When implanted medical devices like urinary stents and catheters get clogged with biofilms, the usual solution is to take them out and replace them with new ones. Now, however, researchers at the University of Bern and ETH Zurich, Switzerland have developed an alternative. By incorporating ultrasound-activated moving structures into their prototype “stent-on-a-chip” device, they showed it is possible to remove biofilms without removing the device itself. If translated into clinical practice, the technology could increase the safe lifespan of implants, saving money and avoiding operations that are uncomfortable and sometimes hazardous for patients.

Biofilms are communities of bacterial cells that adhere to natural surfaces in the body as well as artificial structures such as catheters, stents and other implants. Because they are encapsulated by a protective, self-produced extracellular matrix made from polymeric substances, they are mechanically robust and resistant to standard antibacterial measures. If not removed, they can cause infections, obstructions and other complications.

Intense, steady flows push away impurities

The new technology, which was co-developed by Cornel Dillinger, Pedro Amado and other members of Francesco Clavica and Daniel Ahmed’s research teams, takes advantage of recent advances in the fields of robotics and microfluidics. Its main feature is a coating made from microscopic hair-like structures known as cilia. Under the influence of an acoustic field, which is applied externally via a piezoelectric transducer, these cilia begin to move. This movement produces intense, steady fluid flows with velocities of up to 10 mm/s – enough to break apart encrusted deposits (made from calcium carbonate, for example) and flush away biofilms from the inner and outer surfaces of implanted urological devices.

“This is a major advance compared to existing stents and catheters, which require regular replacements to avoid obstruction and infections,” Clavica says.

The technology is also an improvement on previous efforts to clear implants by mechanical means, Ahmed adds. “Our polymeric cilia in fact amplify the effects of ultrasound by allowing for an effect known as acoustic streaming at frequencies of 20 to 100 kHz,” he explains. “This frequency is lower than that possible with previous microresonator devices developed to work in a similar way that had to operate in the MHz-frequency range.”

The lower frequency achieves the desired therapeutic effects while prioritizing patient safety and minimizing the risk of tissue damage, he adds.

Wider applications

In creating their technology, the researchers were inspired by biological cilia, which are a natural feature of physiological systems such as the reproductive and respiratory tracts and the central nervous system. Future versions, they say, could apply the ultrasound probe directly to a patient’s skin, much as handheld probes of ultrasound scanners are currently used for imaging. “This technology has potential applications beyond urology, including fields like visceral surgery and veterinary medicine, where keeping implanted medical devices clean is also essential,” Clavica says.

The researchers now plan to test new coatings that would reduce contact reactions (such as inflammation) in the body. They will also explore ways of improving the device’s responsiveness to ultrasound – for example by depositing thin metal layers. “These modifications could not only improve acoustic streaming performance but could also provide additional antibacterial benefits,” Clavica tells Physics World.

In the longer term, the team hope to translate their technology into clinical applications. Initial tests that used a custom-built ultrasonic probe coupled to artificial tissue have already demonstrated promising results in generating cilia-induced acoustic streaming, Clavica notes. “In vivo animal studies will then be critical to validate safety and efficacy prior to clinical adoption,” he says.

The present study is detailed in PNAS.

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Physicists at CERN have completed a “test run” for taking antimatter out of the laboratory and transporting it across the site of the European particle-physics facility. Although the test was carried out with ordinary protons, the team that performed it says that antiprotons could soon get the same treatment. The goal, they add, is to study antimatter in places other than the labs that create it, as this would enable more precise measurements of the differences between matter and antimatter. It could even help solve one of the biggest mysteries in physics: why does our universe appear to be made up almost entirely of matter, with only tiny amounts of antimatter?

According to the Standard Model of particle physics, each of the matter particles we see around us – from baryons like protons to leptons such as electrons – should have a corresponding antiparticle that is identical in every way apart from its charge and magnetic properties (which are reversed). This might sound straightforward, but it leads to a peculiar prediction. Under the Standard Model, the Big Bang that formed our universe nearly 14 billion years ago should have generated equal amounts of antimatter and matter. But if that were the case, there shouldn’t be any matter left, because whenever pairs of antimatter and matter particles collide, they annihilate each other in a burst of energy.

Physicists therefore suspect that there are other, more subtle differences between matter particles and their antimatter counterparts – differences that could explain why the former prevailed while the latter all but disappeared. By searching for these differences, they hope to shed more light on antimatter-matter asymmetry – and perhaps even reveal physics beyond the Standard Model.

Extremely precise measurements

At CERN’s Baryon-Antibaryon Symmetry Experiment (BASE) experiment, the search for matter-antimatter differences focuses on measuring the magnetic moment (or charge-to-mass ratio) of protons and antiprotons. These measurements need to be extremely precise, but this is difficult at CERN’s “Antimatter Factory” (AMF), which manufactures the necessary low-energy antiprotons in profusion. This is because essential nearby equipment – including the Antiproton Decelerator and ELENA, which reduce the energy of incoming antiprotons from GeV to MeV – produces magnetic field fluctuations that blur the signal.

To carry out more precise measurements, the team therefore needs a way of transporting the antiprotons to other, better-shielded, laboratories. This is easier said than done, because antimatter needs to be carefully isolated from its environment to prevent it from annihilating with the walls of its container or with ambient gas molecules.

The BASE team’s solution was to develop a device that can transport trapped antiprotons on a truck for substantial distances. It is this device, known as BASE-STEP (for Symmetry Tests in Experiments with Portable Antiprotons), that has now been field-tested for the first time.

Protons on the go

During the test, the team successfully transported a cloud of about 105 trapped protons out of the AMF and across CERN’s Meyrin campus over a period of four hours. Although protons are not the same as antiprotons, BASE-STEP team leader Christian Smorra says they are just as sensitive to disturbances in their environment caused by, say, driving them around. “They are therefore ideal stand-ins for initial tests, because if we can transport protons, we should also be able to transport antiprotons,” he says.

The BASE-STEP device is mounted on an aluminium frame and measures 1.95 m x 0.85 m x 1.65 m. At 850‒900 kg, it is light enough to be transported using standard forklifts and cranes.

Like BASE, it traps particles in a Penning trap composed of gold-plated cylindrical electrode stacks made from oxygen-free copper. To further confine the protons and prevent them from colliding with the trap’s walls, this trap is surrounded by a superconducting magnet bore operated at cryogenic temperatures. The second electrode stack is also kept at ultralow pressures of 10^-19 bar, which Smorra says is low enough to keep antiparticles from annihilating with residual gas molecules. To transport antiprotons instead of protons, Smorra adds, they would just need to switch the polarity of the electrodes.

The transportable trap system, which is detailed in Nature, is designed to remain operational on the road. It uses a carbon-steel vacuum chamber to shield the particles from stray magnetic fields, and its frame can handle accelerations of up to 1g (9.81 m/s²) in all directions over and above the usual (vertical) force of gravity. This means it can travel up and down slopes with a gradient of up to 10%, or approximately 6°.

Once the BASE-STEP device is re-configured to transport antiprotons, the first destination on the team’s list is a new Penning-trap system currently being constructed at the Heinrich Heine University in Düsseldorf, Germany. Here, physicists hope to search for charge-parity-time (CPT) violations in protons and antiprotons with a precision at least 100 times higher than is possible at CERN’s AMF.

“At BASE, we are currently performing measurements with a precision of 16 parts in a trillion,” explains BASE spokesperson Stefan Ulmer, an experimental physicist at Heinrich Heine and a researcher at CERN and Japan’s RIKEN laboratory. “These experiments are the most precise tests of matter/antimatter symmetry in the baryon sector to date, but to make these experiments better, we have no choice but to transport the particles out of CERN’s antimatter factory,” he tells Physics World.

