Scientists in the US have developed a new type of photovoltaic battery that runs on the energy given off by nuclear waste. The battery uses a scintillator crystal to transform the intense gamma rays from radioisotopes into electricity and can produce more than a microwatt of power. According to its developers at Ohio State University and the University of Toledo, it could be used to power microelectronic devices such as microchips.

The idea of a nuclear waste battery is not new. Indeed, Raymond Cao, the Ohio State nuclear engineer who led the new research effort, points out that the first experiments in this field date back to the early 1950s. These studies, he explains, used a 50 milli-Curie ⁹⁰Sr-⁹⁰Y source to produce electricity via the electron-voltaic effect in p-n junction devices.

However, the maximum power output of these devices was just 0.8 μW, and their power conversion efficiency (PCE) was an abysmal 0.4 %. Since then, the PCE of nuclear voltaic batteries has remained low, typically in the 1–3% range, and even the most promising devices have produced, at best, a few hundred nanowatts of power.

Exploiting the nuclear photovoltaic effect

Cao is confident that his team’s work will change this. “Our yet-to-be-optimized battery has already produced 1.5 μW,” he says, “and there is much room for improvement.”

To achieve this benchmark, Cao and colleagues focused on a different physical process called the nuclear photovoltaic effect. This effect captures the energy from highly-penetrating gamma rays indirectly, by coupling a photovoltaic solar cell to a scintillator crystal that emits visible light when it absorbs radiation. This radiation can come from several possible sources, including nuclear power plants, storage facilities for spent nuclear fuel, space- and submarine-based nuclear reactors or, really, anyplace that happens to have large amounts of gamma ray-producing radioisotopes on hand.

The scintillator crystal Cao and colleagues used is gadolinium aluminium garnet (GAGG), and they attached it to a solar cell made from polycrystalline CdTe. The resulting device measures around 2 x 2 x 1 cm, and they tested it using intense gamma rays emitted by two different radioactive sources, ¹³⁷Cs and ⁶⁰Co, that produced 1.5 kRad/h and 10 kRad/h, respectively. ¹³⁷Cs is the most common fission product found in spent nuclear fuel, while ⁶⁰Co is an activation product.

Enough power for a microsensor

The Ohio-Toledo team found that the maximum power output of their battery was around 288 nW with the ¹³⁷Cs source. Using the ⁶⁰Co irradiator boosted this to 1.5 μW. “The greater the radiation intensity, the more light is produced, resulting in increased electricity generation,” Cao explains.

The higher figure is already enough to power a microsensor, he says, and he and his colleagues aim to scale the system up to milliwatts in future efforts. However, they acknowledge that doing so presents several challenges. Scaling up the technology will be expensive, and gamma radiation gradually damages both the scintillator and the solar cell. To overcome the latter problem, Cao says they will need to replace the materials in their battery with new ones. “We are interested in finding alternative scintillator and solar cell materials that are more radiation-hard,” he tells Physics World.

The researchers are optimistic, though, arguing that optimized nuclear photovoltaic batteries could be a viable option for harvesting ambient radiation that would otherwise be wasted. They report their work in Optical Materials X.

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A new technique could reduce the risk of blood clots associated with medical implants, making them safer for patients. The technique, which was developed by researchers at the University of Sydney, Australia, involves coating the implants with highly hydrophilic molecules known as zwitterions, thereby inhibiting the build-up of clot-triggering proteins.

Proteins in blood can stick to the surfaces of medical implants such as heart valves and vascular stents. When this happens, it produces a cascade effect in which multiple mechanisms lead to the formation of extensive clots and fibrous networks. These clots and networks can impair the function of implanted medical devices so much that invasive surgery may be required to remove or replace the implant.

To prevent this from happening, the surfaces of implants are often treated with polymeric coatings that resist biofouling. Hydrophilic polymeric coatings such as polyethylene glycol are especially useful, as their water-loving nature allows a thin layer of water to form between them and the surface of the implants, held in place via hydrogen and/or electrostatic bonds. This water layer forms a barrier that prevents proteins from sticking, or adsorbing, to the implant.

An extra layer of zwitterions

Recently, researchers discovered that polymers coated with an extra layer of small molecules called zwitterions provided even more protection against protein adsorption. “Zwitter” means “hybrid” in German; hence, zwitterions are molecules that carry both positive and negative charge, making them neutrally charged overall. These molecules are also very hydrophilic and easily form tight bonds with water molecules. The resulting layer of water has a structure that is similar to that of bulk water, which is energetically stable.

A further attraction of zwitterionic coatings for medical implants is that zwitterions are naturally present in our bodies. In fact, they make up the hydrophilic phospholipid heads of mammalian cell membranes, which play a vital role in regulating interactions between biological cells and the extracellular environment.

Plasma functionalization

In the new work, researchers led by Sina Naficy grafted nanometre-thick zwitterionic coatings onto the surfaces of implant materials using a technique called plasma functionalization. They found that the resulting structures reduce the amount of fibrinogen proteins that adsorb onto the implants by roughly nine-fold and decrease blood clot formation (thrombosis) by almost 75%.

Naficy and colleagues achieved their results by optimizing the density, coverage and thickness of the coating. This was critical for realizing the full potential of these materials, they say, because a coating that is not fully optimized would not reduce clotting.

Naficy tells Physics World that the team’s main goal is to enhance the surface properties of medical devices. “These devices when implanted are in contact with blood and can readily cause thrombosis or infection if the surface initiates certain biological cascade reactions,” he explains. “Most such reactions begin when specific proteins adsorb on the surface and activate the next stage of cascade. Optimizing surface properties with the aid of zwitterions can control / inhibit protein adsorption, hence reducing the severity of adverse body reactions.”

The researchers say they will now be evaluating the long-term stability of the zwitterion-polymer coatings and trying to scale up their grafting process. They report their work in Communications Materials and Cell Biomaterials.

