A new ultra-precise atomic clock outperforms existing microwave clocks in time-keeping and sturdiness under real-world conditions. The clock, made by a team of researchers from the California, US-based engineering firm Vector Atomic, exploits the precise frequencies of atomic transitions in iodine molecules and recently passed a three-week trial aboard a ship sailing around Hawaii.

Atomic clocks are the world’s most precise timekeeping devices, and they are essential to staples of modern life such as global positioning systems, telecommunications and data centres. The most common types of atomic clock used in these real-world applications were developed in the 1960s, and they work by measuring the frequency at which atoms oscillate between two energy states. They are often based on caesium atoms, which absorb and emit radiation at microwave frequencies as they oscillate, and the best of them are precise to within one second in six million years.

Clocks that absorb and emit at higher, visible, frequencies are even more precise, with timing errors of less than 1 second in 30 billion years. These optical atomic clocks are, however, much bulkier than their microwave counterparts, and their sensitivity to disturbances in their surroundings means they only work properly under well-controlled conditions.

Prototypes based on iodine

The Vector Atomic work, which the team describe in Nature, represents a step towards overturning these limitations. Led by Vector Atomic co-founder and study co-author Jamil-Abo-Shaeer, the team developed three robust optical clock prototypes based on transitions in iodine molecules (I₂). These transitions occur at wavelengths conveniently near those of routinely-employed commercial frequency-doubled lasers, and the iodine itself is confined in a vapour cell, doing away with the need to cool atoms to extremely cold temperatures or keep them in an ultrahigh vacuum. With a volume of around 30 litres, the clocks are also compact enough to fit on a tabletop.

While the precision of these prototype optical clocks lags behind that of the best lab-based versions, it is still 1000 times better than clocks of a similar size that ships currently use, says Abo-Shaeer. The prototype clocks are also 100 times more precise than existing microwave clocks of the same size.

Sea trials

The researchers tested their clocks aboard a Royal New Zealand Navy ship, HMNZS Aotearoa, during a three-week voyage around Hawaii. They found that the clocks performed almost as well as in the laboratory, despite the completely different conditions. Indeed, two of the larger devices recorded errors of less than 400 picoseconds (10^-12 seconds) over 24 hours.

The team describe the prototypes as a “key building block” for upgrading the world’s timekeeping networks from the nanosecond to the picosecond regime. According to team member Jonathan Roslund, the goal is to build the world’s first fully integrated optical atomic clock with the same “form factor” as a microwave clock, and then demonstrate that it outperforms microwave clocks under real-world conditions.

“Iodine optical clocks are certainly not new,” he tells Physics World. “In fact, one of the very first optical clocks utilized iodine, but researchers moved onto more exotic atoms with better timekeeping properties. Iodine does have a number of attractive properties, however, for making a compact and simple portable optical clock.”

The most finicky parts of any atomic-clock system, Roslund explains, are the lasers, but iodine can rely on industrial-grade lasers operating at both 1064 nm and 1550 nm. “The vapour cell architecture we employ also uses no consumables and requires neither laser cooling nor a pre-stabilization cavity,” Roslund adds.

The next generation

After testing their first-generation clocks on HMNZS Aotearoa, the researchers developed a second-generation device that is 2.5 times more precise. With a volume of just 30 litres including the power supply and computer control, the upgraded version is now a commercial product called Evergreen-30. “We are also hard at work on a 5-litre version targeting the same performance, and an ultracompact 1-litre version,” Roslund reveals.

As well as travelling aboard ships, Roslund says these smaller clocks could have applications in airborne and space-based systems. They might also make a scientific impact: “We have just finished an exciting demonstration in collaboration with the University of Arizona, in which our Evergreen-30 clocks served as the timebase for a radio observatory in the Event Horizon Telescope Array, which is imaging distant supermassive blackholes.”

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The substrates on which semiconductor chip are grown usually get ignored, but they may be more important than we think. This is the finding of researchers in the US and Germany, who used high-energy X-rays to study titanium dioxide – a common substrate for insulator-to-metal semiconductors. The discovery that this material is far more than just a passive platform could help scientists develop next-generation electronics.

Materials that switch from metal-like to insulating very quickly offer a promising route for developing super-fast electronic transistors. To this end, a team led by materials scientist and physicist Venkatraman Gopalan of Pennsylvania State University, US, began studying a leading candidate for such devices, vanadium dioxide (VO₂). Vanadium dioxide is unusual in that its electrons are strongly correlated. This means that, unlike in silicon-based electronics, the repulsion between electrons cannot be ignored.

Crucially, though, the researchers did not look at the VO₂ layer on its own. They also analysed how it interacts with the titanium dioxide (TiO₂) substrate upon which it is grown. To their surprise, they found that the substrate contains an active layer that behaves just like the semiconductor when the VO₂ switches between an insulating state and a metallic one.

