Science
Cuprate superconductivity mechanism may be coming into focus
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Since their dramatic debut in 1986, cuprate superconductors have been some of the best-studied materials in existence. Nonetheless, many mysteries about the materials have persisted, including perhaps the key question: What mechanism compels electrons to overcome their repulsion and pair up?
In conventional superconductors, Bardeen-Cooper-Schrieffer (BCS) theory describes how phonon vibrations coax electrons together into Cooper pairs. The material properties of those superconductors often abide by “Matthias’s rules”—no magnetism, no oxides, no insulators. Apart from sulfur hydrides, no BCS superconductor exceeds temperatures of 40 K. None of that has stopped doped copper oxides, whose parent compounds are insulating antiferromagnets, from remaining superconducting at temperatures as high as 135 K. As further evidence against a BCS pairing mechanism, cuprate superconductors are mostly insensitive to changes in phonon frequency.
Cuprate superconductors vary in their chemical formulas, but all contain the same essential building block: planes with one copper atom sandwiched between two oxygen atoms. Hypotheses abound for the mechanism behind cuprates’ superconductivity. Some theorists have suggested spin fluctuations; others believe phonons are the answer. Less than a year after Georg Bednorz and Alex Müller’s discovery of cuprate superconductivity, Philip Anderson proposed that the glue that binds electrons comes from superexchange, in which the spins of copper atoms are coupled, creating a magnetic attraction among their electrons despite the nonmagnetic oxygen atom in between.
Recently, several studies have begun to connect the key factors behind a potential superexchange pairing mechanism. One important factor is the charge-transfer gap (CTG), the energy required (usually a few eV) for an oxygen atom to take an electron from a copper atom. The larger the gap, which exists between the copper d orbital and the oxygen p orbital, the less likely the oxygen is to nab an electron from the copper. Last year, theorists at the University of Sherbrooke in Québec computed the rate at which electron pairing varies with the CTG.
That prediction provided a key target for a team led by J. C. Séamus Davis, who has labs at Oxford University, University College Cork in Ireland, and Cornell University. In a recent study in Proceedings of the National Academy of Sciences (PNAS), Davis and his colleagues report evidence that agrees with the Canadian theorists’ predictions, suggesting the mechanism behind cuprate superconductivity is CTG-mediated superexchange.
Re-cuperating models
Although BCS theory can be solved analytically—John Schrieffer famously solved the key equation for the Cooper pairs on the subway—the theory behind high-Tc superconductors is more complex. To simplify the picture, researchers have often turned to a one-band Hubbard model in which the cuprate is approximated as a square lattice of spins. Anderson was able to use the model to show how superexchange might work; others have even used it to predict where cuprate phase transitions take place. But the one-band Hubbard model does not consider multiple electron orbitals between copper and oxygen because it essentially smashes the oxygen and copper into one effective molecule.
As early as 1989, Vic Emery at Brookhaven National Laboratory introduced a more realistic three-band Hubbard model to address those dynamics. At the same time, other theorists were beginning to point to oxygen’s importance. Jeff Tallon, an experimentalist at Victoria University of Wellington, New Zealand, proposed that there was a correlation between oxygen hole content—the amount of electron holes present on an oxygen atom—and maximum Tc.
Extracting answers from three-band Hubbard models has remained out of reach until recently. Since the early 1990s, new algorithms and exponential increases in computing power have allowed theorists to capture the dynamics of far more atoms and previously intractable problems about magnetic impurities. With those tools, the University of Sherbrooke theorists returned to the problem.
The theorists began by trying to understand two experimental findings: that a large CTG is correlated with low Tc and that low oxygen hole content is correlated with low Tc. By solving the three-band Hubbard model for the lattice, the Sherbrooke researchers demonstrated the connection between those results. They found that increasing the CTG lowers oxygen hole content by compressing oxygen p orbitals, leaving less room for holes. A larger CTG also limits the strength of the superexchange interaction because it presents a barrier to coupling. Putting everything together, the authors concluded that the electron pairing mechanism is superexchange, which in turn depends on the CTG and oxygen hole content.
