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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.

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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.

Relationship between oxygen height, charge-transfer gap, and density of Cooper pairs.
Top graph: Oxygen atoms (red dots) vary in height above copper atoms (blue dots). Bottom graphs: Measurements reveal that changes in the height of out-of-plane oxygen atoms (gray) lead to decreases in the charge-transfer gap (green) and increased density of Cooper pairs (orange). Credit: Wangping Ren & Shane O’Mahony

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.

Relationship between charge-transfer gap (left) and density of Cooper pairs (right).
The relationship between the charge-transfer gap (left) and density of Cooper pairs (right), visualized in the wavy undulations of BSCCO layers. Where the CTG is largest (light), the density of Cooper pairs is lowest (dark); where the CTG is smallest (dark), the density of Cooper pairs is highest (light). Adjoined, the two measurements depict a visible link that suggests CTG-mediated superexchange as the mechanism for electron pairing. Credit: Wangping Ren & Shane O’Mahony

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.

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NASA posts high-resolution images of Orion’s final lunar flyby

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Orion just made its final pass around the moon on its way to Earth, and NASA has released some of the spacecraft’s best photos so far. Taken by a high-resolution camera (actually a heavily modified GoPro Hero 4) mounted on the tip of Orion’s solar arrays, they show the spacecraft rounding the Moon then getting a closeup shot of the far side.

The photos Orion snapped on its first near pass to the Moon were rather grainy and blown out, likely because they were captured with Orion’s Optical Navigation Camera rather than the solar array-mounted GoPros. Other GoPro shots were a touch overexposed, but NASA appears to have nailed the settings with its latest series of shots.

Space photos were obviously not the primary goal of the Artemis I mission, but they’re important for public relations, as NASA learned many moons ago. It was a bit surprising that NASA didn’t show some high-resolution closeups of the Moon’s surface when it passed by the first time, but better late than never.

Orion’s performance so far has been “outstanding,” program manager Howard Hu told reporters last week. It launched on November 15th as part of the Artemis 1 mission atop NASA’s mighty Space Launch System. Days ago, the craft completed a three and a half minute engine burn (the longest on the trip so far) to set it on course for a splashdown on December 11th.

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The next mission, Artemis II, is scheduled in 2024 to carry astronauts on a similar path to Artemis I without landing on the moon. Then, humans will finally set foot on the lunar surface again with Artemis III, slated for launch in 2025.

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Biosignatures: Discovery Of Earth’s Oldest DNA Breaks Record By One Million Years

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Taxonomic profiles of the plant assemblage found in the metagenomes. Taxa in bold are genera only found as DNA and not as macrofossil or pollen. Asterisks indicate those that are found at other Pliocene Arctic sites. Extinct species as identified by either macrofossils or phylogenetic placements are marked with a dagger. Reads classified as Pyrus and Malus are marked with a pound symbol, and are probably over-classified DNA sequences belonging to another species within Rosaceae that are not present as a reference genome. — University of Cambridge

Two-million-year-old DNA has been identified for the first time – opening a ‘game-changing’ new chapter in the history of evolution.

 

Microscopic fragments of environmental DNA were found in Ice Age sediment in northern Greenland. Using cutting-edge technology, researchers discovered the fragments are one million years older than the previous record for DNA sampled from a Siberian mammoth bone.

The ancient DNA has been used to map a two-million-year-old ecosystem which weathered extreme climate change. Researchers hope the results could help to predict the long-term environmental toll of today’s global warming.

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The discovery was made by a team of scientists led by Professor Eske Willerslev and Professor Kurt H. Kjær. Professor Willerslev is a Fellow of St John’s College, University of Cambridge, and Director of the Lundbeck Foundation GeoGenetics Centre at the University of Copenhagen where Professor Kjær, a geology expert, is also based.

The results of the 41 usable samples found hidden in clay and quartz are published today (7 DECEMBER 2022) in Nature.

Professor Willerslev said: “A new chapter spanning one million extra years of history has finally been opened and for the first time we can look directly at the DNA of a past ecosystem that far back in time..