Timed X-ray pulse

Gopalan and colleagues obtained their results by growing a very thin film of VO₂ atop a thick TiO₂ single crystal substrate. They then fabricated a device channel on the ensemble across which they could apply the voltage pulses that switch the semiconductor from insulating to conducting. During this switching, they applied high-energy X-ray pulses from the Advanced Photon Source (APS) at Argonne National Laboratory to the channel and observed the lattice planes of the semiconducting film and the substrate.

“The X-ray pulse was timed so that it could arrive before, at and after the electrical pulse so that we see what happens with time,” Gopalan explains. “It was also raster scanned across the channel to map what happens to the entire channel when the material switches from being an insulator to a metal.”

This technique, known as spatio-temporal X-ray diffraction microscopy, is good at revealing the behaviour of materials at the atomic level. In this case, it showed the researchers that the VO₂ film bulges as it changes to a metal. This was unexpected: according to Gopalan, the material was supposed to shrink. “What is more, the substrate, which is usually thought to be electrically and mechanically passive, also bulges along with the VO₂ film,” he says. “It is like the tail wagging the dog, and shows that a mechanism that was missed before is at play.”

Native oxygen vacancies are responsible

According to the researchers’ theoretical calculations and modelling, this mechanism involves atomic sites in the material lattice that are missing oxygen atoms. These native oxygen vacancies, as they are known, are present in both the semiconductor and substrate and they ionize and deionize in concert with the applied electric field.

“Neutral oxygen vacancies hold a charge of two electrons, which they can release when the material switches from an insulator to a metal,” Gopalan explains. “The oxygen vacancy left behind is now charged and swells up, leading to the observed swelling in the device. This can also happen in the substrate.”

The experiment itself was very challenging, Gopalan says. One of the X-ray beamlines at the APS had to be specially rigged and it took the team several years to complete the set-up. Then, he adds, “The results were so intriguing and unexpected that it took us several more years to analyse the data and come up with a theory to understand the results.”

According to Gopalan, there is tremendous interest in next-generation electronics based on correlated electronic materials such as VO₂ that exhibit a fast insulator-to-metal transition. “While previous studies have analysed this material using various techniques, including using X-rays, our is the first to study a functioning device geometry under realistic conditions, while mapping its response in space and time,” he tells Physics World. “This study is unique in that respect, and it paid off in what it revealed.”

The researchers are now trying to understand the mechanisms behind the substrate’s surprising response, and they plan to revisit their experiment to this end. “We are thinking, for example, of intentionally adding ionizing defects that release electrons and trigger a metal-to-insulator transition when a voltage is applied,” Gopalan reveals.

The present study – which also involved collaborators at Cornell University and Georgia Tech in the US, and the Paul Drude Institute in Germany – is detailed in Advanced Materials.

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Researchers at Princeton University in the US say they have made the first direct observation of a Wigner crystal – a structure consisting solely of electrons arranged in a lattice-like configuration. The finding, made by using scanning tunnelling microscopy to examine a material known as Bernal-stacked graphene, confirms a nearly century-old theory that electrons can assemble into a closely-packed lattice without having to orbit around an atom. The work could help scientists discover other phases of exotic matter in which electrons behave collectively.

Although electrons repel each other, at room temperatures their kinetic energy is high enough to overcome this, so they flow together as electric currents. At ultralow temperatures, however, repulsive forces dominate, and electrons spontaneously crystallize into an ordered quantum phase of matter. This, at least, is what the physicist Eugene Wigner predicted 90 years ago would happen. But while scientists have seen evidence of this type of crystalline lattice forming before (for example, in a one-dimensional carbon nanotube and in a quantum wire), it had never been observed directly.

A pristine sample of graphene

In the new work, which is detailed in Nature, researchers led by Princeton’s Ali Yazdani used a scanning tunnelling microscope (STM) to study electrons in a pristine sample of graphene (a sheet of carbon one atom thick). To keep the material as pure as possible, and so avoid the possibility of electron crystals forming in lattice defects or imperfections, they placed one sheet of graphene atop another in a configuration known as a bilayer Bernal stack.

Next, they cooled the sample down to just above absolute zero, which reduced the kinetic energy of the electrons. They also applied a magnetic field perpendicular to the sample’s layers, which suppresses kinetic energy still further by restricting the electrons’ possible orbits. The result was a two-dimensional gas of electrons located between the graphene layers, with a density the researchers could tune by applying a voltage across the sample.

Scanning tunnelling microscopy involves scanning a sharp metallic tip across a sample. When the tip passes over an electron, the particle tunnels through the gap between the sample surface and the tip, thereby creating an electric current. By measuring this current, researchers can determine the local density of electrons. Yazdani and colleagues found that when they increased this density, they observed a phase transition during which the electrons spontaneously assembled into an ordered triangular lattice structure – just as Wigner predicted.