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Physicists have set a new upper bound on the interaction strength of dark matter by simulating the collision of two clouds of interstellar plasma. The result, from researchers at Ruhr University Bochum in Germany, CINECA in Italy and the Instituto Superior Tecnico in Portugal, could force a rethink on theories describing this mysterious substance, which is thought to make up more than 85% of the mass in the universe.

Since dark matter has only ever been observed through its effect on gravity, we know very little about what it’s made of. Indeed, various theories predict that dark matter particles could have masses ranging from around 10⁻²² eV to around 10¹⁹ GeV — a staggering 50 orders of magnitude.

Another major unknown about dark matter is whether it interacts via forces other than gravity, either with itself or with other particles. Some physicists have hypothesized that dark matter particles might possess positive and negative “dark charges” that interact with each other via “dark electromagnetic forces”. According to this supposition, dark matter could behave like a cold plasma of self-interacting particles.

Bullet Cluster experiment

In the new study, the team searched for evidence of dark interactions in a cluster of galaxies located several billion light years from Earth. This galactic grouping is known as the Bullet Cluster, and it contains a subcluster that is moving away from the main body after passing through it at high speed.

Since the most basic model of dark-matter interactions relies on the same equations as ordinary electromagnetism, the researchers chose to simulate these interactions in the Bullet Cluster system using the same computational tools they would use to describe electromagnetic interactions in a standard plasma. They then compared their results with real observations of the Bullet Cluster galaxy.

The new work builds on a previous study in which members of the same team simulated the collision of two clouds of standard plasma passing through one another. This study found that as the clouds merged, electromagnetic instabilities developed. These instabilities had the effect of redistributing energy from the opposing flows of the clouds, slowing them down while also broadening the temperature range within them.

Ruling out many of the simplest dark matter theories

The latest study showed that, as expected, the plasma components of the subcluster and main body slowed down thanks to ordinary electromagnetic interactions. That, however, appeared to be all that happened, as the data contained no sign of additional dark interactions. While the team’s finding doesn’t rule out dark electromagnetic interactions entirely, team member Kevin Schoeffler explains that it does mean that these interactions, which are characterized by a parameter known as 𝛼𝐷, must be far weaker than their ordinary-matter counterpart. “We can thus calculate an upper limit for the strength of this interaction,” he says.

This limit, which the team calculated as 𝛼𝐷 < 4 x 10^-25 for a dark matter particle with a mass of 1 TeV, rules out many of the simplest dark matter theories and will require them to be rethought, Schoeffler says. “The calculations were made possible thanks to detailed discussions with scientists working outside of our speciality of physics, namely plasma physicists,” he tells Physics World. “Throughout this work, we had to overcome the challenge of connecting with very different fields and interacting with communities that speak an entirely different language to ours.”

As for future work, the physicists plan to compare the results of their simulations with other astronomical observations, with the aim of constraining the upper limit of the dark electromagnetic interaction even further. More advanced calculations, such as those that include finer details of the cloud models, would also help refine the limit. “These more realistic setups would include other plasma-like electromagnetic scenarios and ‘slowdown’ mechanisms, leading to potentially stronger limits,” Schoeffler says.

The present study is detailed in Physical Review D.

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Researchers in Germany report that they have directly measured a superconducting gap in a hydride sulphide material for the first time. The new finding represents “smoking gun” evidence for superconductivity in these materials, while also confirming that the electron pairing that causes it is mediated by phonons.

Superconductors are materials that conduct electricity without resistance. Many materials behave this way when cooled below a certain transition temperature T_c, but in most cases this temperature is very low. For example, solid mercury, the first superconductor to be discovered, has a T_c of 4.2 K. Superconductors that operate at higher temperatures – perhaps even at room temperature – are thus highly desirable, as an ambient-temperature superconductor would dramatically increase the efficiency of electrical generators and transmission lines.

The rise of the superhydrides

The 1980s and 1990s saw considerable progress towards this goal thanks to the discovery of high-temperature copper oxide superconductors, which have T_cs between 30–133 K. Then, in 2015, the maximum known critical temperature rose even higher thanks to the discovery that a sulphide material, H₃S, has a T_c of 203 K when compressed to pressures of 150 GPa.

This result sparked a flurry of interest in solid materials containing hydrogen atoms bonded to other elements. In 2019, the record was broken again, this time by lanthanum decahydride (LaH₁₀), which was found to have a T_c of 250–260 K, again at very high pressures.

A further advance occurred in 2021 with the discovery of high-temperature superconductivity in cerium hydrides. These novel phases of CeH₉ and another newly-synthesized material, CeH₁₀, are remarkable in that they are stable and display high-temperature superconductivity at lower pressures (about 80 GPa, or 0.8 million atmospheres) than the other so-called “superhydrides”.

But how does it work?

One question left unanswered amid these advances concerned the mechanism for superhydride superconductivity. According to the Bardeen–Cooper–Schrieffer (BCS) theory of “conventional” superconductivity, superconductivity occurs when electrons overcome their mutual electrical repulsion to form pairs. These electron pairs, which are known as Cooper pairs, can then travel unhindered through the material as a supercurrent without scattering off phonons (quasiparticles arising from vibrations of the material’s crystal lattice) or other impurities.

Cooper pairing is characterized by a tell-tale energy gap near what’s known as the Fermi level, which is the highest energy level that electrons can occupy in a solid at a temperature of absolute zero. This gap is equivalent to the maximum energy required to break up a Cooper pair of electrons, and spotting it is regarded as unambiguous proof of that material’s superconducting nature.

For the superhydrides, however, this is easier said than done, because measuring such a gap requires instruments that can withstand the extremely high pressures required for superhydrides to exist and behave as superconductors. Traditional techniques such as scanning tunnelling spectroscopy or angle-resolved photoemission spectroscopy do not work, and there was little consensus on what might take their place.

Planar electron tunnelling spectroscopy

A team led by researchers at Germany’s Max Planck Institute for Chemistry has now stepped in by developing a form of spectroscopy that can operate under extreme pressures. The technique, known as planar electron tunnelling spectroscopy, required the researchers to synthesize highly pure planar tunnel junctions of H₃S and its deuterated equivalent D₃S under pressures of over 100 GPa. Using a technique called laser heating, they created junctions with three parts: a metal, tantalum; a barrier made of tantalum pentoxide, Ta₂O₅; and the H₃S or D₃S superconductors. By measuring the differential conductance across the junctions, they determined the density of electron states in H₃S and D₃S near the Fermi level.

These tunnelling spectra revealed that both H₃S and D₃S have fully open superconducting gaps of 60 meV and 44 meV respectively. According to team member Feng Du, the smaller gap in D₃S confirms that the superconductivity in H₃S comes about thanks to interactions between electrons and phonons – a finding that backs up long-standing predictions.

The researchers hope their work, which they report on in Nature, will inspire more detailed studies of superhydrides. They now plan to measure the superconducting gap of other metal superhydrides and compare them with the covalent superhydrides they studied in this work. “The results from such experiments could help us understand the origin of the high T_c in these superconductors,” Du tells Physics World.

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Physicists have succeeded in making neutrons travel in a curved parabolic waveform known as an Airy beam. This behaviour, which had previously been observed in photons and electrons but never in a non-elementary particle, could be exploited in fundamental quantum science research and in advanced imaging techniques for materials characterization and development.

