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Physicists move closer to an ultra-precise ‘nuclear’ clock

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A Strontium optical clock pictured at the National Physical Laboratory in Teddington, UK.

In principle, a nuclear clock should be more precise and more stable than an optical clock (pictured).Credit: Andrew Brookes, National Physical Laboratory/Science Photo Library

Scientists have taken a major leap towards making an entirely new type of clock — one based on tiny shifts in energy in an atomic nucleus. In principle, a nuclear clock could be even more precise than the world’s current best timekeepers, known as optical clocks, and less sensitive to disturbances.

A nuclear timekeeper could also allow physicists to study fundamental forces of nature in new ways. “We will be able to probe scenarios of dark matter and of fundamental physics that are currently inaccessible to other methods,” says Elina Fuchs, a theoretical physicist at CERN, Europe’s particle-physics laboratory outside Geneva, Switzerland.

The long-sought breakthrough — made by a collaboration between the Vienna University of Technology and Germany’s national metrology institute, the PTB, in Braunschweig — involved using an ultraviolet laser to prompt a nucleus of the radioactive metal thorium-229 to switch between energy states. The frequency of light absorbed and emitted by the nucleus functions as the clock’s tick. The researchers published their work in Physical Review Letters on 29 April1.

“This is major,” says Adriana Pálffy-Buß, a theoretical physicist at the University of Würzburg in Germany. Driving the transition with a laser is “the milestone you need to say ‘I’ll be able to build a clock’”.“It is a culmination of nearly a half a century of effort of many scientific groups,” says Olga Kocharovskaya, a physicist at Texas A&M University in College Station.

Precision timing

Optical clocks keep time so well that they waver by just 1 second every roughly 30 billion years. Their ticks are governed by the frequency of the visible light needed to shift an electron orbiting an atom such as strontium between energy states.

But a nuclear clock could do even better. It would use the more energetic transition of boosting the nucleus’s protons and neutrons to a higher energy state. This would use slightly higher frequency radiation, meaning that time could be sliced even more finely to create a more precise clock. More importantly, such a clock would be much more stable than an optical clock, because particles in the nucleus are less sensitive than electrons to external fields or temperature.

But finding a material with a suitable nucleus has proved difficult. Energy transitions in most nuclei tend to be huge, requiring much more than the nudge of a tabletop laser. In the 1970s, physicists discovered that thorium-229 is an anomaly — its first energy state is extremely close to its lowest, ground state. And in 2003, physicists proposed using thorium-229 as the basis of a super-stable clock, but they needed to find the precise energy of the transition and its corresponding laser frequency, which would have been impossible to predict with any accuracy using theory. Since then, experimentalists have used range of methods to narrow down the figures.

To observe the transition, researchers placed radioactive thorium atoms into crystals of calcium fluoride that were a few millimetres wide. Scanning across the expected region with a purpose-built laser, they eventually hit upon the right frequency — around 2 petahertz (1015 oscillations per second) — which they detected by spotting the photons emitted as the nuclei returned to the lower energy state. Co-author Thorsten Schumm, an atomic physicist at the Vienna University of Technology, recalls scrawling “found it” in large red letters across his lab book at a meeting convened the next day to discuss the promising-looking signal. “It was crystal clear,” he says.

The team pinpointed the frequency with a resolution 800 times better that the next best attempt. A team at the University of California, Los Angeles, has since reproduced the result using a different crystal, but the same frequency, says co-author Ekkehard Peik, a physicist at PTB. It’s “a very nice confirmation”, he says.

Fundamental physics boost

To turn the system into an actual clock, physicists will need to markedly reduce the resolution of the laser, so that it stimulates the nucleus at almost exactly the right frequency to be read off reliably, says Peik. Building such a laser “remains a big challenge, but there are little doubts that it will be achievable in the near future”, adds Kocharovskaya.

If all goes well, the team says that a thorium-based nuclear clock could end up being around 10 times more accurate than the best optical clocks. “It’s the robustness with respect to external perturbations that will make this a better clock,” says Schumm. Hosting the nuclei in a solid crystal could also help to make the clock more compact and portable than optical systems.

