Rare “Lazarus Superconductivity” Observed in Rediscovered Material

 
Researchers from the University of Maryland, the National Institute of Standards and Technology (NIST), the National High Magnetic Field Laboratory (National MagLab) and the University of Oxford have observed a rare phenomenon called re-entrant superconductivity in the material uranium ditelluride. The discovery furthers the case for uranium ditelluride as a promising material for use in quantum computers.

A team of researchers has observed a rare phenomenon called re-entrant superconductivity in the material uranium ditelluride. Nicknamed “Lazarus superconductivity,” the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field. The discovery furthers the case for uranium ditelluride as a promising material for use in quantum computers. Image credit: Emily Edwards/JQI (Click image to download hi-res version.)

A team of researchers has observed a rare phenomenon called re-entrant superconductivity in the material uranium ditelluride. Nicknamed “Lazarus superconductivity,” the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field. The discovery furthers the case for uranium ditelluride as a promising material for use in quantum computers. Image credit: Emily Edwards/JQI (Click image to download hi-res version.)

Nicknamed “Lazarus superconductivity” after the biblical figure who rose from the dead, the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field. The researchers published their results on October 7, 2019, in the journal Nature Physics.

Once dismissed by physicists for its apparent lack of interesting physical properties, uranium ditelluride is having its own Lazarus moment. The current study is the second in as many months (both published by members of the same research team) to demonstrate unusual and surprising superconductivity states in the material.

“This is a very recently discovered superconductor with a host of other unconventional behavior, so it's already weird,” said Nicholas Butch, an adjunct assistant professor of physics at UMD and a physicist at the NIST Center for Neutron Research. “[Lazarus superconductivity] almost certainly has something to do with the novelty of the material. There's something different going on in there.”

The previous research, published on August 16, 2019 in the journal Science, described the rare and exotic ground state known as spin-triplet superconductivity in uranium ditelluride. The discovery marked the first clue that uranium ditelluride is worth a second look, due to its unusual physical properties and its high potential for use in quantum computers.

“This is indeed a remarkable material and it’s keeping us very busy,” said Johnpierre Paglione, a professor of physics at UMD, the director of UMD’s Center for Nanophysics and Advanced Materials (CNAM; soon to be renamed the Quantum Materials Center) and a co-author of the paper. “Uranium ditelluride may very well become the ‘textbook’ spin-triplet superconductor that people have been seeking for dozens of years and it likely has more surprises in store. It could be the next strontium ruthenate—another proposed spin-triplet superconductor that has been studied for more than 25 years.”

Superconductivity is a state in which electrons travel through a material with perfect efficiency. By contrast, copper—which is second only to silver in terms of its ability to conduct electrons—loses roughly 20% power over long-distance transmission lines, as the electrons bump around within the material during travel.

Lazarus superconductivity is especially strange, because strong magnetic fields usually destroy the superconducting state in the vast majority of materials. In uranium ditelluride, however, a strong magnetic field coupled with specific experimental conditions caused Lazarus superconductivity to arise not just once, but twice.

For Butch, Paglione and their team, the discovery of this rare form of superconductivity in uranium ditelluride was serendipitous; the study’s lead author, CNAM Research Associate Sheng Ran, synthesized the crystal accidentally while attempting to produce another uranium-based compound. The team decided to try some experiments anyway, even though previous research on the compound hadn’t yielded anything unusual.

The team’s curiosity was soon rewarded many times over. In the earlier Science paper, the researchers reported that uranium ditelluride’s superconductivity involved unusual electron configurations called spin triplets, in which pairs of electrons are aligned in the same direction. In the vast majority of superconductors, the orientations—called spins—of paired electrons point in opposite directions. These pairs are (somewhat counterintuitively) called singlets. Magnetic fields can more easily disrupt singlets, killing superconductivity.

Spin triplet superconductors, however, can withstand much higher magnetic fields. The team’s early findings led them to the National MagLab, where a unique combination of very high-field magnets, capable instrumentation and resident expertise allowed the researchers to push uranium ditelluride even further.

At the lab, the team tested uranium ditelluride in some of the highest magnetic fields available. By exposing the material to magnetic fields up to 65 teslas—more than 30 times the strength of a typical MRI magnet—the team attempted to find the upper limit at which the magnetic fields crushed the material’s superconductivity. Butch and his team also experimented with orienting the uranium ditelluride crystal at several different angles in relation to the direction of the magnetic field.