Forcing a lattice to form

The team explains that this spontaneous assembly is the natural outcome of a “battle” between the electrons’ increased density (which pushes them closer together) and their mutual repulsion (which pushes them apart). An organized lattice configuration – a Wigner crystal – is, in effect, a compromise that lets electrons maintain a degree of distance from each other even when their density is relatively high. If the density increases still further, this crystalline phase melts, producing a phase known as a fractional quantum Hall electron liquid as well as an anisotropic quantum fluid in which the electrons organize themselves into stripes.

By analysing the size of each electron site in the Wigner crystal, the researchers also found evidence for the crystal’s “zero-point” motion. This motion, which comes about because of the Heisenberg uncertainty principle, occupies a “remarkable” 30% of the lattice constant of a crystal site, Yazdani explains, and highlights the crystal’s quantum nature.

The Princeton team now aims to use this same STM technique to image a Wigner crystal made of “holes”, which are regions of positive charge where electrons are absent. “We also plan to image other types of electron solid phases, so-called skyrme crystals and ‘bubble phases’,” Yazdani says. “In addition to even more exotic phases such as quasiparticle Wigner crystals made of fractional charges, there is also the possibility to study how these quantum crystals would change in the presence of a net electrical current.”

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A novel detector designed to search for so-called “dark” photons has placed unprecedented constraints on where this type of dark matter might be found. The detector uses a coaxial dish antenna to trap and funnel photons, and its US-based developers say it could easily be scaled up and made more sensitive in the future.

Current theories of physics suggest that dark matter makes up roughly 85% of the universe’s mass. This form of matter may have played an important role in the formation of galaxies thanks to its gravitational pull, but we have not been able to detect it directly as yet. Indeed, scientists are not sure what form dark matter might take, or even where precisely to look for it. The main approach involves using detectors that search for particles with a specific range of masses (or frequencies) in the hopes that even if they see nothing, we will at least learn more about what dark matter is not.

Searching a broader band of masses

Researchers led by David Miller of the University of Chicago and Andrew Sonnenschein of the Fermi National Accelerator Laboratory (Fermilab) have now put forward a slightly different approach involving a detector that searches over a broader range of masses, albeit with slight less precision. Known as the Broadband Reflector Experiment for Axion Detection (BREAD), this experiment looks for dark matter in the form of particles known as axions and dark photons. These particles are extremely light and can be converted into visible photons under certain circumstances. For example, when they hit a metallic wall, visible photons are emitted in a direction perpendicular to the wall.

BREAD consists of a cylindrical metal tube that traps and funnels dark photons, and the outer part of this cylindrical tube corresponds to the wall, explains team member Stefan Knirck, a physicist at Fermilab. “The photons are focused onto a small spot behind which you can place a light detector or antenna to search for a signal,” he explains. “In BREAD, the combination of an inner teardrop-shaped reflector and the outer cylinder take care of the focusing.”

Very high sensitivity in the chosen frequency range

The team describe the results from the experiment in its present form as “very promising”, noting that it shows very high sensitivity at frequencies from 10.7 to 12.5 GHz. In this range, the detector exceeds existing constraints by a factor of ~100, placing the most stringent bound yet on dark photons at these frequencies.

The UChicago/Fermilab team is now developing the technology to make it more sensitive and scalable. “At present, the experiment is sensitive to only quite contrived dark matter models,” Knirck says, “but, ultimately, the method might enable us to explore other axion models.”

To make the detector sensitive to these other axionic versions of dark matter, Knirck notes that he and his colleagues will need to add a magnetic field parallel to the wall. They plan to do this by placing the instrument in a metre-scale, high-field (multi-Tesla) solenoid magnet, and they are currently performing trials using a 4T magnet at the Argonne National Laboratory.

“We are also building more prototypes combining the concept with different cutting-edge quantum technology to be sensitive to single particles of light at the focus,” Knirck tells Physics World. “At Fermilab we soon expect to receive an even more powerful magnet which will make our experiments much more sensitive. The long-term goal is a large-scale experimental program with a setup on the 10-metre scale inside a huge magnet.”

The study is published in Physical Review Letters.

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Pulses of laser light can cause any material – including insulators – to develop a relatively large magnetic moment. This effect, which has been demonstrated for the first time by an international team of researchers, shows that laser light can induce quantum behaviour even at room temperature, not just under the extremely cold conditions usually required. While primarily of interest for fundamental science, the technique could also have applications for faster, more efficient magnetic data storage.

In their experiments, Stefano Bonetti of Stockholm University and the Ca’ Foscari University of Venice and colleagues started with a relatively simple idea. By applying laser light that is both circularly polarized – that is, its polarization traces out a corkscrew-like shape as it propagates – and resonant with the frequency of atomic oscillations within a material, they figured they could drive these oscillations in a circular pattern and thus induce a magnetic moment.

The researchers were encouraged in their thinking by theoretical research, which predicted that atoms moving in circular patterns could indeed induce magnetization in almost any material. “Given my expertise in magnetism and my recent investigations into phonon dynamics (lattice vibrations), I believed that my laboratory would be an ideal setting to experiment with this concept,” Bonetti says.