Scientific methods that were made possible by super precise optical clocks, such as probing Earth’s gravitational field by measuring differences in clock speed, “could get a major boost”, says Kocharovskaya.

Physics could also benefit at a deeper level. A nuclear clock would be around 10,000 times more sensitive to changes in fundamental constants — such as the strength of the electromagnetic and strong nuclear forces — than an optical clock is, says Fuchs. This means that they could detect proposed forms of dark matter, an invisible substance that physicists think accounts for 85% of material in the Universe, and which are predicted to make minuscule changes in the strength of these forces.

“It could be that there’s very ‘light’ dark matter that wiggles around and that could make these fundamental constants wiggle,” says Fuchs. Nuclear clocks might be able to detect that wiggle, she says, because the energy of their transition is governed by these forces, and any change in their strength would alter the clock’s tick in a measurable way. Nuclear clocks could also detect whether some particle masses change over time, she adds. Fuchs and her collaborators are already working on their first paper, on the basis of the frequency measurement. “This is exciting us quite a lot,” she says.

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Judge dismisses superconductivity physicist’s lawsuit against university

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A judge has dismissed a lawsuit brought by superconductivity physicist Ranga Dias against his employer, the University of Rochester in New York. In February, a university investigation found that he had committed scientific misconduct by, among other things, fabricating data to claim the discovery of superconductors — materials with zero electrical resistance — at room temperature. Dias filed the lawsuit against the university for allegedly violating his academic freedom and conducting a biased investigation into his work.

On 19 April, Monroe County Supreme Court justice Joseph Waldorf denied Dias’s petitions and dismissed the lawsuit as premature. The matter “is not ripe for judicial review”, Waldorf wrote (see Supplementary information), because, although Rochester commissioned an independent review that found Dias had committed misconduct, it has not yet finished taking administrative action. The university provost has recommended that Dias be fired, but a final decision is still forthcoming.

A spokesperson for the university said Rochester was “pleased” with the justice’s ruling, and reiterated that its investigation was “carried out in a fair manner” and reached a conclusion that it thinks is correct.

Dias did not respond to requests for comment. His lawyer, Morgan Levy, referred Nature’s news team to documents filed with the lawsuit in which Dias responded to the university’s investigation.

Nature’s news team reported on Rochester’s investigation previously: three scientists external to the university conducted a 10-month probe into 16 allegations against Dias and determined that the physicist had committed plagiarism, and data fabrication and falsification related to four scientific papers, including two published in Nature1,2. (Nature’s news team is editorially independent of its journals team.) Normally, the details of the investigation would probably have remained confidential. But in response to Dias’s lawsuit, the university submitted the entire report as a court exhibit, making it public.

Other documents and e-mails from Dias made public owing to the lawsuit reveal more details about the physicist’s attempts to halt the investigation and to cast doubt on former graduate students from his laboratory who had shared concerns with investigators about data in one of the blockbuster Nature papers2, and who later requested its retraction. Nature’s news team spoke about the lawsuit to four of Dias’s former students, who requested anonymity because they were concerned about the negative impact on their careers. They disagree with Dias’s characterization of events in the e-mails submitted to the court. One student described Dias’s attitude as “it’s not me that’s wrong, it’s everyone around me”.

Toxic environment

In March 2023, the National Science Foundation (NSF), which funds US academic research — including much of Dias’s — ordered Rochester to investigate allegations that Dias committed scientific misconduct when he claimed to have discovered room-temperature superconductivity in a material made of carbon, sulfur and hydrogen at room temperature1. This order followed three internal ‘inquiries’ into Dias’s work by the university, which did not evidence of misconduct. Prompted by the NSF, Stephen Dewhurst, the then-interim vice-president for research at Rochester, organized a committee of three external experts to undertake the investigation.