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A new artificial intelligence/machine learning method rapidly and accurately characterizes binary neutron star mergers based on the gravitational wave signature they produce. Though the method has not yet been tested on new mergers happening “live”, it could enable astronomers to make quicker estimates of properties such as the location of mergers and the masses of the neutron stars. This information, in turn, could make it possible for telescopes to target and observe the electromagnetic signals that accompany such mergers.

When massive objects such as black holes and neutron stars collide and merge, they emit ripples in spacetime known as gravitational waves (GWs). In 2015 scientists on Earth began observing these ripples using kilometre-scale interferometers that measure the minuscule expansion and contraction of space–time that occurs when a gravitational wave passes through our planet. These interferometers are located in the US, Italy and Japan and are known collectively as the LVK observatories after their initials: the Laser Interferometer GW Observatory (LIGO), the Virgo GW Interferometer (Virgo) and the Kamioka GW Detector (KAGRA).

When two neutron stars in a binary pair merge, they emit electromagnetic waves as well as GWs. While both types of wave travel at the speed of light, certain poorly understood processes that occur within and around the merging pair cause the electromagnetic signal to be slightly delayed. This means that the LVK observatories can detect the GW signal coming from a binary neutron star (BNS) merger seconds, or even minutes, before its electromagnetic counterpart arrives. Being able to identify GWs quickly and accurately therefore increases the chances of detecting other signals from the same event.

This is no easy task, however. GW signals are long and complex, and the main technique currently used to interpret them, Bayesian inference, is slow. While faster alternatives exist, they often make algorithmic approximations that negatively affect their accuracy.

Trained with millions of GW simulations

Physicists led by Maximilian Dax of the Max Planck Institute for Intelligent Systems in Tübingen, Germany have now developed a machine learning (ML) framework that accurately characterizes and localizes BNS mergers within a second of a GW being detected, without resorting to such approximations. To do this, they trained a deep neural network model with millions of GW simulations.

Once trained, the neural network can take fresh GW data as input and predict corresponding properties of the merging BNSs – for example, their masses, locations and spins – based on its training dataset. Crucially, this neural network output includes a sky map. This map, Dax explains, provides a fast and accurate estimate for where the BNS is located.

The new work built on the group’s previous studies, which used ML systems to analyse GWs from binary black hole (BBH) mergers. “Fast inference is more important for BNS mergers, however,” Dax says, “to allow for quick searches for the aforementioned electromagnetic counterparts, which are not emitted by BBH mergers.”

The researchers, who report their work in Nature, hope their method will help astronomers to observe electromagnetic counterparts for BNS mergers more often and detect them earlier – that is, closer to when the merger occurs. Being able to do this could reveal important information on the underlying processes that occur during these events. “It could also serve as a blueprint for dealing with the increased GW signal duration that we will encounter in the next generation of GW detectors,” Dax says. “This could help address a critical challenge in future GW data analysis.”

So far, the team has focused on data from current GW detectors (LIGO and Virgo) and has only briefly explored next-generation ones. They now plan to apply their method to these new GW detectors in more depth.

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Researchers at the EMBL in Germany have dramatically reduced the time required to create images using Brillouin microscopy, making it possible to study the viscoelastic properties of biological samples far more quickly and with less damage than ever before. Their new technique can image samples with a field of view of roughly 10 000 pixels at a speed of 0.1 Hz – a 1000-fold improvement in speed and throughput compared to standard confocal techniques.

Mechanical properties such as the elasticity and viscosity of biological cells are closely tied to their function. These properties also play critical roles in processes such as embryo and tissue development and can even dictate how diseases such as cancer evolve. Measuring these properties is therefore important, but it is not easy since most existing techniques to do so are invasive and thus inherently disruptive to the systems being imaged.

Non-destructive, label- and contact-free

In recent years, Brillouin microscopy has emerged as a non-destructive, label- and contact-free optical spectroscopy method for probing the viscoelastic properties of biological samples with high resolution in three dimensions. It relies on Brillouin scattering, which occurs when light interacts with the phonons (or collective vibrational modes) that are present in all matter. This interaction produces two additional peaks, known as Stokes and anti-Stokes Brillouin peaks, in the spectrum of the scattered light. The position of these peaks (the Brillouin shift) and their linewidth (the Brillouin width) are related to the elastic and viscous properties, respectively, of the sample.

The downside is that standard Brillouin microscopy approaches analyse just one point in a sample at a time. Because the scattering signal from a single point is weak, imaging speeds are slow, yielding long light exposure times that can damage photosensitive components within biological cells.

“Light sheet” Brillouin imaging

To overcome this problem, EMBL researchers led by Robert Prevedel began exploring ways to speed up the rate at which Brillouin microscopy can acquire two- and three-dimensional images. In the early days of their project, they were only able to visualize one pixel at a time. With typical measurement times of tens to hundreds of milliseconds for a single data point, it therefore took several minutes, or even hours, to obtain two-dimensional images of 50–250 square pixels.

In 2022, however, they succeeded in expanding the field of view to include an entire spatial line — that is, acquiring image data from more than 100 points in parallel. In their latest work, which they describe in Nature Photonics, they extended the technique further to allow them to view roughly 10 000 pixels in parallel over the full plane of a sample. They then used the  new approach to study mechanical changes in live zebrafish larvae.

“This advance enables much faster Brillouin imaging, and in terms of microscopy, allows us to perform ‘light sheet’ Brillouin imaging,” says Prevedel. “In short, we are able to ‘under-sample’ the spectral output, which leads to around 1000 fewer individual measurements than normally needed.”

Towards a more widespread use of Brillouin microscopy

Prevedel and colleagues hope their result will lead to more widespread use of Brillouin microscopy, particularly for photosensitive biological samples. “We wanted to speed-up Brillouin imaging to make it a much more useful technique in the life sciences, yet keep overall light dosages low. We succeeded in both aspects,” he tells Physics World.

