and oxygen. It was synthesized in the lab of Stanford Professor and SIMES investigator Ian Fisher by Paula Giraldo-Gallo, a PhD student at the time.

Stanford University Professor and SIMES investigator Hari Manoharan. (L. Cicero/Stanford University)

As researchers there put it through routine tests, including determining the transition temperature at which it flips between a superconducting and an insulating phase – like water changing to steam or ice – they realized that their data showed electrons behaving as if they were confined to ultrathin, 2D layers or stripes within the material. This was a puzzle, because BPBO is a 3D superconductor whose electrons are normally free to move in any direction they like.

Intrigued, Manoharan’s team took a closer look with a scanning tunneling microscope, or STM – an instrument that can identify and even move individual atoms in the top few atomic layers of a material.

Interacting puddles

The stripes, they discovered, seemed to have no relationship with the way the material’s atoms were organized or with tiny bumps and dips on its surface.

“Instead, the stripes were layers where electrons behave as if they are confined to 2D, puddle-like areas in the material,” Parra said. “The distance between puddles is short enough that the electrons can ‘see’ and interact with each other in a way that allows them to move without resistance, which is the hallmark of superconductivity.”

Carolina Parra (center), who as a Stanford postdoc carried out the experiments that led to the visualization of these intriguing results, now heads a lab at the Federico Santa María Technical University in Valparaíso, Chile, focusing on interdisciplinary studies of nanoscale biological materials. She recently won a grant to acquire and operate the first-ever low-temperature scanning tunneling microscope in South America, which she plans to use to continue this line of research. (Photo courtesy of Carolina Parra)

The 2D puddles emerged as the scientists carefully adjusted the temperature and other conditions toward the transition point where the superconductor would become an insulator.

Their observations closely match a theory of “emergent electronic granularity” in superconductors, developed by Nandini Trivedi of Ohio State University and colleagues.

“The predictions we had made went against the standard paradigm for superconductors,” Trivedi said. “Usually, the stronger a superconductor is, the more the energy needed to break the bond between its superconducting electron pairs – a factor we call the energy gap. But my group had predicted that in this particular type of disordered superconductor, the opposite would be true: The system would form emergent puddles where superconductivity was strong but the pairs could be broken with much less energy than expected.

“It was quite thrilling to see those predictions being confirmed by the STM measurements from the Stanford group!”

Spreading the science

The results have practical implications for crafting 2D materials, Parra said.

“Most of the methods for making 2D materials are engineering approaches, like growing films a few atomic layers thick or creating a sharp interface between two materials and confining a 2D state there,” she said. “This offers an additional way to get to these 2D superconducting states. It’s cheaper, you don’t need fancy equipment that requires very low temperatures and it doesn’t take days and weeks. The only tricky part would be getting the composition of the material just right.”

Parra now heads a lab at the Federico Santa María Technical University in Valparaíso, Chile, focusing on interdisciplinary studies of nanoscale biological materials. She recently won a grant to acquire and operate the first-ever low-temperature scanning tunneling microscope in South America, which she plans to use to continue this line of research.

“When I have this equipment in the lab,” she said, “I will connect it with all the things I learned in Hari’s lab and use it to teach a new generation of researchers that we’re going to have working in nanoscience and nanotechnology in Chile.”

The research was funded by the DOE Office of Science.

Reference: Carolina Parra et al., “Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor”, Proceedings of the National Academy of Sciences, 12 April 2021 (10.1073/pnas.201781011)

Provided by SLAC

Room Temperature Superconductor? Rochester Lab Sets New Record Toward Long-sought Goal (Physics)

University physical scientists synthesize new superconducting materials, developing processes that may help ‘break down barriers and open the door to many potential applications.’

Compressing simple molecular solids with hydrogen at extremely high pressures, engineers and physicists are setting new records in the race for materials that are superconducting at room temperature.

In a pair of studies published last fall and this spring, the lab of Ranga Dias, an assistant professor of mechanical engineering and of physics and astronomy at the University of Rochester, has reported a new record for the temperature at which materials have superconductivity and has developed a novel way to synthesize superconducting materials at lower pressures than previously reported.

Dias says developing materials that are superconducting—without electrical resistance and expulsion of magnetic field at room temperature—is the “holy grail” of condensed matter physics. Sought for more than a century, such materials “can definitely change the world as we know it,” Dias says.

In a report featured as the cover article in the journal Nature (and as part of the Nature Podcast), Dias and his research team combined hydrogen with carbon and sulfur to photochemically synthesize simple organic-derived carbonaceous sulfur hydride in a diamond anvil cell, a research device used to examine miniscule amounts of materials under extraordinarily high pressure.

The result was a new record: a material that exhibited superconductivity at about 58 degrees Fahrenheit and a pressure of about 39 million pounds per square inch (psi).

In a second study published in Physical Review Letters, the lab described separating hydrogen atoms from yttrium with a thin film of palladium. The resulting yttrium superhydride is superconducting at 12 degrees Fahrenheit and about 26 million pounds per square inch. (Pressure at sea level is about 15 psi.)