Dias initially appeared pleased with the investigators. After his first interview with them, he sent Dewhurst an e-mail on 16 June 2023, writing that he welcomed the university’s “comprehensive neutral unbiased independent investigation into all the allegations”. Later, his opinion of the investigation would change.

When the investigators interviewed Dias’s graduate students the next month, serious issues came to light, according to court documents: the students said that Dias dismissed their concerns about the veracity of certain data and that he had created a culture of fear in the lab. Speaking to Nature‘s news team, one student says that Dias apparently retaliated against them for reporting concerns to another faculty member at Rochester. The news team reviewed a memo written by the student immediately after the incident. The student recorded Dias as saying that “an adviser is like your parents — you can’t remove them, you’re stuck with them”.

In a 3 August 2023 e-mail to Dias, Wendi Heinzelman, dean of Rochester’s engineering school, told the physicist that his students would be moved to new advisers. Dias objected and expressed concern that the decision would affect the ongoing investigation. “Reassignment of my students has inadvertently conveyed a perception of wrongdoing on my part,” he responded. In that e-mail, Dias blamed the decision on two students he said were biased against him, alleging that one created a toxic environment in the lab and that the other was “a distraction to other students”.

Nature’s news team showed the e-mail to other former graduate students, who said that the toxic environment was caused by Dias. The students he accused of being biased against him “were not the issue in the group, and they tried their hardest to make it work”, says one of the former students.

In September 2023, five of Dias’s former students decided to ask for a retraction of a Nature paper that claimed that the team had observed room-temperature superconductivity in a lutetium-based material at relatively low pressures2. Dias found out and sent them each a cease-and-desist letter, as previously reported by Nature’s news team. At the same time, the physicist sent his first formal concerns about the investigation committee to the NSF, court documents show.

He alleged bias, conflicts of interest and a lack of expertise on the part of the investigators. Rochester administrators reviewed the claims and, in a letter to the NSF, concluded that the investigation was fair.

Legal trouble

Dias sued the university in December last year, alleging that his academic freedom was violated when he was stripped of his students. He filed another lawsuit in February, first attempting to stop the investigation, then to prevent it from becoming public. A judge denied both requests.

The case was eventually moved to a new justice, Waldorf, who heard arguments from lawyers representing Dias and Rochester in early April. In his decision to dismiss Dias’s lawsuit, Waldorf cited a previous ruling that “absent extraordinary circumstances, courts are constrained not to interject themselves into ongoing administrative proceedings”. These proceedings will determine whether Dias, who does not yet have tenure, will be fired. The final decision rests with Rochester’s board of trustees.

Nature’s news team spoke with scholars about Waldorf’s ruling, which was based on a cut-and-dry precedent. “The decision is unassailable,” says Matthew Finkin, a labour law and academic-freedom scholar at the University of Illinois at Urbana-Champaign. Scott Gelber, a historian of education at Wheaton College in Norton, Massachusetts, summed up his thoughts: “Academic freedom doesn’t protect academic misconduct.”

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Could JWST solve cosmology’s big mystery? Physicists debate Universe-expansion data

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This image, taken with the Wide Field Planetary Camera 2 on board the NASA/ESA Hubble Space Telescope, shows the globular star cluster Terzan 1.

Observations of the current Universe suggest a faster rate of cosmic expansion than predictions based on early-Universe data.Credit: NASA/ESA/Judy Schmidt

Cosmology seems to be heading for a showdown on one of its most basic questions: how fast is the Universe expanding?

For more than a decade, two types of measurement have been in disagreement. Observations of the current Universe typically find the rate of expansion — called the Hubble constant — to be about 9% faster than predictions based on early-Universe data.

Researchers hoped that the James Webb Space Telescope (JWST), which launched in late 2021, would help to settle the question once and for all. But consensus has so far failed to materialise. Instead, two teams of cosmologists have calculated different values for the Hubble constant — despite both observing the recent Universe using the JWST.

Wendy Freedman, an astronomer at the University of Chicago in Illinois, and her collaborators presented preliminary results from their JWST observations today at a conference at the Royal Society in London. The Hubble constant they measured was 69.1 kilometers per second per megaparsec, meaning that galaxies separated by one million parsec (around 3 million light years) are receding from each other at a rate of 69.1 km/s.