The Sherbrooke theory paper, published last year in PNAS, “is a true landmark in the long journey to understand the cuprates,” Tallon says. The authors also suggested an elegant explanation for why cuprates are special: Among all the transition metals, the strongest covalent bond exists between copper and oxygen. Strong covalent bonds lead to more superexchange than do weak ones or ionic bonds.
Critically, the Sherbrooke theorists also identified a quantifiable target for future experiments: They predicted how much a given change to the CTG would affect the density of Cooper pairs. “From the experimentalist’s point of view, now you have traction,” says Davis. “If the controlling degree of freedom can be measured, and if the response can be measured, then you can do real physics.”
Laboratory labors
To verify the Sherbrooke prediction, Davis and his colleagues chose the cuprate Bi2Sr2CaCu2O8+x (BSCCO, pronounced “bisco”) because of its unique periodic property. The height of the oxygen atom located above the copper atom in BSCCO varies by up to 12%—a huge difference that appears as wavy lines in topographic imaging of the sample. According to the Sherbrooke theorists, increasing the oxygen height would decrease the CTG, and a smaller CTG would lead to a larger superexchange interaction, which is measurable via the local density of the Cooper pairs.
Davis and colleagues used two very different scanning tunneling microscopy (STM) approaches to measure BSCCO at about 15% hole doping. To measure the electron pair, the tip of the probe must come to within picometers of the surface of the flat, flaky material, where the electric field is on the order of 109 V/m. (The Josephson STM technique that Davis used for the measurement took a decade to develop, he says.) To measure the CTG, the probe must be 5000 times farther away—like operating a record player with a stylus on the other side of a room, Davis says. He and his team had to split the experiment into two parts and perform the measurements with different STM tips.
Matching changes in the CTG to differences in Cooper pair density allowed the researchers to demonstrate a strong and compelling correlation, perhaps the clearest evidence yet of a mechanism that underlies cuprate superconductivity.
The Davis group’s paper is “a stunning tour de force,” Tallon says. But that doesn’t mean that one of the biggest questions in condensed-matter physics has been answered. “Is this the clinching experiment for identifying the long-sought microscopic origins of cuprate superconductivity?” he asks. “With deep respect for the authors, my view is—not yet.”
Inna Vishik, a condensed-matter experimentalist at the University of California, Davis, agrees. “It’s a correlation which proposes a mechanism, but ultimately, it motivates further experimental work in terms of assessing this in other compounds,” says Vishik, who was not involved in the recent studies.
Another recent study, published in Nature Communications, points to superexchange as the pairing mechanism in mercury-based cuprates. “We were looking at these two systems with a 30% difference in Tc,” says lead author Yuan Li of Peking University. “The question we wanted to answer is very simple: Is the magnetic energy scale also different between these two by 30%?” They found the difference in magnetic energy corresponded exactly to the difference in Tc, suggesting a magnetic basis such as superexchange for the mechanism.
One issue with any cuprate study is doping. Unlike the dopants in semiconductors, whose amounts are known within a part per million, oxygen is tricky and hard to pin down to better than one part in a hundred. Differences in doping can have large effects on the electronic structure, even pushing the compound into the pseudogap region, making it neither an antiferromagnet nor a superconductor. If even part of the BSCCO crystals slipped out of superconductivity into the pseudogap phase, it would severely compromise the authors’ conclusions. Davis argues that their sample was far from the pseudogap region but acknowledges that the pseudogap remains mysterious.
Additionally, there are exceptions: Some cuprate superconductors, such as La2−xSrxCuO4, have a large superexchange but a low Tc. Performing measurements of those compounds could be extremely difficult because they lack BSCCO’s extreme reaction to changes in the CTG, a property that makes the material easier to measure. For now, Davis says he will focus on repeating the experiment on BSCCO under different conditions, especially doping. He hopes that in the future, theorists will predict more testable parameters that are soft targets for experimentalists. “Even if what we report is not correct, opening the door to measuring the correct degrees of freedom in a falsifiable way is the correct way forward in a complicated field like this,” he says.