Polarized light source induces large magnetic moments

Before they could begin, the researchers first had to develop a new polarized light source with a frequency in the required terahertz (far-infrared) range. Once the source was ready, they used it to fire short, intense pulses at a sample of strontium titanate (SrTiO₃). At room temperature, this material is a paraelectric diamagnet with a cubic perovskite lattice structure. The researchers chose it because some of its atoms vibrate at terahertz frequencies – specifically, at 3 THz with a bandwidth of 0.5 THz.

The team found that these light pulses induced a phenomenon known as dynamic multiferroicity. Multiferroicity occurs when several properties of a material each have their own preferred states. For example, a multiferroic material might have magnetic moments that point in one direction, and electric charge that also shifts in a certain direction. Importantly, the two phenomena are independent of each other.

Though predicted by theory, this phenomenon had never been demonstrated experimentally. Bonetti reports that the experiment also yielded a surprise: the magnetic moments induced in the material were 10 000 times larger than theory predicts.

Magnetic data storage applications

The researchers say their discoveries could find use in magnetic data storage technologies, where there is great interest in novel methods of encoding magnetic information. This is because magnetic domains could be switched by a fast, lower-power electric field, rather than by an electric current (an energy-intensive and relatively slow process) as conventional domains are.

The team, which also includes scientists from the Nordic Institute of Theoretical Physics (NORDITA) in Sweden; the University of Connecticut and the SLAC National Accelerator Laboratory in the US; the Elettra-Sincrotrone Trieste and the ‘Sapienza’ University of Rome, both in Italy; and the National Institute for Materials Science in Tsukuba, Japan, is now working to better understand the physics of dynamic multiferroicity. “This will be essential for better controlling the effect,” Bonetti tells Physics World. “We also aim to make the effect more persistent, as currently it only occurs while the laser light is active.”

The experiments are described in Nature.

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Oil and water famously don’t mix – at least, not without adding a surfactant such as soap to coax them into a stable combination. Now, however, researchers in France and US have turned this conventional wisdom on its head by showing that they can, in fact, mix without a surfactant. The finding could have wide-reaching implications for industries that make heavy use of such mixtures, including food, cosmetics, health, paints and packaging to name just a few.

A mixture of two immiscible liquids such as water and oil is known as an emulsion. When an emulsion is shaken vigorously, one of its component liquids may disperse into small droplets within the other. But if the emulsion is left to stand, its components invariably separate out again.

The main driver of this separation is that as droplets of each liquid move closer to each other, they coalesce into ever-larger droplets. To prevent this, a third component may be added that is amphiphilic, meaning that it has an affinity for the interface between the mixture’s two components. Today’s industrial emulsions rely on the use of such materials, which are termed surfactants. However, many surfactants are toxic for both humans and the environment. Reducing their use (or doing away with them altogether) would therefore be highly beneficial.

A counter-intuitive phenomenon

In the latest work, researchers from the Colloïdes et Matériaux Divisés Laboratory at the ESPCI in Paris, France; the French company Calyxia, which specializes in the design and manufacture of biodegradable microcapsules; and Harvard University in the US studied mixtures composed solely of water and various types of oil. Within these normally immiscible mixtures, they observed ultrathin but abnormally stable films of oil spontaneously appearing between the dispersed droplets of water.

“This phenomenon systematically induces adhesion between the droplets while preventing them from coalescing, so allowing us to disperse large proportions of water (80% by volume or more) in oil,” explains Jérôme Bibette, the chemical physicist and ESPCI laboratory director who led the research.

Stable over several weeks

The phenomenon, which is detailed in Science, works best for highly polar oils that contain both hydrophilic and hydrophobic components and have a high molecular weight. These criteria exclude aliphatic hydrocarbons such as methane and polyethylene, for example, but include oils containing alternating oxygen and carbon atoms – a category that encompasses all vegetable oils.

The researchers found that these oils can change their configuration as soon as they are confined between two drops of water by “choosing” to preferentially locate their hydrophilic parts towards the water and the hydrophobic parts away from it. “The ultrathin adhesive film induced by the affinity of the hydrophobic parts develops spontaneously as soon as the two interfaces approach,” Bibette says. “The film then acquires an enormous viscosity while reducing the free energy of the interface – something that manifests itself by the water and oil drops adhering together.”

Such spontaneous gelling between two immiscible liquids had never been observed before, he adds.

Since most vegetable oils can be polymerized, combining them with water could allow researchers to make perfectly biodegradable polymeric materials. For Bibette, one of the most obvious applications that springs to mind is biodegradable capsules for industries such as cosmetics and fragrances.

The technique could also allow researchers to create new types of plastics comprising biodegradable polymers and up to 90% water by volume, he tells Physics World. “Both phases could be made (and controlled to be) homogenous throughout the entire mixture, which could allow us to produce a unique bi-continuous, coexisting hydrophilic and hydrophobic material,” he says. “This could have applications in areas as diverse as tissue engineering, biodegradable packing and materials for replacing plastics in general.”