This is only slightly larger than the 67 km/s per megaparsec predicted using early-universe data from Europe’s Planck satellite. But it is at odds with recent work by Adam Riess, an astrophysicist at Johns Hopkins University in Baltimore, Maryland, and his collaborators, who calculated a substantially higher Hubble constant, of at least 73 km/s per Mpc1,2,3.

Stars and supernovas

Freedman’s team analyzed three types of star that are used as distance indicators, or ‘standard candles’, in nearby galaxies. Understanding the average brightness of standard candles helps astronomers estimate how far away the same types of star are in more distant galaxies, which appear as they were billions of years ago. Together with observations of supernova explosions in the same galaxies, standard candles can be used to measure the Universe’s current rate of expansion.

Riess, whose observations were based on the same three types of star, warns that it is too early to draw conclusions from any of the JWST data. “The Hubble Space Telescope has collected a mountain of data over several decades, including four separate and direct calibrations of [the Hubble constant],” he says. “Our JWST programme and Wendy’s are tiny by comparison.”

It would be premature to comment on Freedman’s results because they have not yet been published, says Kristin McQuinn, an astronomer at Rutgers University in New Jersey who is leading her own study of standard candles with JWST. “It is hard to evaluate their results without seeing their data.”

Freedman says that multiple techniques will need to agree before the Hubble constant issue is solved. “We need more than one method, and we need more than three if we want to put this issue to rest,” she told delegates at the London meeting.

Cosmologist George Efstathiou, a leading member of the Planck collaboration who is based at the University of Cambridge, UK, sees the glass half full, saying that the latest JWST results are remarkably close to Planck’s. “They are 4 km/s away from each other, which is not a lot,“ he says.

Hiranya Peiris, a cosmologist also at the University of Cambridge, says that she wouldn’t be surprised if the recent-Universe observations were to end up converging towards the Planck early-Universe results. But she agrees that it will be crucial to add a completely new technique to the mix. Observations of gravitational waves could offer a ‘clean’ approach that doesn’t suffer from the confounding factors that are always present when observing stars, she adds.

If the discrepancy is here to stay, it could mean that the current theoretical model of the expansion of the Universe — which relies on Einstein’s general theory of relativity — needs to be amended. Theorists have been busy trying to find explanations for the Hubble-constant discrepancy, but none of them are compatible with every set of observations, says cosmologist Eleonora Di Valentino at the University of Sheffield, UK. “At least 500 models have been proposed, and none of them is satisfactory.”

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Weird new electron behaviour thrills physicists

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Illustration showing four graphene layers.

Electrons in stacked sheets of staggered graphene collectively act as though they have fractional charges at ultralow temperatures.Credit: Ramon Andrade 3DCiencia/Science Photo Library

Two teams have observed that electrons, which usually have a charge of −1, can behave as if they had fractional charges (such as −2/3) — and do so without being nudged by an external magnetic field. It’s the first time this ‘fractional quantum anomalous Hall effect’ has been observed experimentally, and physicists are scratching their heads over exactly how it works. It’s a fundamental discovery that might also someday have practical applications: fractionally charged particles are a key requirement for a certain type of quantum computer. “I don’t know anyone who’s not excited about this,” says condensed-matter physicist Pablo Jarillo-Herrero.

Nature | 7 min read

Reference: Nature paper 1 & paper 2

Japanese tits (Parus minor) flutter their wings to invite their mate to enter the nest first. Scientists who observed eight breeding pairs of wild tits noticed that when one of the birds sat in front of the next box and fluttered its wings, the other would go in first. It’s the first documented evidence of birds using a symbolic gesture: one that has a specific meaning (like waving ‘goodbye’) but isn’t simply pointing at an object of interest. “It implies that birds have a level of understanding of symbolism that probably a lot of people wouldn’t have given them credit for before,” says ornithologist Mike Webster.