In free space, beams of light propagate in straight lines. When they pass through an aperture, they diffract, becoming wider and less intense. Airy beams, however, are different. Named after the 19^th-century British scientist George Biddell Airy, who developed the mathematics behind them while studying rainbows, they follow a parabola-shaped path – a property known as self-acceleration – and do not spread out as they travel. Airy beams are also “self-healing”, meaning that they reconstruct themselves after passing through an obstacle that blocked part of the beam.

Scientists have been especially interested in Airy beams since 1979, when theoretical work by the physicist Michael Berry suggested several possible applications for them, says Dmitry Pushin, a physicist at the Institute for Quantum Computing (IQC) and the University of Waterloo, Canada. Researchers created the first Airy beams from light in 2007, followed by an electron Airy beam in 2013.

“Inspired by the unusual properties of these beams in optics and electron experiments, we wondered whether similar effects could be harnessed for neutrons,” Pushin says.

Making such beams out of neutrons turned out to be challenging, however. Because neutrons have no charge, they cannot be shaped by electric fields. Also, lenses that focus neutron beams do not exist.

A holographic approach

A team led by Pushin and Dusan Sarenac of the University at Buffalo’s Department of Physics in the US has now overcome these difficulties using a holographic approach based on a custom-microfabricated silicon diffraction grating. The team made this grating from an array of 6 250 000 micron-sized cubic phase patterns etched onto a silicon slab. “The grating modulates incoming neutrons into an Airy form and the resulting beam follows a curved trajectory, exhibiting the characteristics of a two-dimensional Airy profile at a neutron detector,” Sarenac explains.

According to Pushin, it took years of work to figure out the correct dimensions for the array. Once the design was optimized, however, fabricating it took just 48 hours at the IQC’s nanofabrication facility. “Developing a precise wave phase modulation method using holography and silicon microfabrication allowed us to overcome the difficulties in manipulating neutrons,” he says.

The researchers say the self-acceleration and self-healing properties of Airy beams could improve existing neutron imaging techniques (including neutron scattering and diffraction), potentially delivering sharper and more detailed images. The new beams might even allow for new types of neutron optics and could be particularly useful, for example, when targeting specific regions of a sample or navigating around structures.

Creating the neutron Airy beams required access to international neutron science facilities such as the US National Institute of Standards and Technology’s Center for Neutron Research; the US Department of Energy’s Oak Ridge National Laboratory; and the Paul Scherrer Institute in Villigen, Switzerland. To continue their studies, the researchers plan to use the UK’s ISIS Neutron and Muon Source to explore ways of combining neutron Airy beams with other structured neutron beams (such as helical waves of neutrons or neutron vortices). This could make it possible to investigate complex properties such as the chirality, or handedness, of materials. Such work could be useful in drug development and materials science. Since a material’s chirality affects how its electrons spin, it could be important for spintronics and quantum computing, too.

“We also aim to further optimize beam shaping for specific applications,” Sarenac tells Physics World. “Ultimately, we hope to establish a toolkit for advanced neutron optics that can be tailored for a wide range of scientific and industrial uses.”

The present work is detailed in Physical Review Letters.

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The first clear images of Yellowstone’s shallowest magma reservoir have revealed its depth with unprecedented precision, providing information that could help scientists determine how dangerous it is. By pinpointing the reservoir’s location, geophysicists and seismologists from Rice University and the universities of Utah, New Mexico and Texas at Dallas, hope to develop more accurate predictions of when this so-called “supervolcano” will erupt again.

Yellowstone is America’s oldest national park, and it owes its spectacular geysers and hot springs to its location above one of the world’s largest volcanoes. The last major eruption of the Yellowstone supervolcano happened around 630 000 years ago, and was violent enough to create a collapsed crater, or caldera, over 60 km across. Though it shows no sign of repeating this cataclysm anytime soon, it is still an active volcano, and it is slowly forming a new magma reservoir.

Previous estimates of the depth of this magma reservoir were highly imprecise, ranging from three to eight kilometres. Scientists also lacked an accurate location for the reservoir’s top and were unsure how its properties changed with increasing depth.

The latest results, from a team led by Brandon Schmandt and Chenglong Duan at Rice and Jamie Farrell at Utah, show that the reservoir’s top lies 3.8 km below the surface. They also show evidence of an abrupt downward transition into a mixture of gas bubbles and magma filling the pore space of volcanic rock. The gas bubbles are made of mostly H₂O in supercritical form and the magma comprises molten silicic rock such as rhyolite.

Creating artificial seismic waves

Duan and colleagues obtained their result by using a mechanical vibration source (a specialized truck built by the oil and gas firm Dawson Geophysical) to create artificial seismic waves across the ground beneath the northeast portion of Yellowstone’s caldera. They also deployed a network of hundreds of portable seismometers capable of recording both vertical and ground vibrations, spaced at 100 to 150-m intervals, across the national park. “Researchers already knew from previous seismic and geochemical studies that this region was underlain by magma, but we needed new field data and an innovative adaptation of conventional seismic imaging techniques,” explains Schmandt. The new study, he tells Physics World, is “a good example of how the same technologies are relevant to energy industry imaging and studies of natural hazards”.

Over a period of a few days, the researchers created artificial earthquakes at 110 different locations using 20 shocks lasting 40 seconds apiece. This enabled them to generate two types of seismic wave, known as S- and P-waves, which reflect off molten rock at different velocities. Using this information, they were able to locate the top of the magma chamber and determine that 86% of this upper portion was solid rock.

The rest, they discovered, was made up of pores filled with molten material such as rhyolite and volatile gases (mostly water in supercritical form) and liquids in roughly equal proportion. Importantly, they say, this moderate concentration of pores allows the volatile bubbles to gradually escape to the surface so they do not accumulate and increase the buoyancy deeper inside the chamber. This is good news as it means that the Yellowstone supervolcano is unlikely to erupt any time soon.

A key aspect of this analysis was a wave-equation imaging method that Duan developed, which substantially improved the spatial resolution of the features observed. “This was important since we had to adapt the data we obtained to its less than theoretically ideal properties,” Schmandt explains.

The work, which is detailed in Nature, could also help scientists monitor the eruption potential of other volcanos, Schmandt adds. This is because estimating the accumulation and buoyancy of volatile material beneath sharp magmatic cap layers is key to assessing the stability of the system. “There are many types of similar hazardous magmatic systems and their older remnants on our planet that are important for resources like metal ores and critical minerals,” he explains. “We therefore have plenty of targets left to understand and now some refined ideas about how we might approach them in the field and on the computer.”

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Physicists have observed axion quasiparticles for the first time in a two-dimensional quantum material. As well as having applications in materials science, the discovery could aid the search for fundamental axions, which are a promising (but so far hypothetical) candidate for the unseen dark matter pervading our universe.

Theorists first proposed axions in the 1970s as a way of solving a puzzle involving the strong nuclear force and charge-parity (CP) symmetry. In systems that obey this symmetry, the laws of physics are the same for a particle and the spatial mirror image of its oppositely charged antiparticle. Weak interactions are known to violate CP symmetry, and the theory of quantum chromodynamics (QCD) allows strong interactions to do so, too. However, no-one has ever seen evidence of this happening, and the so-called “strong CP problem” remains unresolved.

More recently, the axion has attracted attention as a potential constituent of dark matter – the mysterious substance that appears to make up more than 85% of matter in the universe. Axions are an attractive dark matter candidate because while they do have mass, and theory predicts that the Big Bang should have generated them in large numbers, they are much less massive than electrons, and they carry no charge. This combination means that axions interact only very weakly with matter and electromagnetic radiation – exactly the behaviour we expect to see from dark matter.