At about 16 teslas, the material’s superconducting state abruptly changed. While it died in most of the experiments, it persisted when the crystal was aligned at a very specific angle in relation to the magnetic field. This unusual behavior continued until about 35 teslas, at which point all superconductivity vanished and the electrons shifted their alignment, entering a new magnetic phase.

As the researchers increased the magnetic field while continuing to experiment with angles, they found that a different orientation of the crystal yielded yet another superconducting phase that persisted to at least 65 teslas, the maximum field strength the team tested. It was a record-busting performance for a superconductor and marked the first time two field-induced superconducting phases have been found in the same compound. 

Instead of killing superconductivity in uranium ditelluride, high magnetic fields appeared to stabilize it. While it is not yet clear exactly what is happening at the atomic level, Butch said the evidence points to a phenomenon fundamentally different than anything scientists have seen to date.

“I'm going to go out on a limb and say that these are probably different—quantum mechanically different—from other superconductors that we know about,” Butch said. “It is sufficiently different, I think, to expect it will take a while to figure out what's going on.”

On top of its convention-defying physics, uranium ditelluride shows every sign of being a topological superconductor, as are other spin-triplet superconductors, Butch added. Its topological properties suggest it could be a particularly accurate and robust component in the quantum computers of the future.

“The discovery of this Lazarus superconductivity at record-high fields is likely to be among the most important discoveries to emerge from this lab in its 25-year history,” said National MagLab Director Greg Boebinger. “I would not be surprised if unraveling the mysteries of uranium ditelluride leads to even stranger manifestations of superconductivity in the future.”

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This release was adapted from text provided by the National High Magnetic Field Laboratory.

In addition to Butch, Paglione and Ran, UMD-affiliated co-authors of the research paper include physics postdoctoral researcher Yun Suk Eo; physics graduate students I-Lin LiuDaniel Campbell and Christopher Eckberg; undergraduate physics major Paul Neves, physics faculty assistant Wesley Fuhrman; CNAM (QMC) Assistant Research Scientist Hyunsoo Kim and CNAM (QMC) Associate Research Scientist Shanta Saha.

The research paper, “Extreme magnetic field-boosted superconductivity,” Sheng Ran, I-Lin Liu, Yun Suk Eo, Daniel Campbell, Paul Neves, Wesley Fuhrman, Shanta Saha, Christopher Eckberg, Hyunsoo Kim, Johnpierre Paglione, David Graf, Fedor Balakirev, John Singleton and Nicholas Butch, was published in the journal Nature Physics on October 7, 2019.

This work was supported by the Schmidt Science Fellows program (in partnership with the Rhodes Trust), the National Science Foundation (Award Nos. DMR-1610349, DMR-1157490, DMR-1644779), the U.S. Department of Energy (Award No. DE-SC-0019154), the Gordon and Betty Moore Foundation’s EPiQS Initiative (Award No. GBMF4419), and the State of Florida. The content of this article does not necessarily reflect the views of these organizations.

Media Relations Contact: Matthew Wright, 301-405-9267, This email address is being protected from spambots. You need JavaScript enabled to view it.

Original story: https://cmns.umd.edu/news-events/features/4500

 

Ions Clear Another Hurdle Toward Scaled-up Quantum Computing

parallel gates

Scientists at the Joint Quantum Institute (JQI) have been steadily improving the performance of ion trap systems, a leading platform for future quantum computers. Now, a team of researchers led by JQI Fellows Norbert Linke and Christopher Monroe has performed a key experiment on five ion-based quantum bits, or qubits. They used laser pulses to simultaneously create quantum connections between different pairs of qubits—the first time these kinds of parallel operations have been executed in an ion trap. The new study, which is a critical step toward large-scale quantum computation, was published on July 24 in the journal Nature.  

“When it comes to the scaling requirements for a quantum computer, trapped ions check all of the boxes,” says Monroe, who is also the Bice-Sechi Zorn professor in the UMD Department of Physics and co-founder of the quantum computing startup IonQ. “Getting these parallel operations to work further illustrates that advancing ion trap quantum processors is not limited by the physics of qubits and is instead tied to engineering their controllers.” 