Scientific American | 4 min read

Reference: Current Biology paper

Offering researchers more money or time doesn’t seem to push them towards higher-risk, higher-reward projects. A survey asked more than 4,000 US-based academics how various hypothetical grants would influence their research strategy. Only tenured professors were willing to pursue riskier research with longer-running grants. And respondents were very unwilling to take less money in exchange for a longer grant. The results seem to suggest that “fairly reasonable changes in the structure of one particular individual grant don’t do enough to change the overall incentive structure”, says Carl Bergstrom, a biologist who has studied science-funding models.

Nature | 6 min read

Reference: arXiv preprint (not peer reviewed)

Infographic of the week

An infographic illustrating projections of space objects onto Earth

A. Williams et al./Nature Sustainability

The number of satellites and other space debris in low-Earth orbit (a) is on track to exceed 100,000 in the next ten years as SpaceX, OneWeb, GuoWang and Amazon plan to launch around 65,000 satellites (b). Policymakers must come up with a plan for space sustainability, argues a group of astronomers. (Nature Sustainability | 10 min read) (A. Williams et al./Nature Sustainability)

Features & opinion

More than two billion people worldwide lack access to reliable and safe drinking water. Changing that means tackling complex challenges that can only be solved by working alongside local communities, say four researchers with first-hand knowledge of water scarcity. “People are not voiceless, they simply remain unheard,” says water-governance scholar Farhana Sultana. “The way forward is through listening.”

Nature | 8 min read

You might have heard this one before: astronomer Fred Hoyle coined the phrase ‘Big Bang’ to make fun of a theory of the Universe’s origins that he disliked. Wrong, writes historian Helge Kragh. Hoyle did originate the catchy term — in a 1949 popular-science talk for BBC radio — but it was never intended as ridicule. And most people, including Hoyle, pretty much ignored it for decades afterwards. In 1965, the discovery of the cosmic microwave background signalled the triumph of the theory, ‘Big Bang’ made it into a New York Times headline, and the term snowballed into the popular lexicon.

Nature | 9 min read

Climate forecasting powered by artificial-intelligence (AI) algorithms could replace the equation-based systems that guide global policy. Some scientists are developing AI emulators that produce the same results as conventional models but do so much faster, using less energy. Others are hoping that AI systems can pick up on hidden patterns in climate data to make better predictions. Hybrids could embed machine-learning components inside physics-based models to gain better performance while being more trustworthy than models built entirely from AI. “I think the holy grail really is to use machine learning or AI tools to learn how to represent small-scale processes,” says climate scientist Tapio Schneider.

Nature | 8 min read

Quote of the day

Nature artist and scientific illustrator Zoe Keller says she is particularly drawn to “less charismatic” species such as snakes, amphibians, invertebrates, bats and fungi. (Nautilus | 6 min read)

Tonight, I’ll be keeping an eye on Corona Borealis, a constellation that is home to a white dwarf star. Once every 80 years or so, the white dwarf blows off the material it has syphoned off from a nearby red giant star in a spectacular cosmic explosion visible to the naked eye — and it’s expected to take place sometime before September.

Help to keep this newsletter on a stable trajectory by sending your feedback to [email protected].

Thanks for reading,

Katrina Krämer, associate editor, Nature Briefing

With contributions by Flora Graham, Smriti Mallapaty and Sarah Tomlin

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Weird new electron behaviour in stacked graphene thrills physicists

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Illustration showing four graphene layers.

Electrons in stacked sheets of staggered graphene collectively act as though they have fractional charges at ultra-low temperatures.Credit: Ramon Andrade 3DCiencia/Science Photo Library

Minneapolis, Minnesota

Last May, a team led by physicists at the University of Washington in Seattle observed something peculiar. When the scientists ran an electrical current across two atom-thin sheets of molybdenum ditelluride (MoTe2), the electrons acted in concert, like particles with fractional charges. Resistance measurements showed that, rather than the usual charge of –1, the electrons behaved similar to particles with charges of –2/3 or –3/5, for instance. What was truly odd was that the electrons did this entirely because of the innate properties of the material, without any external magnetic field coaxing them. The researchers published the results a few months later, in August1.