Far from the adrenaline-fueled early days of high-Tc superconductors, the latest efforts are the culmination of steady, normal science. André-Marie Tremblay, an author on the Sherbrooke group’s paper, credits programs like the Canadian Institute for Advanced Research for continued support even after the honeymoon phase of cuprate superconductors was over. After all, many mysteries remain.
“Even if we are correct, in 30 years people will still say that the theory is not understood,” Tremblay says.
Science
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In a remarkable development, NASA has given the green light to the Dragonfly mission, a revolutionary rotorcraft designed to investigate the complex chemistry of Saturn‘s moon Titan.
This confirmation allows the mission to proceed with the final design, construction, and testing of the spacecraft and its scientific instruments.
Deciphering the prebiotic chemistry on Titan
The Dragonfly mission, led by Dr. Melissa Trainer of NASA’s Goddard Space Flight Center, will carry a cutting-edge instrument called the Dragonfly Mass Spectrometer (DraMS).
This powerful tool will help scientists delve into the intricate chemistry at work on Titan, potentially shedding light on the chemical processes that led to the emergence of life on Earth, known as prebiotic chemistry.
“We want to know if the type of chemistry that could be important for early pre-biochemical systems on Earth is taking place on Titan,” explains Dr. Trainer, a planetary scientist and astrobiologist specializing in Titan.
Titan: Dragonfly’s target
Titan, the largest moon of Saturn, is shrouded in a dense nitrogen-rich atmosphere, bears a striking resemblance to Earth in many ways. With a diameter of 5,150 kilometers, Titan is the second-largest moon in our solar system, surpassed only by Jupiter’s Ganymede.
Dense atmosphere and unique climate
One of Titan’s most distinctive features is its thick atmosphere, which is composed primarily of nitrogen and methane. This dense atmosphere creates a surface pressure 1.5 times higher than Earth’s, making it the only moon in our solar system with a substantial atmosphere.
The presence of methane in Titan’s atmosphere leads to a fascinating hydrological cycle, similar to Earth’s water cycle, but with methane as the primary liquid.
Titan’s surface is dotted with numerous lakes and seas of liquid hydrocarbons, predominantly methane and ethane. These liquid bodies, some of which are larger than the Great Lakes on Earth, are the result of Titan’s unique climate and atmospheric conditions.
The Cassini mission, which explored the Saturn system from 2004 to 2017, provided stunning images and data of these extraterrestrial lakes and seas.
Dragonfly mission to search Titan for prebiotic chemistry and life
The complex chemistry occurring on Titan’s surface and in its atmosphere has drawn significant attention from astrobiologists.
With its abundant organic compounds and the presence of liquid methane, Titan is considered a prime candidate for studying prebiotic chemistry and the potential for life to emerge in environments different from Earth.
Beneath Titan’s icy crust lies another intriguing feature: a global subsurface ocean of liquid water and ammonia. This ocean, which is believed to be salty and have a high pH, may potentially host microbial life.
The presence of this subsurface ocean, along with the unique chemistry on Titan’s surface, makes this moon a fascinating target for future exploration and scientific research.
Pushing the boundaries of rotorcraft exploration
Nicky Fox, associate administrator of the Science Mission Directorate at NASA Headquarters, emphasized the significance of the Dragonfly mission, stating, “Exploring Titan will push the boundaries of what we can do with rotorcraft outside of Earth.”
Titan’s unique characteristics, including its abundant complex carbon-rich chemistry, interior ocean, and past presence of liquid water on the surface, make it an ideal destination for studying prebiotic chemical processes and the potential habitability of an extraterrestrial environment.
Innovative design and cutting-edge technology
The Dragonfly robotic rotorcraft will leverage Titan’s low gravity and dense atmosphere to fly between different points of interest on the moon’s surface, spanning several miles apart.
This innovative approach allows the entire suite of instruments to be relocated to new sites once the previous one has been thoroughly explored, providing access to samples from diverse geological environments.
DraMS, developed by the same team responsible for the Sample Analysis at Mars (SAM) instrument suite aboard the Curiosity rover, will analyze surface samples using techniques tested on Mars.