Despite many searches, though, axions have never been detected directly. Now, however, a team of physicists led by Jianxiang Qiu of Harvard University has proposed a new detection strategy based on quasiparticles that are axions’ condensed-matter analogue. According to Qiu and colleagues, these quasiparticle axions, as they are known, could serve as axion “simulators”, and might offer a route to detecting dark matter in quantum materials.

Topological antiferromagnet

To detect axion quasiparticles, the Harvard team constructed gated electronic devices made from several two-dimensional layers of manganese bismuth telluride (MnBi₂Te₄). This material is a rare example of a topological antiferromagnet – that is, a material that is insulating in its bulk while conducting electricity on its surface, and that has magnetic moments that point in opposite directions. These properties allow quasiparticles known as magnons (collective oscillations of spin magnetic moments) to appear in and travel through the MnBi₂Te₄. Two types of magnon mode are possible: one in which the spins oscillate in sync; and another in which they are out of phase.

Qiu and colleagues applied a static magnetic field across the plane of their MnBi₂Te₄ sheets and bombarded the devices with sub-picosecond light pulses from a laser. This technique, known as ultrafast pump-probe spectroscopy, allowed them to observe the 44 GHz coherent oscillation of the so-called condensed-matter field. This field is the CP-violating term in QCD, and it is proportional to a material’s magnetoelectric coupling constant. “This is uniquely enabled by the out-of-phase magnon in this topological material,” explains Qiu. “Such coherent oscillations are the smoking-gun evidence for the axion quasiparticle and it is the combination of topology and magnetism in MnBi₂Te₄ that gives rise to it.”

A laboratory for axion studies

Now that they have detected axion quasiparticles, Qiu and colleagues say their next step will be to do experiments that involve hybridizing them with particles such as photons. Such experiments would create a new type of “axion-polariton” that would couple to a magnetic field in a unique way – something that could be useful for applications in ultrafast antiferromagnetic spintronics, in which spin-polarized currents can be controlled with an electric field.

The axion quasiparticle could also be used to build an axion dark matter detector. According to the team’s estimates, the detection frequency for the quasiparticle is in the milli-electronvolt (meV) range. While several theories for the axion predict that it could have a mass in this range, most existing laboratory detectors and astrophysical observations search for masses outside this window.

“The main technical barrier to building such a detector would be grow high-quality large crystals of MnBi₂Te₄ to maximize sensitivity,” Qiu tells Physics World. “In contrast to other high-energy experiments, such a detector would not require expensive accelerators or giant magnets, but it will require extensive materials engineering.”

The research is described in Nature.

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Researchers at Linköping University in Sweden have developed a new fluid electrode and used it to make a soft, malleable battery that can recharge and discharge over 500 cycles while maintaining its high performance. The device, which continues to function even when stretched to twice its length, might be used in next-generation wearable electronics.

Futuristic wearables such as e-skin patches, e-textiles and even internal e-implants on the organs or nerves will need to conform far more closely to the contours of the human body than today’s devices can. To fulfil this requirement of being soft and stretchable as well as flexible, such devices will need to be made from mechanically pliant components powered by soft, supple batteries. Today’s batteries, however, are mostly rigid. They also tend to be bulky because long-term operations and power-hungry functions such as wireless data transfer, continuous sensing and complex processing demand plenty of stored energy.

To overcome these barriers, researchers led by the Linköping chemist Aiman Rahmanudin decided to rethink the very concept of battery electrode design. Instead of engineering softness and stretchability into a solid electrode, as was the case in most previous efforts, they made the electrode out of a fluid. “Bulky batteries compromise the mechanical compliance of wearable devices, but since fluids can be easily shaped into any configuration, this limitation is removed, opening up new design possibilities for next-generation wearables,” Rahmanudin says.

A “holistic approach”

Designing a stretchable battery requires a holistic approach, he adds, as all the device’s components need to be soft and stretchy. For example, they used a modified version of the wood-based biopolymer lignin as the cathode and a conjugated poly(1-amino-5-chloroanthraquinone) (PACA) as the anode. They made these electrodes fluid by dispersing them separately with conductive carbon fillers in an aqueous electrolyte medium consisting of 0.1 M HClO₄.

To integrate these electrodes into a complete cell, they had to design a stretchable current collector and an ion-selective membrane to prevent the cathodic and anodic fluids from crossing over. They also encapsulated the fluids in a robust, low-permeability elastomer to prevent them from drying up.

Designing energy storage devices from the “inside out”

Previous flexible, high-performance electrode work by the Linköping team focused on engineering the mechanical properties of solid battery electrodes by varying their Young’s modulus. “For example, think of a rubber composite that can be stretched and bent,” explains Rahmanudin. “The thicker the rubber, however, the higher the force required to stretch it, which affects mechanical compliancy.

“Learning from our past experience and work on electrofluids (which are conductive particles dispersed in a liquid medium employed as stretchable conductors), we figured that mixing redox particles with conductive particles and suspending them in an electrolyte could potentially work as battery electrodes. And we found that it did.”

Rahmanudin tells Physics World that fluid-based electrodes could lead to entirely new battery designs, including batteries that could be moulded into textiles, embedded in skin-worn patches or integrated into soft robotics.

After reporting their work in Science Advances, the researchers are now working on increasing the voltage output of their battery, which currently stands 0.9 V. “We are also looking into using Earth-abundant and sustainable materials like zinc and manganese oxide for future versions of our device and aim at replacing the acidic electrolyte we used with a safer pH neutral and biocompatible equivalent,” Rahmanudin says.

Another exciting direction, he adds, will be to exploit the fluid nature of such materials to build batteries with more complex three-dimensional shapes, such as spirals or lattices, that are tailored for specific applications. “Since the electrodes can be poured, moulded or reconfigured, we envisage a lot of creative potential here,” Rahmanudin says.

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Researchers in Singapore and the US have independently developed two new types of photonic computer chips that match existing purely electronic chips in terms of their raw performance. The chips, which can be integrated with conventional silicon electronics, could find use in energy-hungry technologies such as artificial intelligence (AI).

For nearly 60 years, the development of electronic computers proceeded according to two rules of thumb: Moore’s law (which states that the number of transistors in an integrated circuit doubles every two years) and Dennard scaling (which says that as the size of transistors decreases, their power density will stay constant). However, both rules have begun to fail, even as AI systems such as large language models, reinforcement learning and convolutional neural networks are becoming more complex. Consequently, electronic computers are struggling to keep up.

Light-based computation, which exploits photons instead of electrons, is a promising alternative because it can perform multiplication and accumulation (MAC) much more quickly and efficiently than electronic devices. These operations are crucial for AI, and especially for neural networks. However, while photonic systems such as photonic accelerators and processors have made considerable progress in performing linear algebra operations such as matrix multiplication, integrating them into conventional electronics hardware has proved difficult.

A hybrid photonic-electronic system

The Singapore device was made by researchers at the photonic computing firm Lightelligence and is called PACE, for Photonic Arithmetic Computing Engine. It is a hybrid photonic-electronic system made up of more than 16 000 photonic components integrated on a single silicon chip and performs matrix MAC on 64-entry binary vectors.

“The input vector data elements start in electronic form and are encoded as binary intensities of light (dark or light) and fed into a 64 x 64 array of optical weight modulators that then perform multiply and summing operations to accumulate the results,” explains Maurice Steinman, Lightelligence’s senior vice president and general manager for product strategy. “The result vectors are then converted back to the electronic domain where each element is compared to its corresponding programmable 8-bit threshold, producing new binary vectors that subsequently re-circulate optically through the system.”