Looking ahead, the researchers plan to further optimize the design of their approach and merge it with microscopes that enable more robust and straightforward imaging. “We then want to start applying it to various real-world biological structures and so help shed more light on the role mechanical properties play in biological processes,” Prevedel says.

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New data from the NOvA experiment at Fermilab in the US contain no evidence for so-called “sterile” neutrinos, in line with results from most – though not all – other neutrino detectors to date. As well as being consistent with previous experiments, the finding aligns with standard theoretical models of neutrino oscillation, in which three active types, or flavours, of neutrino convert into each other. The result also sets more stringent limits on how much an additional sterile type of neutrino could affect the others.

“The global picture on sterile neutrinos is still very murky, with a number of experiments reporting anomalous results that could be attributed to sterile neutrinos on one hand and a number of null results on the other,” says NOvA team member Adam Lister of the University of Wisconsin, Madison, US. “Generally, these anomalous results imply we should see large amounts of sterile-driven neutrino disappearance at NOvA, but this is not consistent with our observations.”

Neutrinos were first proposed in 1930 by Wolfgang Pauli as a way to account for missing energy and spin in the beta decay of nuclei. They were observed in the laboratory in 1956, and we now know that they come in (at least) three flavours: electron, muon and tau. We also know that these three flavours oscillate, changing from one to another as they travel through space, and that this oscillation means they are not massless (as was initially thought).

Significant discrepancies

Over the past few decades, physicists have used underground detectors to probe neutrino oscillation more deeply. A few of these detectors, including the LSND at Los Alamos National Laboratory, BEST in Russia, and Fermilab’s own MiniBooNE, have observed significant discrepancies between the number of neutrinos they detect and the number that mainstream theories predict.

One possible explanation for this excess, which appears in some extensions of the Standard Model of particle physics, is the existence of a fourth flavour of neutrino. Neutrinos of this “sterile” type do not interact with the other flavours via the weak nuclear force. Instead, they interact only via gravity.

Detecting sterile neutrinos would fundamentally change our understanding of particle physics. Indeed, some physicists think sterile neutrinos could be a candidate for dark matter – the mysterious substance that is thought to make up around 85% of the matter in the universe but has so far only made itself known through the gravitational force it exerts.

Near and far detectors

The NOvA experiment uses two liquid scintillator detectors to monitor a stream of neutrinos created by firing protons at a carbon target. The near detector is located at Fermilab, approximately 1 km from the target, while the far detector is 810 km away in northern Minnesota. In the new study, the team measured how many muon-type neutrinos survive the journey through the Earth’s crust from the near detector to the far one. The idea is that if fewer neutrinos survive than the conventional three-flavour oscillations picture predicts, some of them could have oscillated into sterile neutrinos.

The experimenters studied two different interactions between neutrinos and normal matter, says team member V Hewes of the University of Cincinnati, US. “We looked for both charged current muon neutrino and neutral current interactions, as a sterile neutrino would manifest differently in each,” Hewes explains. “We then compared our data across those samples in both detectors to simulations of neutrino oscillation models with and without the presence of a sterile neutrino.”

No excess of neutrinos seen

Writing in Physical Review Letters, the researchers state that they found no evidence of neutrinos oscillating into sterile neutrinos. What is more, introducing a fourth, sterile neutrino did not provide better agreement with the data than sticking with the standard model of three active neutrinos.

This result is in line with several previous experiments that looked for sterile neutrinos, including those performed at T2K, Daya Bay, RENO and MINOS+. However, Lister says it places much stricter constraints on active-sterile neutrino mixing than these earlier results. “We are really tightening the net on where sterile neutrinos could live, if they exist,” he tells Physics World.

The NOvA team now hopes to tighten the net further by reducing systematic uncertainties. “To that end, we are developing new data samples that will help us better understand the rate at which neutrinos interact with our detector and the composition of our beam,” says team member Adam Aurisano, also at the University of Cincinnati. “This will help us better distinguish between the potential imprint of sterile neutrinos and more mundane causes of differences between data and prediction.”

NOvA co-spokesperson Patricia Vahle, a physicist at the College of William & Mary in Virginia, US, sums up the results. “Neutrinos are full of surprises, so it is important to check when anomalies show up,” she says. “So far, we don’t see any signs of sterile neutrinos, but we still have some tricks up our sleeve to extend our reach.”

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Physicists in Germany have found an alternative explanation for an anomaly that had previously been interpreted as potential evidence for a mysterious “dark force”. Originally spotted in ytterbium atoms, the anomaly turns out to have a more mundane cause. However, the investigation, which involved high-precision measurements of shifts in ytterbium’s energy levels and the mass ratios of its isotopes, could help us better understand the structure of heavy atomic nuclei and the physics of neutron stars.

Isotopes are forms of an element that have the same number of protons and electrons, but different numbers of neutrons. These different numbers of neutrons produce shifts in the atom’s electronic energy levels. Measuring these so-called isotope shifts is therefore a way of probing the interactions between electrons and neutrons.

In 2020, a team of physicists at the Massachusetts Institute of Technology (MIT) in the US observed an unexpected deviation in the isotope shift of ytterbium. One possible explanation for this deviation was the existence of a new “dark force” that would interact with both ordinary, visible matter and dark matter via hypothetical new force-carrying particles (bosons).

Although dark matter is thought to make up about 85 percent of the universe’s total matter, and its presence can be inferred from the way light bends as it travels towards us from distant galaxies, it has never been detected directly. Evidence for a new, fifth force (in addition to the known strong, weak, electromagnetic and gravitational forces) that acts between ordinary and dark matter would therefore be very exciting.

A team led by Tanja Mehlstäubler from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and Klaus Blaum from the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg has now confirmed that the anomaly is real. However, the PTB-MPIK researchers say it does not stem from a dark force. Instead, it arises from the way the nuclear structure of ytterbium isotopes deforms as more neutrons are added.