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Convention solar cells have a maximum external quantum efficiency (EQE) of 100%: for every photon incident on the cell, they generate one photoexcited electron. In recent years, scientists have sought to improve on this by developing materials that “free up” more than one electron for every photon they absorb. A team led by physicist Chinedu Ekuma of Lehigh University in the US has now achieved this goal, producing a material with an EQE of up to 190% – nearly double that of silicon solar cells.

The team made the new compound by inserting copper atoms between atomically thin layers of germanium selenide (GeSe) and tin sulfide (SnS). The resulting material has the chemical formula Cu_xGeSe/SnS, and the researchers developed it by taking advantage of so-called van der Waals gaps. These atomically small gaps exist between layers of two-dimensional materials, and they form “pockets” into which other elements can be inserted (or “intercalated”) to tune the material’s properties.

Intermediate bandgap states

The Lehigh researchers attribute the material’s increased EQE to the presence of intermediate bandgap states. These distinct electronic energy levels arise within the material’s electronic structure in a way that enables them to absorb light very efficiently over a broad spectrum of solar radiation wavelengths. In the new material, these energy levels exist at around 0.78 and 1.26 electron volts (eV), which lie within the range over which the material can efficiently absorb sunlight.

The material works particularly well in the infrared and visible regions of the electromagnetic spectrum, producing, on average, nearly two photoexcited charge carriers (electrons and holes bound in quasiparticles known as excitons) for every incident photon. According to Ekuma, such “multiple exciton generation” materials can serve as the active layer within solar cell devices, where their performance is fundamentally governed by exciton physics. “This active layer is crucial for enhancing the solar cell’s efficiency by facilitating the generation and transport of excitons in the material,” Ekuma explains.

Further research needed for practical devices

The researchers used advanced computational models to optimize the thickness of the photoactive layer in the material. They calculated that its EQE can be enhanced by making sure that it remains thin (in the so-called quasi-2D limit) to prevent quantum confinement losses. This is a key factor that affects efficient exciton generation and transport through a process known as nonradiative recombination, in which electrons and holes have time to recombine instead of being whisked apart to produce useful current, Ekuma explains. “By maintaining quantum confinement, we preserve the material’s ability to effectively convert absorbed sunlight into electrical energy and operate at peak efficiency,” he says.

While the new material is a promising candidate for the development of next-generation, high-efficient solar cells, the researchers acknowledge that further research will be needed before it can be integrated into existing solar energy systems. “We are now further exploring this family of intercalated materials and optimizing their efficiency via various materials engineering processes to this end,” Ekuma tells Physics World.

The study is detailed in Science Advances.

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Dark matter makes up about 85 percent of the universe’s total matter, and cosmologists believe it played a major role in the formation of galaxies. We know the location of this so-called galactic dark matter thanks to astronomical surveys that map how light from distant galaxies bends as it travels towards us. But so far, efforts to detect dark matter trapped within the Earth’s gravitational field have come up empty-handed, even though this type of dark matter – known as thermalized dark matter – should be present in greater quantities.

The problem is that thermalized dark matter travels much more slowly than galactic dark matter, meaning its energy may be too low for conventional instruments to detect. Physicists at the SLAC National Laboratory in the US have now proposed an alternative that involves searching for thermalized dark matter in an entirely new way, using quantum sensors made from superconducting quantum bits (qubits).

An entirely new approach

The idea for the new method came from SLAC’s Noah Kurinsky, who was working on re-designing transmon qubits as active sensors for photons and phonons. Transmon qubits needs to be cooled to temperatures near absolute zero (- 273 °C) before they become stable enough to store information, but even at these extremely low temperatures, energy often re-enters the system and disrupts the qubits’ quantum states. The unwanted energy is typically blamed on imperfect cooling apparatus or some source of heat in the environment, but it occurred to Kurinsky that it could have a much more interesting origin: “What if we actually have a perfectly cold system, and the reason we can’t cool it down effectively is because it’s constantly being bombarded by dark matter?”

While Kurinsky was pondering this novel possibility, his SLAC colleague Rebecca Leane was developing a new framework for calculating the expected density of dark matter inside Earth. According to these new calculations, which Leane performed with Anirban Das (now a postdoctoral researcher at Seoul National University, Korea), this local dark-matter density could be extremely high at the Earth’s surface – much higher than previously thought.

“Das and I had been discussing what possible low threshold devices could probe this high predicted dark matter density, but with little previous experience in this area, we turned to Kurinsky for vital input,” Leane explains. “Das then performed scattering calculations using new tools that allow the dark matter scattering rate to be calculated using the phonon (lattice vibration) structure of a given material.”