That same month, this phenomenon, known as the fractional quantum anomalous Hall effect (FQAHE), was also observed in a completely different material. A team led by Long Ju, a condensed-matter physicist at the Massachusetts Institute of Technology (MIT) in Cambridge, saw the effect when they sandwiched five layers of graphene between sheets of boron nitride. They published their results in February this year2 — and physicists are still buzzing about it.

At the American Physical Society (APS) March Meeting, held in Minneapolis, Minnesota, from 3 to 8 March, Ju presented the team’s findings, which haven’t yet been replicated by other researchers. Attendees, including Raquel Queiroz, a theoretical physicist at Columbia University in New York City, said that they thought the results were convincing, but were scratching their heads over the discovery. “There is a lot we don’t understand,” Queiroz says. Figuring out the exact mechanism of the FQAHE in the layered graphene will be “a lot of work ahead of theorists”, she adds.

Although the FQAHE might have practical applications down the line — fractionally charged particles are a key requirement for a certain type of quantum computer — the findings are capturing physicists’ imagination because they are fundamentally new discoveries about how electrons behave.

“I don’t know anyone who’s not excited about this,” says Pablo Jarillo-Herrero, a condensed-matter physicist at MIT who was not involved with the studies. “I think the question is whether you’re so excited that you switch all your research and start working on it, or if you’re just very excited.”

Strange maths

Strange behaviour by electrons isn’t new.

In some materials, usually at temperatures near absolute zero, electrical resistance becomes quantized. Specifically it’s the material’s transverse resistance that does this. (An electrical current encounters opposition to its flow in both the same direction as the current — called longitudinal resistance — and in the perpendicular direction — what’s called transverse resistance.)

Quantized ‘steps’ in the transverse resistance occur at multiples of electron charge: 1, 2, 3 and so on. These plateaus are the result of a strange phenomenon: the electrons maintain the same transverse resistance even as charge density increases. That’s a little like vehicles on a highway moving at the same speed, even with more traffic. This is known as the quantum Hall effect.

In a different set of materials, with less disorder, the transverse resistance can even display plateaus at fractions of electron charge: 2/5, 3/7 and 4/9, for example. The plateaus take these values because the electrons collectively act like particles with fractional charges — hence the fractional quantum Hall effect (FQHE).

Key to both phenomena is a strong external magnetic field, which prevents electrons from crashing into each other and enables them to interact.

A photo of the team. From left to right: Long Ju, Postdoc Zhengguang Lu, visiting undergraduate Yuxuan Yao, graduate student Tonghang Hang.

(Left to right) Long Ju, Zhengguang Lu, Yuxuan Yao and Tonghang Hang are all part of the team at MIT that demonstrated the FQAHE in layered graphene.Credit: Jixiang Yang

The FQHE, discovered in 1982, revealed the richness of electron behaviour. No longer could physicists think of electrons as single particles; in delicate quantum arrangements, the electrons could lose their individuality and act together to create fractionally charged particles. “I think people don’t appreciate how different [the fractional] is from the integer quantum Hall effect,” says Ashvin Vishwanath, a theoretical physicist at Harvard University in Cambridge. “It’s a new world.”

Over the next few decades, theoretical physicists came up with models to explain the FQHE and predict its effects. During their exploration, a tantalizing possibility appeared: perhaps a material could exhibit resistance plateaus without any external magnetic field. The effect, now dubbed the quantum anomalous Hall effect — ‘anomalous’, for the lack of a magnetic field — was finally observed in thin ferromagnetic films by a team at Tsinghua University in Beijing, in 20123.

Carbon copy

Roughly a decade later, the University of Washington team reported the FQAHE for the first time1, in a specially designed 2D material: two sheets of MoTe2 stacked on top of one another and offset by a twist.