The process repeats until the resultant vectors reach “convergence” with settled values, Steinman tells Physics World. Each recurrent step requires only a few nanoseconds and the entire process completes quickly.

The Lightelligence device, which the team describe in Nature, can solve complex computational problems known as max-cut/optimization problems that are important for applications in areas such as logistics. Notably, its greatly reduced minimum latency – a key measure of computation speed – means it can solve a type of problem known as an Ising model in just five nanoseconds. This makes it 500 times faster than today’s best graphical-processing-unit-based systems at this task.

High level of integration achieved

Independently, researchers led by Nicholas Harris at Lightmatter in Mountain View, California, have fabricated the first photonic processor capable of executing state-of-the-art neural network tasks such as classification, segmentation and running reinforcement learning algorithms. Lightmatter’s design consists of six chips in a single package with high-speed interconnects between vertically aligned photonic tensor cores (PTCs) and control dies. The team’s processor integrates four 128 x 128 PTCs, with each PTC occupying an area of 14 x 24.96 mm. It contains all the photonic components and analogue mixed-signal circuits required to operate and members of the team say that the current architecture could be scaled to 512 x 512 computing units in a single die.

The result is a device that can perform 65.5 trillion adaptive block floating-point 35 (ABFP) 16-bit operations per second with just 78 W of electrical power and 1.6 W of optical power. Writing in Nature, the researchers claim that this represents the highest level of integration achieved in photonic processing.

The team also showed that the Lightmatter processor can implement complex AI models such as the neural network ResNet (used for image processing) and the natural language processing model BERT (short for Bidirectional Encoder Representations from Transformers) – all with an accuracy rivalling that of standard electronic processors. It can also compute reinforcement learning algorithms such as DeepMind’s Atari. Harris and colleagues have already applied their device to several real-world AI applications, such as generating literary texts and classifying film reviews, and they say that their photonic processor marks an essential step in post-transistor computing.

Both teams fabricated their photonic and electronic chips using standard complementary metal-oxide-semiconductor (CMOS) processing techniques. This means that existing infrastructures could be exploited to scale up their manufacture. Another advantage: both systems were fully integrated in a standard chip interface – a first.

Given these results, Steinman says he expects to see innovations emerging from algorithm developers who seek to exploit the unique advantages of photonic computing, including low latency. “This could benefit the exploration of new computing models, system architectures and applications based on large-scale integrated photonics circuits.”

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A new all-electrical way of controlling spin-polarized currents has been developed by researchers at the Singapore University of Technology and Design (SUTD). By using bilayers of recently-discovered materials known as altermagnets, the researchers developed a tuneable and magnetic-free alternative to current approaches – something they say could bring spintronics closer to real-world applications.

Spintronics stores and processes information by exploiting the quantum spin (or intrinsic angular momentum) of electrons rather than their charge. The technology works by switching electronic spins, which can point either “up” or “down”, to perform binary logical operations in much the same way as electronic circuits use electric charge. One of the main advantages is that when an electron’s spin switches direction, its new state is stored permanently; it is said to be “non-volatile”. Spintronics circuits therefore do not require any additional input power to keep their states stable, which could make them more efficient and faster than the circuits in conventional electronic devices.

The problem is that the spin currents that carry information in spintronics circuits are usually generated using ferromagnetic materials and the magnetization of these materials can only be switched using very strong magnetic fields. Doing this requires bulky apparatus, which hinders the creation of ultracompact devices – a prerequisite for real-world applications.

“Notoriously difficult to achieve”

Controlling the spins with electric fields instead would be ideal, but Ang Yee Sin, who led the new research, says it has proved notoriously difficult to achieve – until now. “We have now shown that we can generate and reverse the spin direction of the electron current in an altermagnet made of two very thin layers of chromium sulphide (CrS) at room temperature using only an electric field,” Ang says.

Altermagnets, which were only discovered in 2024, are different from the conventional magnetically-ordered materials, ferromagnets and antiferromagnets. In ferromagnets, the magnetic moments (or spins) of atoms line up parallel to each other. In antiferromagnets, they line up antiparallel. The spins in altermagnets are also antiparallel, but the atoms that host these spins are rotated with respect to their neighbours. This combination gives altermagnets some properties of both ferromagnets and antiferromagnets, plus new properties of their own.

In bilayers of CrS, explains Ang, the electrons in each layer naturally prefer to spin in opposite directions, essentially cancelling each other out. “When we apply an electric field across the layers, however, one layer becomes more ‘active’ than the other. The current flowing through the device therefore becomes spin-polarized.”

A new device concept

The main challenge the researchers faced in their work was to identify a suitable material and a stacking arrangement in which spin and layers intertwined just right. This required detailed quantum-level simulations and theoretical modelling to prove that CrS bilayers could do the job, says Ang.

The work opens up a new device concept that the team calls layer-spintronics in which spin control is achieved via layer selection using an electric field. According to Ang, this concept has clear applications for next-generation, energy-efficient, compact and magnet-free memory and logic devices. And, since the technology works at room temperature and uses electric gating – a common approach in today’s electronics – it could make it possible to integrate spintronics devices with current semiconductor technology. This could lead to novel spin transistors, reconfigurable logic gates, or ultrafast memory cells based entirely on spin in the future, he says.

The SUTD researchers, who report their work in Materials Horizons, now aim to identify other 2D altermagnets that can host similar or even more robust spin-electric effects. “We are also collaborating with experimentalists to synthesize and characterize CrS bilayers to validate our predictions in the lab and investigating how to achieve non-volatile spin control by integrating them with ferroelectric materials,” reveals Ang. “This could potentially allow for memory devices that can retain information for longer.”

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A burst of solar wind triggered a planet-wide heatwave in Jupiter’s upper atmosphere, say astronomers at the University in Reading, UK. The hot region, which had a temperature of over 750 K, propagated at thousands of kilometres per hour and stretched halfway around the planet.

“This is the first time we have seen something like a travelling ionospheric disturbance, the likes of which are found on Earth, at a giant planet,” says James O’Donoghue, a Reading planetary scientist and lead author of a study in Geophysical Research Letters on the phenomenon. “Our finding shows that Jupiter’s atmosphere is not as self-contained as we thought, and that the Sun can drive dramatic, global changes, even this far out in the solar system.”

Jupiter’s upper atmosphere begins hundreds of kilometres above its surface and has two components. One is a neutral thermosphere composed mainly of molecular hydrogen. The other is a charged ionosphere comprising electrons and ions. Jupiter also has a protective magnetic shield, or magnetosphere.

When emissions from Jupiter’s volcanic moon, Io, become ionized by extreme ultraviolet radiation from the Sun, the resulting plasma becomes trapped in the magnetosphere. This trapped plasma then generates magnetosphere-ionosphere currents that heat the planet’s polar regions and produce aurorae. Thanks to this heating, the hottest places on Jupiter, at around 900 K, are its poles. From there, temperatures gradually decrease, reaching 600 K at the equator.

Quite a different temperature-gradient pattern

In 2021, however, O’Donoghue and colleagues observed quite a different temperature-gradient pattern in near-infrared spectral data recorded by the 10-metre Keck II telescope in Hawaii, US, during an event in 2017. When they analysed these data, they found an enormous hot region far from Jupiter’s aurorae and stretching across 180° in longitude – half the planet’s circumference.

“At the time, we could not definitively explain this hot feature, which is roughly 150 K hotter than the typical ambient temperature of Jupiter,” says O’Donoghue, “so we re-analysed the Keck data using updated solar wind propagation models.”