Low energy threshold

The researchers calculated that a quantum dark-matter sensor would activate at extremely low energies of just one thousandth of an electronvolt (1 meV). This threshold is much lower than that of any comparable dark matter detector, and it implies that a quantum dark-matter sensor could detect low-energy galactic dark matter as well as thermalized dark matter particles trapped around the Earth.

The researchers acknowledge that much work remains before such a detector ever sees the light of day. For one, they will have to identify the best material for making it. “We were looking at aluminium to start with, and that’s just because that’s probably the best characterized material that’s been used for detectors so far,” Leane says. “But it could turn out that for the sort of mass range we’re looking at, and the sort of detector we want to use, maybe there’s a better material.”

The researchers now aim to extend their results to a broader class of dark matter models. “On the experimental side, Kurinsky’s lab is testing the first round of purpose-built sensors that aim to build better models of quasiparticle generation, recombination and detection and study the thermalization dynamics of quasiparticles in qubits, something that is little understood,” Leane tells Physics World. “Quasiparticles in a superconductor seem to cool much less efficiently than previously thought, but as these dynamics are calibrated and modelled better, the results will become less uncertain and we may understand how to make more sensitive devices.”

The study is detailed in Physical Review Letters.

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Stacking layers of two-dimensional materials on top of each other and varying the twist angle between them massively alters their electronic properties. The trick is to get the twist angle just right, and to know when you’ve done so. Researchers in China have now developed a technique that helps with the second part of this challenge. By allowing scientists to directly visualize the variations in local twist angles, the new technique shed light on the electronic structure of twisted materials and accelerate the development of devices that exploit their properties.

Graphene (a 2D form of carbon just one atom thick) does not have an electronic band gap. Neither does a pair of graphene layers stacked on top of each other. However, if you add another 2D material called hexagonal boron nitride (hBN) to the stack, a band gap emerges. This is because the lattice constant of hBN – a measure of how its atoms are arranged – is nearly the same as that of graphene, but not exactly. The slightly mismatched layers of graphene and hBN form a larger structure known as a moiré superlattice, and the interactions between nearby atoms in this superlattice allow a gap to form. If the layers are then twisted so that they are further misaligned, the lattice interactions weaken, and the band gap disappears.

Achieving such changes in conventional materials usually requires scientists to alter the materials’ chemical composition. Varying the twist angle between layers of a 2D material is an entirely different approach, and the associated possibilities kickstarted a new field of device engineering known as twistronics. The problem is that twist angles are hard to control, and if different areas of a sample contain unevenly distributed twisted angles, the sample’s electronic properties will vary from location to location. This is far from ideal for high-performance devices, so researchers have been exploring ways to visualize such inhomogeneities more precisely.

A new method based on sMIM

In the new work, a team led by Hong-Jun Gao and Shiyu Zhu of the Institute of Physics, Chinese Academy of Sciences, adapted a method called scanning microwave impedance microscopy (sMIM) that was recently developed by Zhixun Shen and colleagues at Stanford University in the US. The adapted method involves applying a range of gate voltages to the sample and analysing conductivity fluctuations in the sMIM data at different positions in it. “This process provides the gate voltages corresponding to moiré band gaps, which are indicative of fully filled electronic bands, directly unveiling details about the moiré superlattice and local twist angles,” Zhu explains.

When the researchers tested this method on high-quality samples of twisted bilayer graphene fabricated by their colleagues Qianying Hu, Yang Xu and Jiawei Hu, they were able to detect variations of twist angles directly. They also gleaned information on the conductivity of localized areas, and they characterized other electronic states such as quantum Hall states and Chern insulators by applying out-of-plane magnetic fields. “We made these measurements concurrently,” Zhu notes. “This allowed us to directly obtain quantum state information under different local twist angle conditions.”

The new technique revealed pronounced variations in the local twist angles of around 0.3° over distances of several microns, he adds. It also enabled the team to measure local conductivity, which is not possible with alternative methods that use single-electron transistors to measure compressibility or nanoSQUIDs to measure magnetic fields. What is more, for samples of twisted bilayer graphene covered by an insulating BN layer, the new method has a significant advantage over conventional scanning tunnelling microscopy, as it can penetrate the insulating layer.

Exploring novel quantum states

“Our work has revealed the local twist angle variation within and between domains of a twisted two-dimensional material,” Zhu tells Physics World. “This has deepened our understanding of the microscopic state of the sample, allowing us to explain many experimental phenomena previously observed in ‘bulk-averaging’ measurements. It also provides a way to explore novel quantum states that are difficult to observe macroscopically, offering insights from a microscopic perspective.”

Thanks to these measurements, the unevenness of local twist angles in twisted two-dimensional materials should no longer be a hindrance to the study of novel quantum states, he adds. “Instead, thanks to the rich distribution of local twist angles we have observed, it should now be possible to simultaneously compare various quantum states under multiple local twist angle conditions and band structure conditions in a single sample.”

The researchers now aim to extend their technique to a wider range of twisted systems and heterostructure moiré systems – for example, in materials like twisted bilayer MoTe₂ and WSe₂/WS₂. They would also like to conduct bulk-averaging measurements and compare these results with local measurements using their new method.