Measuring ytterbium isotope shifts and atomic masses

Mehlstäubler, Blaum and colleagues came to this conclusion after measuring shifts in the atomic energy levels of five different ytterbium isotopes: ^{168,170,172,174,176}Yb. They did this by trapping ions of these isotopes in an ion trap at the PTB and then using an ultrastable laser to drive certain electronic transitions. This allowed them to pin down the frequencies of specific transitions (²S_1/2→²D_5/2 and ²S_1/2→²F_7/2) with a precision of 4 ×10⁻⁹, the highest to date.

They also measured the atomic masses of the ytterbium isotopes by trapping individual highly-charged Yb⁴²⁺ ytterbium ions in the cryogenic PENTATRAP Penning trap mass spectrometer at the MPIK. In the strong magnetic field of this trap, team member and study lead author Menno Door explains, the ions are bound to follow a circular orbit. “We measure the rotational frequency of this orbit by amplifying the miniscule inducted current in surrounding electrodes,” he says. “The measured frequencies allowed us to very precisely determine the related mass ratios of the various isotopes with a precision of 4 ×10⁻¹².”

From these data, the researchers were able to extract new parameters that describe how the ytterbium nucleus deforms. To back up their findings, a group at TU Darmstadt led by Achim Schwenk simulated the ytterbium nuclei on large supercomputers, calculating their structure from first principles based on our current understanding of the strong and electromagnetic interactions. “These calculations confirmed that the leading signal we measured was due to the evolving nuclear structure of ytterbium isotopes, not a new fifth force,” says team member Matthias Heinz.

“Our work complements a growing body of research that aims to place constraints on a possible new interaction between electrons and neutrons,” team member Chih-Han Yeh tells Physics World. “In our work, the unprecedented precision of our experiments refined existing constraints.”

The researchers say they would now like to measure other isotopes of ytterbium, including rare isotopes with high or low neutron numbers. “Doing this would allow us to control for uncertain ‘higher-order’ nuclear structure effects and further improve the constraints on possible new physics,” says team member Fiona Kirk.

Door adds that isotope chains of other elements such as calcium, tin and strontium would also be worth investigating. “These studies would allow to further test our understanding of nuclear structure and neutron-rich matter, and with this understanding allow us to probe for possible new physics again,” he says.

The work is detailed in Physical Review Letters.

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Orthopaedic implants that bear loads while bones heal, then disappear once they’re no longer needed, could become a reality thanks to a new technique for enhancing the mechanical properties of zinc alloys. Developed by researchers at Monash University in Australia, the technique involves controlling the orientation and size of microscopic grains in these strong yet biodegradable materials.

Implants such as plates and screws provide temporary support for fractured bones until they knit together again. Today, these implants are mainly made from sturdy materials such as stainless steel or titanium that remain in the body permanently. Such materials can, however, cause discomfort and bone loss, and subsequent injuries to the same area risk additional damage if the permanent implants warp or twist.

To address these problems, scientists have developed biodegradable alternatives that dissolve once the bone has healed. These alternatives include screws made from magnesium-based materials such as MgYREZr (trade name MAGNEZIX), MgYZnMn (NOVAMag) and MgCaZn (RESOMET). However, these materials have compressive yield strengths of just 50 to 260 MPa, which is too low to support bones that need to bear a patient’s weight. They also produce hydrogen gas as they degrade, possibly affecting how biological tissues regenerate.

Zinc alloys do not suffer from the hydrogen gas problem. They are biocompatible, dissolving slowly and safely in the body. There is even evidence that Zn²⁺ ions can help the body heal by stimulating bone formation. But again, their mechanical strength is low: at less than 30 MPa, they are even worse than magnesium in this respect.

Making zinc alloys strong enough for load-bearing orthopaedic implants is not easy. Mechanical strategies such as hot-extruding binary alloys have not helped much. And methods that focus on reducing the materials’ grain size (to hamper effects like dislocation slip) have run up against a discouraging problem: at body temperature (37 °C), ultrafine-grained Zn alloys become mechanically weaker as their so-called “creep resistance” decreases.

Grain size goes bigger

In the new work, a team led by materials scientist and engineer Jian-Feng Nei tried a different approach. By increasing grain size in Zn alloys rather than decreasing it, the Monash team was able to balance the alloys’ strength and creep resistance – something they say could offer a route to stronger zinc alloys for biodegradable implants.

In compression tests of extruded Zn–0.2 wt% Mg alloy samples with grain sizes of 11 μm, 29 μm and 47 μm, the team measured stress-strain curves that show a markedly higher yield strength for coarse-grained samples than for fine-grained ones. What is more, the compressive yield strengths of these coarser-grained zinc alloys are notably higher than those of MAGNEZIX, NOVAMag and RESOMET biodegradable magnesium alloys. At the upper end, they even rival those of high-strength medical-grade stainless steels.

The researchers attribute this increased compressive yield to a phenomenon called the inverse Hall–Petch effect. This effect comes about because larger grains favour metallurgical effects such as intra-granular pyramidal slip as well as a variation of a well-known metal phenomenon called twinning, in which a specific kind of defect forms when part of the material’s crystal structure flips its orientation. Larger grains also make the alloys more flexible, allowing them to better adapt to surrounding biological tissues. This is the opposite of what happens with smaller grains, which facilitate inter-granular grain boundary sliding and make alloys more rigid.

The new work, which is detailed in Nature, could aid the development of advanced biodegradable implants for orthopaedics, cardiovascular applications and other devices, says Nei. “With improved biocompatibility, these implants could be safer and do away with the need for removal surgeries, lowering patient risk and healthcare costs,” he tells Physics World. “What is more, new alloys and processing techniques could allow for more personalized treatments by tailoring materials to specific medical needs, ultimately improving patient outcomes.”

The Monash team now aims to improve the composition of the alloys and achieve more control over how they degrade. “Further studies on animals and then clinical trials will test their strength, safety and compatibility with the body,” says Nei. “After that, regulatory approvals will ensure that the biodegradable metals meet medical standards for orthopaedic implants.”