While that pressure is still too high for practical applications, the new material is a significant improvement over the room temperature superconducting materials the researchers reported last fall in Nature. And both results demonstrate progress toward eventually creating a room temperature superconductor.

“We will continue to use this new method to synthesize new superconducting materials at ambient pressure,” Dias says.

Dias, who is also affiliated with the University’s materials science and high-energy-density physics programs, says if the extraordinary properties of superconductivity could be made more practical, such materials would open the door to many potential applications.

How a diamond anvil works to create a room temperature superconductor. (Courtesy of the Dias Lab)

Applications include:

  • Power grids that transmit electricity without the loss of up to 200 million megawatt hours (MWh) of the energy that now occurs due to resistance in the wires
  • A new way to propel levitated trains and other forms of transportation
  • Medical imaging and scanning techniques such as MRI and magnetocardiography
  • Faster, more efficient electronics for digital logic and memory device technology

The amount of superconducting material created by the diamond anvil cells is measured in picoliters—about the size of a single inkjet particle. The next challenge, Dias says, is finding ways to create the room temperature superconducting materials at lower pressures, so they will be economical to produce in greater volume.

Why room temperature matters for superconductivity

First discovered in 1911, superconductivity gives materials two key properties. Electrical resistance vanishes. And any semblance of a magnetic field is expelled, due to a phenomenon called the Meissner effect. The magnetic field lines have to pass around the superconducting material, making it possible to levitate such materials, something that could be used for frictionless high-speed trains, known as maglev trains.

Powerful superconducting electromagnets are already critical components of maglev trains, magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines, particle accelerators and other advanced technologies, including early quantum supercomputers.

But the superconducting materials used in the devices usually work only at extremely low temperatures—lower than any natural temperatures on Earth. This restriction makes them costly to maintain—and too costly to extend to other potential applications. “The cost to keep these materials at cryogenic temperatures is so high you can’t really get the full benefit of them,” Dias says.

Previously, the highest temperature for a superconducting material was achieved last year in the lab of Mikhail Eremets at the Max Planck Institute for Chemistry in Mainz, Germany, and the Russell Hemley group at the University of Illinois at Chicago. That team reported superconductivity at -10 to 8 degrees Fahrenheit using lanthanum superhydride.

Researchers have also explored copper oxides and iron-based chemicals as potential candidates for high temperature superconductors in recent years. However, hydrogen—the most abundant element in the universe—also offers a promising building block.

“To have a high temperature superconductor, you want stronger bonds and light elements. Those are the two very basic criteria,” Dias says. “Hydrogen is the lightest material, and the hydrogen bond is one of the strongest.

“Solid metallic hydrogen is theorized to have high Debye temperature and strong electron-phonon coupling that is necessary for room temperature superconductivity,” Dias says.

However, extraordinarily high pressures are needed just to get pure hydrogen into a metallic state, which first achieved in a lab in 2017 by Harvard University professor Isaac Silvera and Dias, then a postdoc in Silvera’s lab.

A ‘paradigm shift’ for superconductors

And so, Dias’s lab at Rochester has pursued a “paradigm shift” in its approach, using as an alternative, hydrogen-rich materials that mimic the elusive superconducting phase of pure hydrogen, and can be metalized at much lower pressures.

Coauthors on the papers include lead author Elliot Snider ’19 (MS), Nathan Dasenbrock-Gammon ’18 (MA), Raymond McBride ’20 (MS), Kevin Vencatasamy ’21, and Hiranya Vindana (MS), all of the Dias lab; Mathew Debessai of Intel Corporation, and Keith Lawlor of the University of Nevada Las Vegas.

The project was supported with funding from the National Science Foundation and the US Department of Energy’s Stockpile Stewardship Academic Alliance Program and its Office of Science, Fusion Energy Sciences. Preparation of the diamond surfaces was performed in part at the University of Rochester Integrated Nanosystems Center (URnano).

Dias and Salamat have started a new company, Unearthly Materials, to find a path to a room temperature superconductor that can be scalable at ambient pressure.

Patents are pending. Anyone interested in licensing the technology can contact Curtis Broadbent, licensing manager at URVentures.

Featured image: The goal of new research led by Ranga Dias, assistant professor of mechanical engineering and of physics and astronomy, is to develop room temperature superconducting materials. Currently, extreme cold is required to achieve superconductivity, as demonstrated in this photo from Dias’s lab, in which a magnet floats above a superconductor cooled with liquid nitrogen. (University of Rochester photo / J. Adam Fenster)

Reference: Elliot Snider, Nathan Dasenbrock-Gammon, Raymond McBride, Xiaoyu Wang, Noah Meyers, Keith V. Lawler, Eva Zurek, Ashkan Salamat, and Ranga P. Dias, “Synthesis of Yttrium Superhydride Superconductor with a Transition Temperature up to 262 K by Catalytic Hydrogenation at High Pressures”, Phys. Rev. Lett. 126, 117003 – Published 19 March 2021. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.117003

Provided by University of Rochester