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Researchers in the US and Canada have detected an effect known as quantum Barkhausen noise for the first time. The effect, which comes about thanks to the cooperative quantum tunnelling of a huge number of magnetic spins, may be the largest macroscopic quantum phenomena yet observed in the laboratory.

In the presence of a magnetic field, electron spins (or magnetic moments) in a ferromagnetic material all line up in the same direction – but not all at once. Instead, alignment occurs piecemeal, with different regions, or domains, falling into line at different times. These domains influence each other in a way that can be likened to an avalanche. Just as one clump of snow pushes on neighbouring clumps until the entire mass comes tumbling down, so does alignment spread through the domains until all spins point in the same direction.

One way of detecting this alignment process is to listen to it. In 1919, the physicist Heinrich Barkhausen did just that. By wrapping a coil around a magnetic material and attaching a loudspeaker to it, Barkhausen transformed changes in the magnetism of the domains into an audible crackling. Known today as Barkhausen noise, this crackling can be understood in purely classical terms as being caused by the thermal motion of the domain walls. Analogous noise phenomena and dynamics also exist in other systems, including earthquakes and photomultiplier tubes as well as avalanches.

Quantum Barkhausen noise

In principle, quantum mechanical effects can also produce Barkhausen noise. In this quantum version of Barkhausen noise, the spin flips occur as the particles tunnel through an energy barrier – a process known as quantum tunnelling – rather than by gaining enough energy to jump over it.

In the new work, which is detailed in PNAS, researchers led by Thomas Rosenbaum of the California Institute of Technology (Caltech) and Philip Stamp at the University of British Columbia (UBC) observed quantum Barkhausen noise in a crystalline quantum magnet cooled to temperatures near absolute zero (- 273 °C). Like Barkhausen in 1919, their detection relied on wrapping a coil around their sample. But instead of hooking the coil up to a loudspeaker, they measured jumps in its voltage as the electron spins flipped orientations. When groups of spins in different domains flipped, Barkhausen noise appeared as a series of voltage spikes.

The Caltech/UBC researchers attribute these spikes to quantum effects because they are not affected by a 600% increase in temperature. “If they were, then we would be in the classical, thermally activated regime,” Stamp says.

Rosenbaum adds that applying a magnetic field transverse to the axis of the spins has “profound effects” on the response, with the field acting like a quantum “knob” for the material. This, he says, is further evidence for the novel quantum nature of the Barkhausen noise. “Classical Barkhausen noise in magnetic systems has been known for over 100 years, but quantum Barkhausen noise, where domain walls tunnel through barriers rather than being thermally activated over them, has not, to the best of our knowledge, been seen before,” he says.

Co-tunnelling effects

Intriguingly, the researchers observed spin flips being driven by groups of tunnelling electrons interacting with each other. The mechanism for this “fascinating” co-tunnelling, they say, involves sections of domain walls known as plaquettes interacting with each other through long-range dipolar forces. These interactions produce correlations between different segments of the same wall, and they also nucleate avalanches on different domain walls simultaneously. The result is a mass cooperative tunnelling event that Stamp and Rosenbaum liken to a crowd of people behaving as a single unit.

“While dipolar forces have been observed to affect the dynamics of the motion of a single wall and drive self-organized criticality, in LiHo_xY_1-xF₄, long-range interactions cause correlations not just between different segments of the same wall, but actually nucleate avalanches on different domain walls simultaneously,” Rosenbaum says.

The result can only be explained as a cooperative macroscopic quantum (tunnelling phenomenon, Stamp says. “This is the first example ever seen in nature of a very large-scale cooperative quantum phenomenon, on the scale of 10¹⁵ spins (that is, a thousand billion billion),” he tells Physics World. “This is huge and is by far the largest macroscopic quantum phenomenon ever seen in the lab.”

Advanced detection skills

Even with billions of spins cascading at once, the researchers say the voltage signals they observed are very small. Indeed, it took them some time to develop the detection ability necessary to accumulate statistically significant data. On the theory side, they had to develop a new approach to investigate magnetic avalanches that had not been formulated previously.

They now hope to apply their technique to systems other than magnetic materials to find out whether such cooperative macroscopic quantum phenomena exist elsewhere.

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A novel experimental platform known as a giant quantum vortex mimics certain behaviours of black holes, giving scientists an opportunity to observe the physics of these astrophysical structures up close. The vortex appears in superfluid helium cooled to near-absolute zero temperatures, and according to the team that made it, studies of its dynamics could offer hints as to how cosmological black holes produce their characteristic rotating curved space–times.

Black holes exert huge gravitational forces on their surroundings, curving the fabric of space–time to an extent that is unprecedented among other structures we observe in the universe. These forces are so big that they drag the fabric of space–time around them as the black hole rotates, creating uniquely turbulent environments.