The team is also setting up a start-up company with the goal of developing and commercializing the materials, he adds.

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How would the climate and the environment on our planet change if an asteroid struck? Researchers at the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea have now tried to answer this question by running several impact simulations with a state-of-the-art Earth system model on their in-house supercomputer. The results show that the climate, atmospheric chemistry and even global photosynthesis would be dramatically disrupted in the three to four years following the event, due to the huge amounts of dust produced by the impact.

Beyond immediate effects such as scorching heat, earthquakes and tsunamis, an asteroid impact would have long-lasting effects on the climate because of the large quantities of aerosols and gases ejected into the atmosphere. Indeed, previous studies on the Chicxulub 10-km asteroid impact, which happened around 66 million years ago, revealed that dust, soot and sulphur led to a global “impact winter” and was very likely responsible for the dinosaurs going extinct during the Cretaceous/Paleogene period.

“This winter is characterized by reduced sunlight, because of the dust filtering it out, cold temperatures and decreased precipitation at the surface,” says Axel Timmermann, director of the ICCP and leader of this new study. “Severe ozone depletion would occur in the stratosphere too because of strong warming caused by the dust particles absorbing solar radiation there.”

These unfavourable climate conditions would inhibit plant growth via a decline in photosynthesis both on land and in the sea and would thus affect food productivity, Timmermann adds.

Something surprising and potentially positive would also happen though, he says: plankton in the ocean would recover within just six months and its abundance could even increase afterwards. Indeed, diatoms (silicate-rich algae) would be more plentiful than before the collision. This might be because the dust created by the asteroid is rich in iron, which would trigger plankton growth as it sinks into the ocean. These phytoplankton “blooms” could help alleviate emerging food crises triggered by the reduction in terrestrial productivity, at least for several years after the impact, explains Timmermann.

The effect of a “Bennu”-sized asteroid impact

In this latest study, published in Science Advances, the researchers simulated the effect of a “Bennu”-sized asteroid impact. Bennu is a so-called medium-sized asteroid with a diameter of around 500 m. This type of asteroid is more likely to impact Earth than the “planet killer” larger asteroids, but has been studied far less.

There is an estimated 0.037% chance of such an asteroid colliding with Earth in September 2182. While this probability is small, such an impact would be very serious, says Timmermann, and would lead to climate conditions similar to those observed after some of the largest volcanic eruptions in the last 100 000 years. “It is therefore important to assess the risk, which is the product of the probability and the damage that would be caused, rather than just the probability by itself,” he tells Physics World. “Our results can serve as useful benchmarks to estimate the range of environmental effects from future medium-sized asteroid collisions.”

The team ran the simulations on the IBS’ supercomputer Aleph using the Community Earth System Model Version 2 (CESM2) and the Whole Atmosphere Community Climate Model Version 6 (WACCM6). The simulations injected up to 400 million tonnes of dust into the stratosphere.

The climate effects of impact-dust aerosols mainly depend on their abundance in the atmosphere and how they evolve there. The simulations revealed that global mean temperatures would drop by 4° C, a value that’s comparable with the cooling estimated as a result of the Toba volcano erupting around 74 000 years ago (which emitted 2000 Tg (2×10¹⁵ g) of sulphur dioxide). Precipitation also decreased 15% worldwide and ozone dropped by a dramatic 32% in the first year following the asteroid impact.

Asteroid impacts may have shaped early human evolution

“On average, medium-sized asteroids collide with Earth about every 100 000 to 200 000 years,” says Timmermann. “This means that our early human ancestors may have experienced some of these medium-sized events. These may have impacted human evolution and even affected our species’ genetic makeup.”

The researchers admit that their model has some inherent limitations. For one, CESM2/WACCM6, like other modern climate models, is not designed and optimized to simulate the effects of massive amounts of aerosol injected into the atmosphere. Second, the researchers only focused on the asteroid colliding with the Earth’s land surface. This is obviously less likely than an impact on the ocean, because roughly 70% of Earth’s surface is covered by water, they say. “An impact in the ocean would inject large amounts of water vapour rather than climate-active aerosols such as dust, soot and sulphur into the atmosphere and this vapour needs to be better modelled – for example, for the effect it has on ozone loss,” they say.

The effect of the impact on specific regions on the planet also needs to be better simulated, the researchers add. Whether the asteroid impacts during winter or summer also needs to be accounted for since this can affect the extent of the climate changes that would occur.

Finally, as well as the dust nanoparticles investigated in this study, future work should also look at soot emissions from wildfires ignited by “impact “spherules”, and sulphur and CO₂ released from target evaporites, say Timmermann and colleagues. “The ‘impact winter’ would be intensified and prolonged if other aerosols such as soot and sulphur were taken into account.”

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A research team headed up at Linköping University in Sweden and Cornell University in the US has succeeded in recycling almost all of the components of perovskite solar cells using simple, non-toxic, water-based solvents. What’s more, the researchers were able to use the recycled components to make new perovskite solar cells with almost the same power conversion efficiency as those created from new materials. This work could pave the way to a sustainable perovskite solar economy, they say.

While solar energy is considered an environmentally friendly source of energy, most of the solar panels available today are based on silicon, which is difficult to recycle. This has led to the first generation of silicon solar panels, which are reaching the end of their life cycles, ending up in landfills, says Xun Xiao, one of the team members at Linköping University.

When developing emerging solar cell technologies, we therefore need to take recycling into consideration, adds one of the leaders of the new study, Feng Gao, also at Linköping. “If we don’t know how to recycle them, maybe we shouldn’t put them on the market at all.”

To this end, many countries around the world are imposing legal requirements on photovoltaic manufacturers, to ensure that they collect and recycle any solar cell waste they produce. These initiatives include the WEEE directive 2012/19/EU in the European Union and equivalent legislation in Asia and the US.