Ion traps are devices for capturing charged atoms and molecules, and they are commonly deployed for chemical analysis. In recent decades, physicists and engineers have combined ion traps with sophisticated laser systems to exert control over single atomic ions. Today, this type of hardware is one of the most promising for building a universal quantum computer.

The JQI ion trap used in this study is made from gold-coated electrodes, which carry the electric fields that confine ytterbium ions. The ions are caught in the middle of the trap where they form a line, each one separated from its neighbor by a few microns. This setup enables researchers to have fine control over individual ions and configure them as qubits.

Each ion has internal energy levels or quantum states that are naturally isolated from outside influences. This feature makes them ideal for storing and controlling quantum information, which is notoriously delicate. In this experiment, the research team uses two of these states, called “0” and “1”, as the qubit.

The researchers aim laser pulses at a string of qubits to execute programs on this small-scale quantum computer. The programs, also called circuits, are broken down into a set of single- and two-qubit gates. A single-qubit gate can, for instance, flip the state of an ion from 1 to 0. This is a straightforward task for a laser pulse. A two-qubit gate requires more sophisticated pulses because it involves tailoring the interactions between qubits. Certain two-qubit operations can create entanglement—a quantum connection necessary for quantum computation—between two qubits. 

Until now, circuits in ion trap quantum computers have been limited to a sequence of individual gates, one after another. With this new demonstration, researchers can now do two-qubit gates in parallel, creating entanglement between different pairs of ions simultaneously. The research team achieved this by optimizing the laser pulse sequences used to perform operations, making sure to cancel out unwanted laser-qubit interactions. In this way, they were able to successfully implement simultaneous entangling gates on two separate ion pairs.

According to the authors, parallel entangling gates will enable programs to correct errors during a quantum computation—a near-certain requirement in quantum computers with many more qubits. In addition, a quantum computer that factors large numbers or simulates quantum physics will likely need parallel entangling operations to achieve a speed advantage over conventional computers. 

Story by E. Edwards

In addition to Monroe and Linke, Caroline Figgatt, former JQI graduate student and scientist at Honeywell, was lead author on this research paper and provided background material for this news story. The research paper was published simultaneous to similar work done by former JQI postdoctoral researcher and Tsinghua University professor Kihwan Kim. 

Newfound Superconductor Material Could Be the ‘Silicon of Quantum Computers’

 We have already found lots of superconductors, but this whimsical illustration shows why one superconductor's newfound properties may make it especially useful. Most known superconductors are spin singlets, found on the island to the left. Uranium ditelluride, however, is a rare spin triplet, found on the island to the right, and also exists at the top of a mountain representing its unusually high resistance to magnetic fields. These properties may make it a good material for making qubits, which could maintain coherence in a quantum computer despite interference from the surrounding environment. Credit: N. Hanacek/NIST

 A collaboration of the NIST Center for Neutron Research, the UMD's Center for Nanophysics and Advanced Materials and the Ames Laboratory has yielded a new superconductor with properties highly advantageous for the development of quantum computers. Uranium ditelluride, or UTe2, described in Science magazine, resists magnetism and could maintain coherence in qubits.  Read more at NIST.gov. 

 

Corkscrew Photons May Leave Behind a Spontaneous Twist

 

Everything radiates. Whether it's a car door, a pair of shoes or the cover of a book, anything hotter than absolute zero (i.e., pretty much everything) is constantly shedding radiation in the form of photons, the quantum particles of light.

A twin process—absorption—is usually also present. As photons carry away energy, passers-by from the environment can be absorbed to replenish it. When absorption and emission occur at the same rate, scientists say that an object is in equilibrium with its environment. This often means that object and environment share the same temperature.

Far away from equilibrium, new behaviors can emerge. In a paper published August 1, 2019 as an Editors’ Suggestion in the journal Physical Review Letters, scientists at JQI and Michigan State University suggest that certain materials may experience a spontaneous twisting force if they are hotter than their surroundings.

"The fact that a material might feel a torque due to a temperature difference with the environment is very unusual," says lead author Mohammad Maghrebi, a former JQI postdoctoral researcher who is now an assistant professor at Michigan State University.