This arrangement of MoTe2 is known as a moiré material. Originally used to refer to a patterned textile, the term has been appropriated by physicists to describe the patterns in 2D materials created from atom-thin lattices when they are stacked and then twisted, or staggered atop one another. The slight offset between atoms in different layers of the material shifts the hills and valleys of its electric potential. And it effectively acts like a powerful magnetic field, taking the place of the one needed in the quantum Hall effect and the FQHE.

Xiaodong Xu, a condensed-matter physicist at the University of Washington, talked about the MoTe2 discovery at the APS meeting. Theory hinted that the FQAHE would appear in the material at about a 1.4º twist angle. “We spent a year on it, and we didn’t see anything,” Xu told Nature.

Anomalous behaviour. Graphic showing the details of new moire material.

Source: Adapted from Ref. 2.

Then, the researchers tried a larger angle — a twist of about 4º. Immediately, they began seeing signs of the effect. Eventually, they measured the electrical resistance and spotted the signature plateaus of the FQAHE. Soon after, a team led by researchers at Shanghai Jiao Tong University in China replicated the results4.

Meanwhile at MIT, Ju was perfecting his technique, sandwiching graphene between layers of boron nitride. Similar to graphene, the sheets of boron nitride that Ju’s team used were a mesh of atoms linked together in a hexagonal pattern. Its lattice has a slightly different size than graphene; the mismatch creates a moiré pattern (see ‘Anomalous behaviour’).

Last month, Ju published a report2 about seeing the characteristic plateaus. “It is a really amazing result,” Xu says. “I’m very happy to see there’s a second system.” Since then, Ju says that he’s also seen the effect when using four and six layers of graphene.

Both moiré systems have their pros and cons. MoTe2 exhibited the effect at a few Kelvin, as opposed to 0.1 Kelvin for the layered graphene sandwich. (Low temperatures are required to minimize disorder in the systems.) But graphene is a cleaner and higher-quality material that is easier to measure. Experimentalists are now trying to replicate the results in graphene and find other materials that behave similarly.

Moiré than bargained for

Theorists are relatively comfortable with the MoTe2 results, for which the FQAHE was partly predicted. But Ju’s layered graphene moiré was a shock to the community, and researchers are still struggling to explain how the effect happens. “There’s no universal consensus on what the correct theory is,” Vishwanath says. “But they all agree that it’s not the standard mechanism.” Vishwanath and his colleagues posted a preprint proposing a theory that the moiré pattern might not be that important to the FQAHE5.

One reason to doubt the importance of the moiré is the location of the electrons in the material: most of the activity is in the topmost layer of graphene, far away from the moiré pattern between the graphene and boron nitride at the bottom of the sandwich that is supposed to most strongly influence the electrons. But B. Andrei Bernevig, a theoretical physicist at Princeton University in New Jersey, and a co-author of another preprint proposing a mechanism for the FQAHE in the layered graphene6, urges caution about theory-based calculations, because they rely on currently unverified assumptions. He says that the moiré pattern probably matters, but less than it does in MoTe2.

For theorists, the uncertainty is exciting. “There are people who would say that everything has been seen in the quantum Hall effect,” Vishwanath says. But these experiments, especially the one using the layered graphene moiré, show that there are still more mysteries to uncover.

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How Hawking’s paradox still puzzles physicists

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An electron microprobe image of a grey sphere on a black background. The sphere has a partially irregular surface and is about 200 micrometres across according to the scale bar.

Avi Loeb and his team say that metallic balls found near Papua New Guinea could be of extraterrestrial origin.Credit: Avi Loeb’s photo collection

Scientists have clashed over whether a research team has indeed found fragments of an interstellar space rock that hit Earth in 2014. A team led by controversial astrophysicist Avi Loeb used magnetic sledges to recover more than 800 metallic spherules from the seafloor near Papua New Guinea last year. They claim that a handful of the tiny blobs are unusually rich in beryllium, lanthanum and uranium, proving that they came from outside the Solar System. But the planetary science community is unconvinced, suggesting alternate origins for the spherules, and disputing whether the 2014 asteroid impact was even interstellar.