Two instruments on NASA’s Juno spacecraft were pivotal in the re-analysis, he explains. The first, called Waves, can measure electron densities locally. Its data showed that these electron densities ramped up as the spacecraft approached Jupiter’s magnetosheath, which is the region between the planet’s magnetic field and the solar wind. The second instrument was Juno’s magnetometer, which recorded measurements that backed up the Waves-based analyses, O’Donoghue says.

A new interpretation

In their latest study, the Reading scientists analysed a burst of fast solar wind that emanated from the Sun in January 2017 and propagated towards Jupiter. They found that a high-speed stream of this wind arrived several hours before the Keck telescope recorded the data that led them to identify the hot region.

“Our analysis of Juno’s magnetometer measurements also showed that this spacecraft exited the magnetosphere of Jupiter early,” says O’Donoghue. “This is a strong sign that strong solar winds probably compressed Jupiter’s magnetic field several hours before the hot region appeared.

“We therefore see the hot region emerging as a response to solar wind compression: the aurorae flared up and heat spilled equatorward.”

The result shows that the Sun can significantly reshape the global energy balance in Jupiter’s upper atmosphere, he tells Physics World. “That changes how we think about energy balance at all giant planets, not just Jupiter, but potentially Saturn, Uranus, Neptune and exoplanets too,” he says. “It also shows that solar wind can trigger complex atmospheric responses far from Earth and it could help us understand space weather in general.”

The Reading researchers say they would now like to hunt for more of these events, especially in the southern hemisphere of Jupiter where they expect a mirrored response. “We are also working on measuring wind speeds and temperatures across more of the planet and at different times to better understand how often this happens and how energy moves around,” O’Donoghue reveals. “Ultimately, we want to build a more complete picture of how space weather shapes Jupiter’s upper atmosphere and drives (or interferes) with global circulation there.”

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Worms move faster in an environment riddled with randomly-placed obstacles than they do in an empty space. This surprising observation by physicists at the University of Amsterdam in the Netherlands can be explained by modelling the worms as polymer-like “active matter”, and it could come in handy for developers of robots for soil aeriation, fertility treatments and other biomedical applications.

When humans move, the presence of obstacles – disordered or otherwise – has a straightforward effect: it slows us down, as anyone who has ever driven through “traffic calming” measures like speed bumps and chicanes can attest. Worms, however, are different, says Antoine Deblais, who co-led the new study with Rosa Sinaasappel and theorist colleagues in Sara Jabbari Farouji’s group. “The arrangement of obstacles fundamentally changes how worms move,” he explains. “In disordered environments, they spread faster as crowding increases, while in ordered environments, more obstacles slow them down.”

A maze of cylindrical pillars

The team obtained this result by placing single living worms at the bottom of a water chamber containing a 50 x 50 cm array of cylindrical pillars, each with a radius of 2.5 mm. By tracking the worms’ movement and shape changes with a camera for two hours, the scientists could see how the animals behaved when faced with two distinct pillar arrangements: a periodic (square lattice) structure; and a disordered array. The minimum distance between any two pillars was set to the characteristic width of a worm (around 0.5 mm) to ensure they could always pass through.

“By varying the number and arrangement of the pillars (up to 10 000 placed by hand!), we tested how different environments affect the worm’s movement,” Sinaasappel explains. “We also reduced or increased the worm’s activity by lowering or raising the temperature of the chamber.”

These experiments showed that when the chamber contained a “maze” of obstacles placed at random, the worms moved faster, not slower. The same thing happened when the researchers increased the number of obstacles. More surprisingly still, the worms got through the maze faster when the temperature was lower, even though the cold reduced their activity.

Active polymer-like filaments

To explain these counterintuitive results, the team developed a statistical model that treats the worms as active polymer-like filaments and accounts for both the worms’ flexibility and the fact that they are self-driven. This analysis revealed that in a space containing disordered pillar arrays, the long-time diffusion coefficient of active polymers with a worm-like degree of flexibility increases significantly as the fraction of the surface occupied by pillars goes up. In regular, square-lattice arrangements, the opposite happens.

The team say that this increased diffusivity comes about because randomly-positioned pillars create narrow tube-like structures between them. These curvilinear gaps guide the worms and allow them to move as if they were straight rods for longer before they reorient. In contrast, ordered pillar arrangements create larger open spaces, or pores, in which worms can coil up. This temporarily traps them and they slow down.

Similarly, the team found that reducing the worm’s activity by lowering ambient temperatures increases a parameter known as its persistence length. This is essentially a measure of how straight the worm is, and straighter worms pass between the pillars more easily.

“Self-tuning plays a key role”

Identifying the right active polymer model was no easy task, says Jabbari Farouji. One challenge was to incorporate the way worms adjust their flexibility depending on their surroundings. “This self-tuning plays a key role in their surprising motion,” says Jabbari Farouji, who credits this insight to team member Twan Hooijschuur.

Understanding how active, flexible objects move through crowded environments is crucial in physics, biology and biophysics, but the role of environmental geometry in shaping this movement was previously unclear, Jabbari Farouji says. The team’s discovery that movement in active, flexible systems can be controlled simply by adjusting the environment has important implications, adds Deblais.

“Such a capability could be used to sort worms by activity and therefore optimize soil aeration by earthworms or even influence bacterial transport in the body,” he says. “The insights gleaned from this study could also help in fertility treatments – for instance, by sorting sperm cells based on how fast or slow they move.”

Looking ahead, the researchers say they are now expanding their work to study the effects of different obstacle shapes (not just simple pillars), more complex arrangements and even movable obstacles. “Such experiments would better mimic real-world environments,” Deblais says.

The present work is detailed in Physical Review Letters.

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Contrary to some theorists’ expectations, water does not form hydrogen bonds in its supercritical phase. This finding, which is based on terahertz spectroscopy measurements and simulations by researchers at Ruhr University Bochum, Germany, puts to rest a long-standing controversy and could help us better understand the chemical processes that occur near deep-sea vents.

Water is unusual. Unlike most other materials, it is denser as a liquid than it is as the ice that forms when it freezes. It also expands rather than contracting when it cools; becomes less viscous when compressed; and exists in no fewer than 17 different crystalline phases.

Another unusual property is that at high temperatures and pressures – above 374 °C and 221 bars – water mostly exists as a supercritical fluid, meaning it shares some properties with both gases and liquids. Though such extreme conditions are rare on the Earth’s surface (at least outside a laboratory), they are typical for the planet’s crust and mantle. They are also present in so-called black smokers, which are hydrothermal vents that exist on the seabed in certain geologically active locations. Understanding supercritical water is therefore important for understanding the geochemical processes that occur in such conditions, including the genesis of gold ore.

Supercritical water also shows promise as an environmentally friendly solvent for industrial processes such as catalysis, and even as a mediator in nuclear power plants. Before any such applications see the light of day, however, researchers need to better understand the structure of water’s supercritical phase.

Probing the hydrogen bonding between molecules

At ambient conditions, the tetrahedrally-arranged hydrogen bonds (H-bonds) in liquid water produce a three-dimensional H-bonded network. Many of water’s unusual properties stem from this network, but as it approaches its supercritical point, its structure changes.

Previous studies of this change have produced results that were contradictory or unclear at best. While some pointed to the existence of distorted H-bonds, others identified heterogeneous structures involving rigid H-bonded dimers or, more generally, small clusters of tetrahedrally-bonded water surrounded by nonbonded gas-like molecules.