The effect, which hasn't yet been observed in an experiment, is predicted to arise in a thin ribbon of a material called a topological insulator (TI)—something that allows electrical current to flow on its surface but not through its innards.

In this case, the researchers made two additional assumptions about the TI. One is that it is hotter than its environment. And another is that the TI has some magnetic impurities that affect the behavior of electrons on its surface.

These magnetic impurities interact with a quantum property of the electrons called spin. Spin is part of the basic character of an electron, much like electric charge, and it describes the particle’s intrinsic angular momentum—the tendency of an object to continue rotating. Photons, too, can carry angular momentum.

Although electrons don’t physically rotate, they can still gain and lose angular momentum, albeit only in discrete chunks. Each electron has two spin values—up and down—and the magnetic impurities ensure that one value sits at a higher energy than the other. In the presence of these impurities, electrons can flip their spin from up to down and vice versa by emitting or absorbing a photon that carries the right amount of energy and angular momentum.

Maghrebi and two colleagues, JQI Fellows Jay Deep Sau and Alexey Gorshkov, showed that radiation emanating from this kind of TI carries angular momentum skewed in one rotational direction, like a corkscrew that twists clockwise. The material gets left with a deficit of angular momentum, causing it to feel a torque in the opposite direction (in this example, counterclockwise).

The authors say that TIs are ideal for spotting this effect because they play host to the right kind of interaction between electrons and light. TIs already link electron spin with the momentum of their motion, and it's through this motion that electrons in the material ordinarily absorb and emit light.

If an electron on the surface of this particular kind of TI starts with its spin pointing up, it can shed energy and angular momentum by changing its spin from up to down and emitting a photon. Since the TI is hotter than its environment, electrons will flip from up to down more often than the reverse. That’s because the environment has a lower temperature and lacks the energy to replace the radiation coming from the TI. The result of this imbalance is a torque on the thin TI sample, driven by the random emission of radiation.

Future experiments might observe the effect in one of two ways, the authors say. The most likely method is indirect, requiring experimenters to heat up a TI by running a current through it and collecting the emitted light. By measuring the average angular momentum of the radiation, an experiment might detect the asymmetry and confirm one consequence of the new prediction.

A more direct—and likely more difficult—observation would involve actually measuring the torque on the thin film by looking for tiny rotations. Maghrebi says that he's brought up the idea to several experimentalists. "They were not horrified by having to measure something like a torque, but, at the same time, I think it really depends on the setup," he says. "It certainly didn't sound like it was impossible."

Story by Chris Cesare: https://jqi.umd.edu/news/corkscrew-photons-may-leave-behind-spontaneous-twist

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Currie to Send Next Gen Retroreflectors to Moon

NGLR corner cube retroreflector seen next to Apollo era corner cube retroreflector 

In 1969, University of Maryland physicist Doug Currie helped design three still-in-use lunar instruments placed on the moon by Apollo 11, 14 and 15. Fifty years later, Currie is lead scientist for a just-approved NASA project to place next-generation versions of these instruments on the Moon.

Known as lunar retroreflectors, the instruments reflect laser pulses sent from Earth back to their exact origin point, allowing precise measurements of the Earth-moon distance; providing data to better understand aspects of the moon’s interior, including its liquid core; testing questions of fundamental physics, and allowing better mapping and navigation of the lunar surface.  

According to Currie, a senior research scientist and professor emeritus at the University of Maryland, the new UMD-led project can lead to improvements in all of these research areas: (1) because of the hundred fold improvement in the accuracy of individual ranges using the new retroreflectors and (2) by the increased accuracy produced by a larger number of reflectors with a wider lunar area covered by the network.  Currently, there are five retroreflectors on the moon: the three placed by Apollo missions and two French-designed instruments placed by Soviet lunar missions. The Currie-led proposal would add three Next Generation Retroreflectors for a total of eight lunar retroreflector arrays. 

“Our Next Generation Lunar Retroreflector is a 21st Century version of the instruments currently on the Moon. Each placement of a Next Generation lunar laser ranging array will greatly enhance the scientific and navigational capabilities of retroreflector network,” said Currie. “These additions improve the mapping and navigation capabilities important for NASA’s plans to return to the Moon and by 2028 establish a sustained human presence.” 