“DNA can degrade quickly but we’ve shown that under the right circumstances, we can now go back further in time than anyone could have dared imagine.”

Professor Kjær said: “The ancient DNA samples were found buried deep in sediment that had built-up over 20,000 years. The sediment was eventually preserved in ice or permafrost and, crucially, not disturbed by humans for two million years.”

The incomplete samples, a few millionths of a millimetre long, were taken from the København Formation, a sediment deposit almost 100 metres thick tucked in the mouth of a fjord in the Arctic Ocean in Greenland’s northernmost point. The climate in Greenland at the time varied between Arctic and temperate and was between 10-17C warmer than Greenland is today. The sediment built up metre by metre in a shallow bay.

Scientists discovered evidence of animals, plants and microorganisms including reindeer, hares, lemmings, birch and poplar trees. Researchers even found that Mastodon, an Ice Age mammal, roamed as far as Greenland before later becoming extinct. Previously it was thought the range of the elephant-like animals did not extend as far as Greenland from its known origins of North and Central America.

Detective work by 40 researchers from Denmark, the UK, France, Sweden, Norway, the USA and Germany, unlocked the secrets of the fragments of DNA. The process was painstaking – first they needed to establish whether there was DNA hidden in the clay and quartz, and if there was, could they successfully detach the DNA from the sediment to examine it? The answer, eventually, was yes. The researchers compared every single DNA fragment with extensive libraries of DNA collected from present-day animals, plants and microorganisms. A picture began to emerge of the DNA from trees, bushes, birds, animals and microorganisms.

Some of the DNA fragments were easy to classify as predecessors to present-day species, others could only be linked at genus level, and some originated from species impossible to place in the DNA libraries of animals, plants and microorganisms still living in the 21st century.

The two-million-year-old samples also help academics build a picture of a previously unknown stage in the evolution of the DNA of a range of species still in existence today.

Professor Kjær said: “Expeditions are expensive and many of the samples were taken back in 2006 when the team were in Greenland for another project, they have been stored ever since.

“It wasn’t until a new generation of DNA extraction and sequencing equipment was developed that we’ve been able to locate and identify extremely small and damaged fragments of DNA in the sediment samples. It meant we were finally able to map a two-million-year-old ecosystem.”

Assistant Professor Mikkel W. Pedersen, co-first author on the paper and also based at the Lundbeck Foundation GeoGenetics Centre, said: “The Kap København ecosystem, which has no present-day equivalent, existed at considerably higher temperatures than we have today – and because, on the face of it, the climate seems to have been similar to the climate we expect on our planet in the future due to global warming.

“One of the key factors here is to what degree species will be able to adapt to the change in conditions arising from a significant increase in temperature. The data suggests that more species can evolve and adapt to wildly varying temperatures than previously thought. But, crucially, these results show they need time to do this. The speed of today’s global warming means organisms and species do not have that time so the climate emergency remains a huge threat to biodiversity and the world – extinction is on the horizon for some species including plants and trees.”

While reviewing the ancient DNA from the Kap København Formation, the researchers also found DNA from a wide range of microorganisms, including bacteria and fungi, which they are continuing to map. A detailed description of how the interaction – between animals, plants and single-cell organisms – within the former ecosystem at Greenland’s northernmost point worked biologically will be presented in a future research paper.

It is now hoped that some of the ‘tricks’ of the two-million-year-old plant DNA discovered may be used to help make some endangered species more resistant to a warming climate.

Professor Kjær said: “It is possible that genetic engineering could mimic the strategy developed by plants and trees two million years ago to survive in a climate characterised by rising temperatures and prevent the extinction of some species, plants and trees. This is one of the reasons this scientific advance is so significant because it could reveal how to attempt to counteract the devastating impact of global warming.”

The findings from the Kap København Formation in Greenland have opened up a whole new period in DNA detection.

Professor Willerslev explained: “DNA generally survives best in cold, dry conditions such as those that prevailed during most of the period since the material was deposited at Kap København. Now that we have successfully extracted ancient DNA from clay and quartz, it may be possible that clay may have preserved ancient DNA in warm, humid environments in sites found in Africa.