Perovskites are one of the most promising materials for making next-generation solar cells. Not only are they relatively inexpensive, they are also easy to fabricate, lightweight, flexible and transparent. This allows them to be placed on top of a variety of surfaces, unlike their silicon counterparts. And since they boast a power conversion efficiency (PCE) of more than 25%, this makes them comparable to existing photovoltaics on the market.

A shorter lifespan

One of their downsides, however, is that perovskite solar cells have a shorter lifespan than silicon solar cells. This means that recycling is even more critical for these materials. Today, perovskite solar cells are disassembled using dangerous solvents such as dimethylformamide, but Gao and colleagues have now developed a technique in which water can be used as the solvent.

Perovskites are crystalline materials with an ABX₃ structure, where A is caesium, methylammonium (MA) or formamidinium (FA); B is lead or tin; and X is chlorine, bromine or iodine. Solar cells made of these materials are composed of different layers: the hole/electron transport layers; the perovskite layer; indium tin oxide substrates; and cover glasses.

In their work, which they detail in Nature, the researchers succeeded in delaminating end-of-life devices layer by layer, using water containing three low-cost additives: sodium acetate, sodium iodide and hypophosphorous acid. Despite being able to dissolve organic iodide salts such as methylammonium iodide and formamidinium iodide, water only marginally dissolves lead iodide (about 0.044 g per 100 ml at 20 °C). The researchers therefore developed a way to increase the amount of lead iodide that dissolves in water by introducing acetate ions into the mix. These ions readily coordinate with lead ions, forming highly soluble lead acetate (about 44.31 g per 100 ml at 20 °C).

Once the degraded perovskites had dissolved in the aqueous solution, the researchers set about recovering pure and high-quality perovskite crystals from the solution. They did this by providing extra iodide ions to coordinate with lead. This resulted in [PbI]⁺ transitioning to [PbI₂]⁰ and eventually to [PbI₃]⁻ and the formation of the perovskite framework.

To remove the indium tin oxide substrates, the researchers sonicated these layers in a solution of water/ethanol (50%/50% volume ratio) for 15 min. Finally, they delaminated the cover glasses by placing the degraded solar cells on a hotplate preheated to 150 °C for 3 min.

They were able to apply their technology to recycle both MAPbI₃ and FAPbI₃ perovskites.

New devices made from the recycled perovskites had an average power conversion efficiency of 21.9 ± 1.1%, with the best samples clocking in at 23.4%. This represents an efficiency recovery of more than 99% compared with those prepared using fresh materials (which have a PCE of 22.1 ± 0.9%).

Looking forward, Gao and colleagues say they would now like to demonstrate that their technique works on a larger scale. “Our life-cycle assessment and techno-economic analysis has already confirmed that our strategy not only preserves raw materials, but also appreciably lowers overall manufacturing costs of solar cells made from perovskites,” says co-team leader Fengqi You, who works at Cornell University. “In particular, reclaiming the valuable layers in these devices drives down expenses and helps reduce the ‘levelized cost’ of electricity they produce, making the technology potentially more competitive and sustainable at scale,” he tells Physics World.

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When a mantis shrimp uses shock waves to strike and kill its prey, how does it prevent those shock waves from damaging its own tissues? Researchers at Northwestern University in the US have answered this question by identifying a structure within the shrimp that filters out harmful frequencies. Their findings, which they obtained by using ultrasonic techniques to investigate surface and bulk wave propagation in the shrimp’s dactyl club, could lead to novel advanced protective materials for military and civilian applications.

Dactyl clubs are hammer-like structures located on each side of a mantis shrimp’s body. They store energy in elastic structures similar to springs that are latched in place by tendons. When the shrimp contracts its muscles, the latch releases, releasing the stored energy and propelling the club forward with a peak force of up to 1500 N.

This huge force (relative to the animal’s size) creates stress waves in both the shrimp’s target – typically a hard-shelled animal such as a crab or mollusc – and the dactyl club itself, explains biomechanical engineer Horacio Dante Espinosa, who led the Northwestern research effort. The club’s punch also creates bubbles that rapidly collapse to produce shockwaves in the megahertz range. “The collapse of these bubbles (a process known as cavitation collapse), which takes place in just nanoseconds, releases intense bursts of energy that travel through the target and shrimp’s club,” he explains. “This secondary shockwave effect makes the shrimp’s strike even more devastating.”

Protective phononic armour

So how do the shrimp’s own soft tissues escape damage? To answer this question, Espinosa and colleagues studied the animal’s armour using transient grating spectroscopy (TGS) and asynchronous optical sampling (ASOPS). These ultrasonic techniques respectively analyse how stress waves propagate through a material and characterize the material’s microstructure. In this work, Espinosa and colleagues used them to provide high-resolution, frequency-dependent wave propagation characteristics that previous studies had not investigated experimentally.

The team identified three distinct regions in the shrimp’s dactyl club. The outermost layer consists of a hard hydroxyapatite coating approximately 70 μm thick, which is durable and resists damage. Beneath this, an approximately 500 μm-thick layer of mineralized chitin fibres arranged in a herringbone pattern enhances the club’s fracture resistance. Deeper still, Espinosa explains, is a region that features twisted fibre bundles organized in a corkscrew-like arrangement known as a Bouligand structure. Within this structure, each successive layer is rotated relative to its neighbours, giving it a unique and crucial role in controlling how stress waves propagate through the shrimp.

“Our key finding was the existence of phononic bandgaps (through which waves within a specific frequency range cannot travel) in the Bouligand structure,” Espinosa explains. “These bandgaps filter out harmful stress waves so that they do not propagate back into the shrimp’s club and body. They thus preserve the club’s integrity and protect soft tissue in the animal’s appendage.”

 The team also employed finite element simulations incorporating so-called Bloch-Floquet analyses and graded mechanical properties to understand the phonon bandgap effects. The most surprising result, Espinosa tells Physics World, was the formation of a flat branch around the 450 to 480 MHz range, which correlates to frequencies arising from bubble collapse originating during club impact.