“And these also will significantly boost scientists’ ability to use the network to conduct important science, such as new tests of general relativity and other theories of gravity. Such studies may help us understand the nature of mysterious dark matter, which appears to constitute almost 27 percent of the Universe,” he said. 

According to a NASA release, the Next Generation Lunar Retroreflectors (NGLR) is one of 12 new science and technology payloads selected by the agency to help humans study the Moon and explore more of its surface as part of the NASA’s Artemis lunar program. The agency says the retroreflector and the other 11 investigations and demonstrations “will help the agency to send astronauts to the Moon by 2024 as a way to prepare to send humans to Mars for the first time.”

The selected investigations will go to the Moon on future flights through NASA's Commercial Lunar Payload Services (CLPS) project. According to the agency, the CLPS project allows rapid acquisition of lunar delivery services for payloads like these that advance capabilities for science, exploration, or commercial development of the Moon. 

NASA has selected the first three commercial Moon landing service providers that will deliver science and technology payloads to the lunar surface. According to Currie, the Next Generation Retroreflectors are not currently scheduled by NASA to be among the payloads carried on those three commercial Moon landings. “However, we believe because of the low size and weight of these retroreflectors, each mission could safely add one to their planned payloads for each of these commercial delivery missions,” he said. 

"The selected lunar payloads represent cutting-edge innovations, and will take advantage of early flights through our commercial services project,” said Thomas Zurbuchen, associate administrator of the agency's Science Mission Directorate in Washington. "Each demonstrates either a new science instrument or a technological innovation that supports scientific and human exploration objectives, and many have broader applications for Mars and beyond.”

The NGLR team consists of: Principal Investigator Douglas Currie Professor Emeritus, Department of Physics, University of Maryland College Park (UMD); Co-I/Co-PI: Simone Dell’Agnello Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Italy; and Co-Investigators Professor Christopher Davis Electrical Engineering Department (UMD); James Williams Jet Propulsion Laboratory Technical Personnel; John Rzasa (UMD); and Dennis Wellnitz Department of Astronomy (UMD). UMD Assistant Research Scientist Chensheng Wu, a member of Professor Chris Davis’s Maryland Optical Group in Electrical and Computer Engineering has also done crucial work on the design of the Next Generation Lunar Retroreflectors.

(Currie was interviewed by ABC News about the 50th anniversary of Apollo 11.)

Links for more information:

What Neil & Buzz Left on the Moon | NASA Science Mission Directorate

NASA Selects 12 New Lunar Science, Technology Investigations 

Dr. Douglas Currie next to the Orbit Beyond lander. Credit Udit Shah of Orbit Beyond

 UMD Professor Dr. Douglas Currie next to the Orbit Beyond lander. Credit Udit Shah of Orbit Beyond

Pictured (L-R) in 1969 UMD physicist Doug Currie, UT McDonald Observatory Director Harlan J. Smith, NASA Scientist-Astronaut Philip Chapman, UMD physicist Carroll Alley, & Scientist-Astronaut Don Lind

Pictured (L-R) in 1969 are University of Maryland (UMD) physicist Doug Currie, University of Texas (UT) McDonald Observatory Director Harlan J. Smith, NASA Scientist-Astronaut Philip Chapman, UMD Professor of Physics Carroll Alley and NASA Scientist-Astronaut Don L. Lind discussing use of the McDonald Lunar Laser Ranging Observatory (MLLRO) to send short laser pulses to the first Lunar Laser Ranging (LLR) Retroreflector left on the lunar surface by the Apollo 11 Astronauts. The MLLRO program was developed and initially operated by Professor Currie (then associate professor) with the support of the Godard Space Craft Center and the University of Texas. UMD’s Professor Alley was the Principle Investigator of the project to place LLRs on the moon to address lunar physics, gravitation, General Relativity and Earth physics.  The LLR Retroreflectors were designed and developed by a team that in addition to UMD physicists Alley and Currie, included scientists from universities and federal institutions that included Princeton University, NASA Goddard Space Craft Center, the National Bureau of Standards, Wesleyan University and the University of California. NASA’s Scientist-Astronaut program trained Ph.D. scientists & engineers as astronauts. Image courtesy Doug Currie.

Original story: https://umdrightnow.umd.edu/news/umd-physicist-apollo-experiment-gets-chance-send-next-gen-version-moon

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