“If we can begin to explore ancient DNA in clay grains from Africa, we may be able to gather ground-breaking information about the origin of many different species – perhaps even new knowledge about the first humans and their ancestors – the possibilities are endless.”

 

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How you can watch Mars disappear behind the full moon tonight

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If you happen to have clear skies on Wednesday night, you’ll be able to catch a planet disappearing behind the moon.

The event occurs at a special time for Mars. On Wednesday night, Mars will be directly opposite the sun’s position in the sky, rising as the sun sets and setting as the sun rises. This is called an opposition and is when Mars is at its brightest in the night sky.

“Having the moon hide a bright planet is rare,” said Alan Dyer, an amateur astronomer and accomplished astrophotographer who will watch the event from his home near Strathmore, Alta.

“Having it do so on the very night a planet is at its brightest, as Mars now is, is very unusual. And with the objects so well-placed high in our sky. Fabulous!”

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When a planet or a star disappears behind another object, it’s called an occultation. The next time this happens between the moon and Mars will be in January 2025, although it will be two days before opposition.

When and where to look

Canada is in a prime location for the event.

Elaina Hyde, director of the Allan I. Carswell Observatory at York University’s department of physics and astronomy in Toronto, is also looking forward to Wednesday night’s occultation.

“Tonight, the occultation of Mars by our moon requires a ‘just right’ alignment,” she said. “In fact, not everyone on Earth will even be able to see this one.”

If you want to watch Mars blink out behind the moon, you just need clear skies. However, binoculars will provide a better view (though, be warned: a full moon is quite bright with binoculars or a telescope).

Because the occultation is between the moon, which is super bright, and Mars, which is at its brightest, they’re easy to find.

All you have to do is look east to find the moon. Mars will appear at the left or lower left, depending on your location.

People across North America in the shaded areas on the map will enjoy the the sight of the moon eclipsing Mars on Wednesday night. (Gregg Dinderman/Sky & Telescope)

You can gradually watch the event unfold right after sunset, when the pair will be farther apart. Over the next few hours, the pair will gradually appear to get closer and closer. The moon will seem to move to the left as they rise in the sky, eventually overtaking Mars.

How long Mars will stay eclipsed behind the moon depends on your location: it could be several minutes or about an hour. This is because it depends on how much of the moon’s disc Mars will need to traverse.

For example, in Toronto, Mars will only cross a small fraction of the moon’s lower disc, beginning at 10:29 p.m. ET and re-emerging roughly 45 minutes later. In Edmonton, it will take more than an hour for the entire event.

Here are the approximate times when Mars will disappear behind the moon. All times are local:

  • Vancouver: 6:55 p.m.
  • Edmonton: 8:04 p.m.
  • Calgary: 7:59 p.m.
  • Regina: 9:01 p.m.
  • Saskatoon: 9:03 p.m.
  • Winnipeg: 9:05 p.m.
  • Toronto: 10:29 p.m.
  • Ottawa: 10:36 p.m.
  • Montreal: 10:40 p.m.
  • Iqaluit: 9:50 p.m.
  • Whitehorse: 8:25 p.m.
  • Yellowknife: 8:23 p.m.

Thursday:

  • Halifax: 12:15 a.m.
  • Charlottetown: 12:07 a.m.
  • Moncton: 12:04 a.m.
  • St. John’s: 12:25 a.m.

You can find more locations here.

Remember, the event occurs all night, so you can take a peek outside once in a while leading up to the occultation and afterwards as it progresses.

You may also notice a bright red star not too far away from the moon and Mars, but to the right. That’s Aldebaran, the brightest star in the constellation Taurus.

This red giant lies near one of the most beautiful open star clusters in the northern sky, Hyades. In a few days’ time, once the moon moves away from that area of the sky, try using a pair of binoculars to check out the cluster.

Also, since you’re outside on Wednesday night, why not take a peak to the southwest where Jupiter will be quite apparent as the brightest object in the sky (aside from the moon). A pair of binoculars will also reveal four of its brightest moons, Io, Callisto, Ganymede and Europa.

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