To resolve this mystery, an experimental team led by Gerhard Schwaab and Martina Havenith, together with Philipp Schienbein and Dominik Marx, investigated how water absorbs light in the far infrared/terahertz (THz) range of the spectrum. They performed their experiments and simulations at temperatures of 20° to 400°C and pressures from 1 bar up to 240 bars. In this way, they were able to investigate the hydrogen bonding between molecules in samples of water that were entering the supercritical state and samples that were already in it.

Diamond and gold cell

Because supercritical water is highly corrosive, the researchers carried out their experiments in a specially-designed cell made from diamond and gold. By comparing their experimental data with the results of extensive ab initio simulations that probed different parts of water’s high-temperature phase diagram, they obtained a molecular picture of what was happening.

The researchers found that the terahertz spectrum of water in its supercritical phase was practically identical to that of hot gaseous water vapour. This, they say, proves that supercritical water is different from both liquid water at ambient conditions and water in a low-temperature gas phase where clusters of molecules form directional hydrogen bonds. No such molecular clusters appear in supercritical water, they note.

The team’s ab initio molecular dynamics simulations also revealed that two water molecules in the supercritical phase remain close to each other for a very limited time – much shorter than the typical lifetime of hydrogen bonds in liquid water – before distancing themselves. What is more, the bonds between hydrogen and oxygen atoms in supercritical water do not have a preferred orientation. Instead, they are permanently and randomly rotating. “This is completely different to the hydrogen bonds that connect the water molecules in liquid water at ambient conditions, which do have a persisting preferred orientation,” Havenith says.

Now that they have identified a clear spectroscopic fingerprint for supercritical water, the researchers want to study how solutes affect the solvation properties of this substance. They anticipate that the results from this work, which is published in Science Advances, will enable them to characterize the properties of supercritical water for use as a “green” solvent.

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Abnormal versions of synchronization patterns known as “Arnold’s tongues” have been observed in a femtosecond fibre laser that generates oscillating light pulses. While these unconventional patterns had been theorized to exist in certain strongly-driven oscillatory systems, the new observations represent the first experimental confirmation.

Scientists have known about synchronization since 1665, when Christiaan Huygens observed that pendulums placed on a table eventually begin to sway in unison, coupled by vibrations within the table. It was not until the mid-20^th century, however, that a Russian mathematician, Vladimir Arnold, discovered that plotting certain parameters of such coupled oscillating systems produces a series of tongue-like triangular shapes.

These shapes are now known as Arnold’s tongues, and they are an important indicator of synchronization. When the system’s parameters are in the tongue region, the system is synchronized. Otherwise, it is not.

Arnold’s tongues are found in all real-world synchronized systems, explains Junsong Peng, a physicist at East China Normal University. They have previously been studied in systems such as nanomechanical and biological resonators to which external driving frequencies are applied. More recently, they have been observed in the motion of two bound solitons (wave packets that maintain their shapes and sizes as they propagate) when they are subject to external forces.

Abnormal synchronization regions

In the new work, Peng, Sonia Boscolo of Aston University in the UK, Christophe Finot of the University of Burgundy in France, and colleagues studied Arnold’s tongue patterns in a laser that emits solitons. Lasers of this type possess two natural synchronization frequencies: the repetition frequency of the solitons (determined by the laser’s cavity length) and the frequency at which the energy of the soliton becomes self-modulating, or “breathing”.

In their experiments, which they describe in Science Advances, the researchers found that as they increased the driving force applied to this so-called breathing-soliton laser, the synchronization region first broadened, then narrowed. These changes produced Arnold’s tongues with very peculiar shapes. Instead of being triangle-like, they appeared as two regions shaped like leaves or rays.

Avoiding amplitude death

Although theoretical studies had previously predicted that Arnold’s-tongue patterns would deviate substantially from the norm as the driving force increased, Peng says that demonstrating this in a real system was not easy. The driving force required to access the anomalous regime is so strong that it can destroy fragile coherent pulsing states, leading to “amplitude death” in which all oscillations are completely suppressed.

In the breathing-soliton laser, however, the two frequencies synchronized without amplitude death even though the repetition frequency is about two orders of magnitude higher than the breathing frequency. “These lasers therefore open up a new frontier for studying synchronization phenomena,” Peng says.

To demonstrate the system’s potential, the researchers explored the effects of using an optical attenuator to modulate the laser’s dissipation while changing the laser’s pump current to modulate its gain. Having precise control over both parameters enabled them to identify “holes” within the ray-shaped tongue regions. These holes appear when the driving force exceeds a certain strength, and they represent quasi-periodic (unsynchronized) states inside the larger synchronized regions.

“The manifestation of holes is interesting not only for nonlinear science, it is also important for practical applications,” Peng explains. “This is because these holes, which have not been realized in experiments until now, can destabilize the synchronized system.”

Understanding when and under which conditions these holes appear, Peng adds, could help scientists ensure that oscillating systems operate more stably and reliably.

Extending synchronization to new regimes

The researchers also used simulations to produce a “map” of the synchronization regions. These simulations perfectly reproduced the complex synchronization structures they observed in their experiments, confirming the existence of the “hole” effect.

Despite these successes, however, Peng says it is “still quite challenging” to understand why such patterns appear. “We would like to do more investigations on this issue and get a better understanding of the dynamics at play,” he says.

The current work extends studies of synchronization into a regime where the synchronized region no longer exhibits a linear relationship with the coupling strength (as is the case for normal Arnold’s-tongue pattern), he adds. “This nonlinear relationship can generate even broader synchronization regions compared to the linear regime, making it highly significant for enhancing the stability of oscillating systems in practical applications,” he tells Physics World.

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A concept from quantum information theory appears to explain at least some of the peculiar behaviour of so-called “strange” metals. The new approach, which was developed by physicists at Rice University in the US, attributes the unusually poor electrical conductivity of these metals to an increase in the quantum entanglement of their electrons. The team say the approach could advance our understanding of certain high-temperature superconductors and other correlated quantum structures.

While electrons can travel through ordinary metals such as gold or copper relatively freely, strange metals resist their flow. Intriguingly, some high-temperature superconductors have a strange metal phase as well as a superconducting one. This phenomenon that cannot be explained by conventional theories that treat electrons as independent particles, ignoring any interactions between them.

To unpick these and other puzzling behaviours, a team led by Qimiao Si turned to the concept of quantum Fisher information (QFI). This statistical tool is typically used to measure how correlations between electrons evolve under extreme conditions. In this case, the team focused on a theoretical model known as the Anderson/Kondo lattice that describes how magnetic moments are coupled to electron spins in a material.

Correlations become strongest when strange metallicity appears

These analyses revealed that electron-electron correlations become strongest at precisely the point at which strange metallicity appears in a material. “In other words, the electrons become maximally entangled at this quantum critical point,” Si explains. “Indeed, the peak signals a dramatic amplification of multipartite electron spin entanglement, leading to a complex web of quantum correlations between many electrons.”

What is striking, he adds, is that this surge of entanglement provides a new and positive characterization of why strange metals are so strange, while also revealing why conventional theory fails. “It’s not just that traditional theory falls short, it is that it overlooks this rich web of quantum correlations, which prevents the survival of individual electrons as the elementary objects in this metallic substance,” he explains.

To test their finding, the researchers, who report their work in Nature Communications, compared their predictions with neutron scattering data from real strange-metal materials. They found that the experimental data was a good match. “Our earlier studies had also led us to suspect that strange metals might host a deeply entangled electron fluid – one whose hidden quantum complexity had yet to be fully understood,” adds Si.