Dr. Trainer emphasized the benefits of this heritage, stating, “This design has given us an instrument that’s very flexible, that can adapt to the different types of surface samples.”
Dragonfly mission challenges and funding
The Dragonfly mission successfully passed its Preliminary Design Review in early 2023. However, due to funding constraints, the mission was asked to develop an updated budget and schedule.
The revised plan, presented and conditionally approved in November 2023, hinged on the outcome of the fiscal year 2025 budget process.
With the release of the president’s fiscal year 2025 budget request, Dragonfly is now confirmed with a total lifecycle cost of $3.35 billion and a launch date set for July 2028.
This reflects a cost increase of approximately two times the initially proposed cost and a delay of more than two years from the original selection in 2019.
Despite the challenges posed by funding constraints, the COVID-19 pandemic, supply chain issues, and an in-depth design iteration, NASA remains committed to the Dragonfly mission.
Additional funding has been provided for a heavy-lift launch vehicle to shorten the mission’s cruise phase and compensate for the delayed arrival at Titan.
Rigorous testing and validation
To ensure the success of the Dragonfly mission, researchers on Earth have conducted extensive testing and validation of the designs and models for the nuclear-powered, car-sized drone.
The mission team has carried out test campaigns at NASA’s Langley Research Center, utilizing the Subsonic Tunnel and the Transonic Dynamics Tunnel (TDT) to validate computational fluid dynamics models and gather data under simulated Titan atmospheric conditions.
Ken Hibbard, Dragonfly mission systems engineer at APL, emphasized the importance of these tests, stating, “All of these tests feed into our Dragonfly Titan simulations and performance predictions.”
As the Dragonfly mission progresses, it marks a new era of exploration and scientific discovery. Dr. Trainer expressed her excitement, saying, “Dragonfly is a spectacular science mission with broad community interest, and we are excited to take the next steps on this mission.”
Turning science fiction into fact with the Dragonfly mission
In summary, the Dragonfly mission embodies the essence of human curiosity and the relentless pursuit of knowledge. As NASA prepares to send this revolutionary rotorcraft to the alien world of Titan, we stand on the brink of a new era of exploration and discovery.
With its innovative design, cutting-edge technology, and the unwavering dedication of the mission team, Dragonfly will unlock the secrets of prebiotic chemistry and shed light on the potential for life beyond Earth.
As we eagerly await the launch of this titanic mission, we can only imagine the wonders that await us on Saturn’s enigmatic moon. The Dragonfly mission is a testament to the indomitable human spirit and our boundless capacity to push the frontiers of knowledge.
In the words of Ken Hibbard, “With Dragonfly, we’re turning science fiction into exploration fact,” and that fact will undoubtedly inspire generations to come.
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Science
Marine plankton could act as alert in mass extinction event: UVic researcher – Langley Advance Times
A University of Victoria micropaleontologist found that marine plankton may act as an early alert system before a mass extinction occurs.
With help from collaborators at the University of Bristol and Harvard, Andy Fraass’ newest paper in the Nature journal shows that after an analysis of fossil records showed that plankton community structures change before a mass extinction event.
“One of the major findings of the paper was how communities respond to climate events in the past depends on the previous climate,” Fraass said in a news release. “That means that we need to spend a lot more effort understanding recent communities, prior to industrialization. We need to work out what community structure looked like before human-caused climate change, and what has happened since, to do a better job at predicting what will happen in the future.”
According to the release, the fossil record is the most complete and extensive archive of biological changes available to science and by applying advanced computational analyses to the archive, researchers were able to detail the global community structure of the oceans dating back millions of years.
A key finding of the study was that during the “early eocene climatic optimum,” a geological era with sustained high global temperatures equivalent to today’s worst case global warming scenarios, marine plankton communities moved to higher latitudes and only the most specialized plankton remained near the equator, suggesting that the tropical temperatures prevented higher amounts of biodiversity.
“Considering that three billion people live in the tropics, the lack of biodiversity at higher temperatures is not great news,” paper co-leader Adam Woodhouse said in the release.
Next, the team plans to apply similar research methods to other marine plankton groups.
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