Nature | 6 min read

Reference: arXiv preprint (not peer reviewed) and arXiv preprint (not peer reviewed)

A preprint study has found that some artificial intelligence (AI) systems, including those that power chatbots such as ChatGPT, are more likely to suggest that a fictional defendant is sentenced to death when they write in African American English (AAE) — a dialect spoken by millions of people in the United States that is associated with the descendants of enslaved African Americans — compared with one written in Standardized American English (SAE). The models also associated AAE speakers with extreme negative stereotypes and were more likely to match them with less-prestigious jobs. Overt racism in AI models (linking a particular group with violence, for example) can be reduced by using human feedback. But such fine-tuning did nothing to remove covert racism based purely on dialect.

Nature | 5 min read

Reference: arXiv preprint (not peer reviewed)

Mobile clinics in rural villages in Sierra Leone sharply boosted the uptake of COVID-19 vaccines compared to villages that did not get the service. When COVID-19 vaccines were first made available, people who live in rural areas had to make, on average, a seven-hour round trip to receive one, at a total cost that could exceed a week’s wages, says economist and study co-author Ahmed Mushfiq Mobarak. “When you’re starting with a baseline vaccination rate of essentially zero, our research shows that the most cost-effective thing to do is just to show up,” Mobarak says. The mobile clinics cost about US$33 per person vaccinated.

Nature | 3 min read

Read an expert analysis by public-health researchers Alison Buttenheim and Harsha Thirumurthy in the Nature News & Views article (Nature | 8 min read)

Reference: Nature paper

The heart is the first organ to develop, but scientists know surprisingly little about how different types of heart cells organize to form a working heart. Researchers combined RNA sequencing and high-resolution fluorescence imaging to map communities of heart cells in enough detail to build a high resolution, 3D ‘heart atlas’. As well as mapping cardiac structures, the study reveals signalling pathways that orchestrate the arrangement of heart cells. The authors hope that this ‘atlas’ will offer new insights into congenital heart disease, a leading cause of death in infants.

Nature | 3 min video

Reference: Nature paper

Features & opinion

Early-onset cancer is on the rise. Colorectal cancer, for example, has become the leading cause of cancer death among men under 50 in the United States and the second leading cause of cancer death in young women. Researchers are looking at tumour genetics, dietary changes and microbiome composition for clues, but so far there is no clear explanation for the shift. “If it had been a single smoking gun, our studies would have at least pointed to one factor,” says gastroenterologist Sonia Kupfer. “It seems to be a combination of many different factors.”

Nature | 11 min read

Nanomedicine researcher Morteza Mahmoudi co-founded the Academic Parity Movement after he was forced to quit his job because of bullying. The non-profit organization targets academic harassment with tailored and context-specific training, monitoring and intervention strategies. “Survey data revealed that academic bullying and harassment do not affect all scientific fields equally,” explains Mahmoudi.

Nature | 5 min read

Physicist Stephen Hawking died six years ago, on 14 March 2018. Fifty years ago, he published a Nature paper with the enigmatic title: Black hole explosions? The paper introduced the concept of ‘Hawking radiation’: the idea that black holes are not truly black because they constantly emit a tiny amount of heat. As Hawking soon realized, this creates a paradox. Hawking radiation doesn’t maintain the details of the original material that went into the hole; therefore, it inexorably erases information from the Universe, contradicting the laws of quantum mechanics. Efforts to solve the conundrum have led to legendary wagers, a theory that wormholes connect the inside of black holes with the outside and the idea that the Universe is a hologram. “Here it is, 50 years after that great paper, and we’re still puzzled,” says theoretical physicist John Preskill.

Nature | 7 min read

Reference: Nature paper (from 1974)

Quote of the day

After his son broke a beaker in his school chemistry lab, Keith Hornberger asked chemists on Twitter to help him feel better by sharing their biggest glassware blunders. Chemist Josh McBee wins by a mile. (Chemistry World | 5 min read)

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