The implications of this work are far-reaching, he tells Physics World. “Strange metals may hold the key to unlocking the next generation of superconductors — materials poised to transform how we transmit energy and, perhaps one day, eliminate power loss from the electric grid altogether.”

The Rice researchers say they now plan to explore how QFI manifests itself in the charge of electrons as well as their spins. “Until now, our focus has only been on the QFI associated with electrons spins, but electrons also of course carry charge,” Si says.

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New measurements by physicists from the University of Surrey in the UK have shed fresh light on where the universe’s heavy elements come from. The measurements, which were made by smashing high-energy protons into a uranium target to generate strontium ions, then accelerating these ions towards a second, helium-filled target, might also help improve nuclear reactors.

The origin of the elements that follow iron in the periodic table is one of the biggest mysteries in nuclear astrophysics. As Surrey’s Matthew Williams explains, the standard picture is that these elements were formed when other elements captured neutrons, then underwent beta decay. The two ways this can happen are known as the rapid (r) and slow (s) processes.

The s-process occurs in the cores of stars and is relatively well understood. The r-process is comparatively mysterious. It occurs during violent astrophysical events such as certain types of supernovae and neutron star mergers that create an abundance of free neutrons. In these neutron-rich environments, atomic nuclei essentially capture neutrons before the neutrons can turn into protons via beta-minus decay, which occurs when a neutron emits an electron and an antineutrino.

From the night sky to the laboratory

One way of studying the r-process is to observe older stars. “Studies on heavy element abundance patterns in extremely old stars provide important clues here because these stars formed at times too early for the s-process to have made a significant contribution,” Williams explains. “This means that the heavy element pattern in these old stars may have been preserved from material ejected by prior extreme supernovae or neutron star merger events, in which the r-process is thought to happen.”

Recent observations of this type have revealed that the r-process is not necessarily a single scenario with a single abundance pattern. It may also have a “weak” component that is responsible for making elements with atomic numbers ranging from 37 (rubidium) to 47 (silver), without getting all the way up to the heaviest elements such as gold (atomic number 79) or actinides like thorium (90) and uranium (92).

This weak r-process could occur in a variety of situations, Williams explains. One scenario involves radioactive isotopes (that is, those with a few more neutrons than their stable counterparts) forming in hot neutrino-driven winds streaming from supernovae. This “flow” of nucleosynthesis towards higher neutron numbers is caused by processes known as (alpha,n) reactions, which occur when a radioactive isotope fuses with a helium nucleus and spits out a neutron. “These reactions impact the final abundance pattern before the neutron flux dissipates and the radioactive nuclei decay back to stability,” Williams says. “So, to match predicted patterns to what is observed, we need to know how fast the (alpha,n) reactions are on radioactive isotopes a few neutrons away from stability.”

The ⁹⁴Sr(alpha,n)⁹⁷Zr reaction

To obtain this information, Williams and colleagues studied a reaction in which radioactive strontium-94 absorbs an alpha particle (a helium nucleus), then emits a neutron and transforms into zirconium-97. To produce the radioactive ⁹⁴Sr beam, they fired high-energy protons at a uranium target at TRIUMF, the Canadian national accelerator centre. Using lasers, they selectively ionized and extracted strontium from the resulting debris before filtering out ⁹⁴Sr ions with a magnetic spectrometer.

The team then accelerated a beam of these ⁹⁴Sr ions to energies representative of collisions that would happen when a massive star explodes as a supernova. Finally, they directed the beam onto a nanomaterial target made of a silicon thin film containing billions of small nanobubbles of helium. This target was made by researchers at the Materials Science Institute of Seville (CSIC) in Spain.

“This thin film crams far more helium into a small target foil than previous techniques allowed, thereby enabling the measurement of helium burning reactions with radioactive beams that characterize the weak r-process,” Williams explains.

To identify the ⁹⁴Sr(alpha,n)⁹⁷Zr reactions, the researchers used a mass spectrometer to select for ⁹⁷Zr while simultaneously using an array of gamma-ray detectors around the target to look for the gamma rays it emits. When they saw both a heavy ion with an atomic mass of 97 and a ⁹⁷Zr gamma ray, they knew they had identified the reaction of interest. In doing so, Williams says, they were able to measure the probability that this reaction occurs at the energies and temperatures present in supernovae.

Williams thinks that scientists should be able to measure many more weak r-process reactions using this technology. This should help them constrain where the weak r-process comes from. “Does it happen in supernovae winds? Or can it happen in a component of ejected material from neutron star mergers?” he asks.

As well as shedding light on the origins of heavy elements, the team’s findings might also help us better understand how materials respond to the high radiation environments in nuclear reactors. “By updating models of how readily nuclei react, especially radioactive nuclei, we can design components for these reactors that will operate and last longer before needing to be replaced,” Williams says.

The work is detailed in Physical Review Letters.

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Researchers from the Institute of Physics of the Chinese Academy of Sciences have produced the first two-dimensional (2D) sheets of metal. At just angstroms thick, these metal sheets could be an ideal system for studying the fundamental physics of the quantum Hall effect, 2D superfluidity and superconductivity, topological phase transitions and other phenomena that feature tight quantum confinement. They might also be used to make novel electronic devices such as ultrathin low-power transistors, high-frequency devices and transparent displays.

Since the discovery of graphene – a 2D sheet of carbon just one atom thick – in 2004, hundreds of other 2D materials have been fabricated and studied. In most of these, layers of covalently-bonded atoms are separated by gaps. The presence of these gaps mean that neighbouring layers are held together only by weak van der Waals (vdW) interactions, making it relatively easy to “shave off” single layers to make 2D sheets.

Making atomically thin metals would expand this class of technologically important structures. However, because each atom in a metal is strongly bonded to surrounding atoms in all directions, thinning metal sheets to this degree has proved difficult. Indeed, many researchers thought it might be impossible.

Melting and squeezing pure metals

The technique developed by Guangyu Zhang, Luojun Du and colleagues involves heating powders of pure metals between two monolayer-MoS₂/sapphire vdW anvils. The team used MoS₂/sapphire because both materials are atomically flat and lack dangling bonds that could react with the metals. They also have high Young’s moduli, of 430 GPa and 300 GPa respectively, meaning they can withstand extremely high pressures.

Once the metal powders melted into a droplet, the researchers applied a pressure of 200 MPa. They then continued this “vdW squeezing” until the opposite sides of the anvils cooled to room temperature and 2D sheets of metal formed.

The team produced five atomically thin 2D metals using this technique. The thinnest, at around 5.8 Å, was tin, followed by bismuth (~6.3 Å), lead (~7.5 Å), indium (~8.4 Å) and gallium (~9.2 Å).

“Arduous explorations”

Zhang, Du and colleagues started this project around 10 years ago after they decided it would be interesting to work on 2D materials other than graphene and its layered vdW cousins. At first, they had little success. “Since 2015, we tried out a host of techniques, including using a hammer to thin a metal foil – a technique that we borrowed from gold foil production processes – all to no avail,” Du recalls. “We were not even able to make micron-thick foils using these techniques.”

After 10 years of what Du calls “arduous explorations”, the team finally moved a crucial step forward by developing the vdW squeezing method.

Writing in Nature, the researchers say that the five 2D metals they’ve realized so far are just the “tip of the iceberg” for their method. They now intend to increase this number. “In terms of novel properties, there is still a knowledge gap in the emerging electrical, optical, magnetic properties of 2D metals, so it would be nice to see how these materials behave physically as compared to their bulk counterparts thanks to 2D confinement effects,” says Zhang. “We would also like to investigate to what extent such 2D metals could be used for specific applications in various technological fields.”

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