Evolution and its applications

For Espinosa and his colleagues, a key goal of their research is to understand how evolution leads to natural composite materials with unique photonic, mechanical and thermal properties. In particular, they seek to uncover how hierarchical structures in natural materials and the chemistry of their constituents produce emergent mechanical properties. “The mantis shrimp’s dactyl club is an example of how evolution leads to materials capable of resisting extreme conditions,” Espinosa says. “In this case, it is the violent impacts the animal uses for predation or protection.”

The properties of the natural “phononic shield” unearthed in this work might inspire advanced protective materials for both military and civilian applications, he says. Examples could include the design of helmets, personnel armour, and packaging for electronics and other sensitive devices.

In this study, which is described in Science, the researchers analysed two-dimensional simulations of wave behaviour. Future research, they say, should focus on more complex three-dimensional simulations to fully capture how the club’s structure interacts with shock waves. “Designing aquatic experiments with state-of-the-art instrumentation would also allow us to investigate how phononic properties function in submerged underwater conditions,” says Espinosa.

The team would also like to use biomimetics to make synthetic metamaterials based on the insights gleaned from this work.

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New statistical analyses of the supermassive black hole M87* may explain changes observed since it was first imaged. The findings, from the same Event Horizon Telescope (EHT) that produced the iconic first image of a black hole’s shadow, confirm that M87*’s rotational axis points away from Earth. The analyses also indicate that turbulence within the rotating envelope of gas that surrounds the black hole – the accretion disc – plays a role in changing its appearance.

The first image of M87*’s shadow was based on observations made in 2017, though the image itself was not released until 2019. It resembles a fiery doughnut, with the shadow appearing as a dark region around three times the diameter of the black hole’s event horizon (the point beyond which even light cannot escape its gravitational pull) and the accretion disc forming a bright ring around it.

Because the shadow is caused by the gravitational bending and capture of light at the event horizon, its size and shape can be used to infer the black hole’s mass. The larger the shadow, the higher the mass. In 2019, the EHT team calculated that M87* has a mass of about 6.5 billion times that of our Sun, in line with previous theoretical predictions. Team members also determined that the radius of the event horizon is 3.8 micro-arcseconds; that the black hole is rotating in a clockwise direction; and that its spin points away from us.

Hot and violent region

The latest analysis focuses less on the shadow and more on the bright ring outside it. As matter accelerates, it produces huge amounts of light. In the vicinity of the black hole, this acceleration occurs as matter is sucked into the black hole, but it also arises when matter is blasted out in jets. The way these jets form is still not fully understood, but some astrophysicists think magnetic fields could be responsible. Indeed, in 2021, when researchers working on the EHT analysed the polarization of light emitted from the bright region, they concluded that only the presence of a strongly magnetized gas could explain their observations.

The team has now combined an analysis of ETH observations made in 2018 with a re-analysis of the 2017 results using a Bayesian approach. This statistical technique, applied for the first time in this context, treats the two sets of observations as independent experiments. This is possible because the event horizon of M87* is about a light-day across, so the accretion disc should present a new version of itself every few days, explains team member Avery Broderick from the Perimeter Institute and the University of Waterloo, both in Canada. In more technical language, the gap between observations exceeds the correlation timescale of the turbulent environment surrounding the black hole.

New result reinforces previous interpretations

The part of the ring that appears brightest to us stems from the relativistic movement of material in a clockwise direction as seen from Earth. In the original 2017 observations, this bright region was further “south” on the image than the EHT team expected. However, when members of the team compared these observations with those from 2018, they found that the region reverted to its mean position. This result corroborated computer simulations of the general relativistic magnetohydrodynamics of the turbulent environment surrounding the black hole.

Even in the 2018 observations, though, the ring remains brightest at the bottom of the image. According to team member Bidisha Bandyopadhyay, a postdoctoral researcher at the Universidad de Concepción in Chile, this finding provides substantial information about the black hole’s spin and reinforces the EHT team’s previous interpretation of its orientation: the black hole’s rotational axis is pointing away from Earth. The analyses also reveal that the turbulence within the accretion disc can help explain the differences observed in the bright region from one year to the next.

Very long baseline interferometry

To observe M87* in detail, the EHT team needed an instrument with an angular resolution comparable to the black hole’s event horizon, which is around tens of micro-arcseconds across. Achieving this resolution with an ordinary telescope would require a dish the size of the Earth, which is clearly not possible. Instead, the EHT uses very long baseline interferometry, which involves detecting radio signals from an astronomical source using a network of individual radio telescopes and telescopic arrays spread across the globe.

The facilities contributing to this work were the Atacama Large Millimeter Array (ALMA) and the Atacama Pathfinder Experiment, both in Chile; the South Pole Telescope (SPT) in Antarctica; the IRAM 30-metre telescope and NOEMA Observatory in Spain; the James Clerk Maxwell Telescope (JCMT) and the Submillimeter Array (SMA) on Mauna Kea, Hawai’I, US; the Large Millimeter Telescope (LMT) in Mexico; the Kitt Peak Telescope in Arizona, US; and the Greenland Telescope (GLT). The distance between these telescopes – the baseline – ranges from 160 m to 10 700 km. Data were correlated at the Max-Planck-Institut für Radioastronomie (MPIfR) in Germany and the MIT Haystack Observatory in the US.

“This work demonstrates the power of multi-epoch analysis at horizon scale, providing a new statistical approach to studying the dynamical behaviour of black hole systems,” says EHT team member Hung-Yi Pu from National Taiwan Normal University. “The methodology we employed opens the door to deeper investigations of black hole accretion and variability, offering a more systematic way to characterize their physical properties over time.”

Looking ahead, the ETH astronomers plan to continue analysing observations made in 2021 and 2022. With these results, they aim to place even tighter constraints on models of black hole accretion environments. “Extending multi-epoch analysis to the polarization properties of M87* will also provide deeper insights into the astrophysics of strong gravity and magnetized plasma near the event horizon,” EHT Management team member Rocco Lico, tells Physics World.

The analyses are detailed in Astronomy and Astrophysics.

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Something extraordinary happened on Earth around 10 million years ago, and whatever it was, it left behind a “signature” of radioactive beryllium-10. This finding, which is based on studies of rocks located deep beneath the ocean, could be evidence for a previously-unknown cosmic event or major changes in ocean circulation. With further study, the newly-discovered beryllium anomaly could also become an independent time marker for the geological record.

Most of the beryllium-10 found on Earth originates in the upper atmosphere, where it forms when cosmic rays interact with oxygen and nitrogen molecules. Afterwards, it attaches to aerosols, falls to the ground and is transported into the oceans. Eventually, it reaches the seabed and accumulates, becoming part of what scientists call one of the most pristine geological archives on Earth.

Because beryllium-10 has a half-life of 1.4 million years, it is possible to use its abundance to pin down the dates of geological samples that are more than 10 million years old. This is far beyond the limits of radiocarbon dating, which relies on an isotope (carbon-14) with a half-life of just 5730 years, and can only date samples less than 50 000 years old.

Almost twice as much ¹⁰Be than expected

In the new work, which is detailed in Nature Communications, physicists in Germany and Australia measured the amount of beryllium-10 in geological samples taken from the Pacific Ocean. The samples are primarily made up of iron and manganese and formed slowly over millions of years. To date them, the team used a technique called accelerator mass spectrometry (AMS) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). This method can distinguish beryllium-10 from its decay product, boron-10, which has the same mass, and from other beryllium isotopes.

The researchers found that samples dated to around 10 million years ago, a period known as the late Miocene, contained almost twice as much beryllium-10 as they expected to see. The source of this overabundance is a mystery, says team member Dominik Koll, but he offers three possible explanations. The first is that changes to the ocean circulation near the Antarctic, which scientists recently identified as occurring between 10 and 12 million years ago, could have distributed beryllium-10 unevenly across the Earth. “Beryllium-10 might thus have become particularly concentrated in the Pacific Ocean,” says Koll, a postdoctoral researcher at TU Dresden and an honorary lecturer at the Australian National University.

Another possibility is that a supernova exploded in our galactic neighbourhood 10 million years ago, producing a temporary increase in cosmic radiation. The third option is that the Sun’s magnetic shield, which deflects cosmic rays away from the Earth, became weaker through a collision with an interstellar cloud, making our planet more vulnerable to cosmic rays. Both scenarios would have increased the amount of beryllium-10 that fell to Earth without affecting its geographic distribution.

To distinguish between these competing hypotheses, the researchers now plan to analyse additional samples from different locations on Earth. “If the anomaly were found everywhere, then the astrophysics hypothesis would be supported,” Koll says. “But if it were detected only in specific regions, the explanation involving altered ocean currents would be more plausible.”

Whatever the reason for the anomaly, Koll suggests it could serve as a cosmogenic time marker for periods spanning millions of years, the likes of which do not yet exist. “We hope that other research groups will also investigate their deep-ocean samples in the relevant period to eventually come to a definitive answer on the origin of the anomaly,” he tells Physics World.

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Quantum-inspired “tensor networks” can simulate the behaviour of turbulent fluids in just a few hours rather than the several days required for a classical algorithm. The new technique, developed by physicists in the UK, Germany and the US, could advance our understanding of turbulence, which has been called one of the greatest unsolved problems of classical physics.

Turbulence is all around us, found in weather patterns, water flowing from a tap or a river and in many astrophysical phenomena. It is also important for many industrial processes. However, the way in which turbulence arises and then sustains itself is still not understood, despite the seemingly simple and deterministic physical laws governing it.

The reason for this is that turbulence is characterized by large numbers of eddies and swirls of differing shapes and sizes that interact in chaotic and unpredictable ways across a wide range of spatial and temporal scales. Such fluctuations are difficult to simulate accurately, even using powerful supercomputers, because doing so requires solving sets of coupled partial differential equations on very fine grids.

An alternative is to treat turbulence in a probabilistic way. In this case, the properties of the flow are defined as random variables that are distributed according to mathematical relationships called joint Fokker-Planck probability density functions. These functions are neither chaotic nor multiscale, so they are straightforward to derive. However, they are nevertheless challenging to solve because of the high number of dimensions contained in turbulent flows.

For this reason, the probability density function approach was widely considered to be computationally infeasible. In response, researchers turned to indirect Monte Carlo algorithms to perform probabilistic turbulence simulations. However, while this approach has chalked up some notable successes, it can be slow to yield results.

Highly compressed “tensor networks”

To overcome this problem, a team led by Nikita Gourianov of the University of Oxford, UK, decided to encode turbulence probability density functions as highly compressed “tensor networks” rather than simulating the fluctuations themselves. Such networks have already been used to simulate otherwise intractable quantum systems like superconductors, ferromagnets and quantum computers, they say.

These quantum-inspired tensor networks represent the turbulence probability distributions in a hyper-compressed format, which then allows them to be simulated. By simulating the probability distributions directly, the researchers can then extract important parameters, such as lift and drag, that describe turbulent flow.

Importantly, the new technique allows an ordinary single CPU (central processing unit) core to compute a turbulent flow in just a few hours, compared to several days using a classical algorithm on a supercomputer.

This significantly improved way of simulating turbulence could be particularly useful in the area of chemically reactive flows in areas such as combustion, says Gourianov. “Our work also opens up the possibility of probabilistic simulations for all kinds of chaotic systems, including weather or perhaps even the stock markets,” he adds.

The researchers now plan to apply tensor networks to deep learning, a form of machine learning that uses artificial neural networks. “Neural networks are famously over-parameterized and there are several publications showing that they can be compressed by orders of magnitude in size simply by representing their layers as tensor networks,” Gourianov tells Physics World.

The study is detailed in Science Advances.

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