Such dramatic effects obviously cannot be studied in the laboratory, so researchers are exploring ways of making structures that mimic them. For example, gravity and fluid dynamics behave somewhat similarly if the viscosity of the fluid is extremely low, as is the case for liquid helium (a superfluid, meaning it flows with little or no friction) and clouds of cold atoms.

Vortex flows created in a kitchen blender

At near zero temperatures (less than –271 °C), liquid helium contains tiny swirling structures known as quantum vortices. Normally, these vortices stay apart, explains Patrik Svancara, a physicist at the University of Nottingham, UK. In the latest study, however, Svancara, co-team leader Silke Weinfurtner, and colleagues at King’s College London and Newcastle University managed to confine tens of thousands of these quanta in a compact object that resembles a tornado.

“The central part of our set-up is a spinning propeller that establishes a continuous circulating loop of superfluid helium, stabilizing the vortex formed above it,” Weinfurtner and Svancara explain. This set-up, they add, was inspired by researchers in Japan, who likewise produced giant vortex flows in a device that resembles a kitchen blender, rather than by placing the entire experimental apparatus on a rotating platform.

From ordinary fluids to superfluids

The researchers began their experiments with rotating fluids back in 2017, when they observed black hole-mimicking wave dynamics in a specially designed “bathtub” containing almost 2000 litres of water. “This was a breakthrough moment for understanding some of the bizarre phenomena that are often challenging, if not impossible, to study otherwise,” says Weinfurtner, a physicist at Nottingham’s Black Hole Laboratory, where the experiment was conceived and developed. “Now, with our more sophisticated experiment, we have taken this research to the next level, which could eventually lead us to predict how quantum fields behave in curved space–times around astrophysical black holes.”

Transitioning from classical fluids like water to quantum ones like superfluid helium was essential, Weinfurtner explains, because the superfluid’s viscosity is much smaller. Superfluids also display unique quantum-mechanical properties like the quantization of vortex strength, meaning that any vortex in superfluid helium must be composed of elementary quanta called quantum vortices. “Setting up large vortices like ours is challenging since individual quanta tend to move apart from each other, as Patrik mentioned,” Weinfurtner tells Physics World, “but we were able to stabilize vortex flows that accommodate tens of thousands of quanta in a compact region, [which] is a record-breaking value in the realm of quantum fluids.”

The new structure will help researchers simulate quantum field dynamics within complex rotating curved space-times, like black holes, and offer an alternative to the two-dimensional ultracold systems conventionally used in such studies until now, she adds.

“Leveraging advanced flow control techniques and high-resolution detection methods to detect the wave dynamics on the superfluid’s surface has allowed us to extract macroscopic flow structures and visualize intricate wave-vortex interactions,” she says. “These observations have revealed the presence of microscopic bound states and phenomena of black-hole-like ringing on the free surface of a giant quantum vortex, which we are currently investigating further.”

The researchers now plan to enhance the accuracy of their detection method and explore regimes in which the quantization of vortex strength becomes important. “This feature could influence the way black holes interact with their surroundings, potentially teaching us about the physics of black holes,” Svancara says.

The present work is detailed in Nature.

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Because there’s not much oxygen deep underground, the bacteria that live there have evolved other ways to get rid of the electrons they produce when they “breathe”. One of these workarounds involves sending out conductive filaments – nanowires – into the soil to disperse the electrons, but important details of this process have eluded biophysicists’ understanding.

Researchers at Yale University, US and NOVA University Lisbon in Portugal have now found that for bacteria in the genus Geobacter, a single protein family acts like a series of electrically connecting “plugs” for charging these microbial nanowires. The finding greatly simplifies the model of how these bacteria export electrons, and the team say this “minimal wiring machinery” may be common among bacterial species.

Bacteria that live in soil have two ways of donating the electrons they produce to external electron acceptors. The first involves transferring the electrons to soil minerals and is known as extracellular electron transfer (EET). The second, direct interspecies electron transfer (DIET), involves partner species. Both processes are vital for the microbes’ ability to survive and form communities, but they can be inefficient. Bacteria like Geobacter have therefore evolved to produce conductive nanowires that facilitate faster, long-range EET.

Five proteins

The protein family the Yale–NOVA team identified as key to the operation of these nanowires contains five proteins. All of them reside in the space between the bacteria’s inner and outer membrane – the bacterial periplasm – and they are known as periplasmic cytochrome ABCDE (PpcA-E). These proteins inject electrons into filaments on bacterial surfaces that act as nanowires, creating an electric connection for “metal breathing” Geobacter.

This electrical connection allows Geobacter to transfer excess electrons produced during metabolism to minerals in soil without the need for intermediaries, explains Yale’s Nikhil Malvankar, who co-led the study with Carlos Salgueiro at NOVA. In essence, the proteins act as plugs within a natural soil-based “electrical grid”. This grid may be responsible for allowing many types of microbes to survive and support life, the researchers say.

Microscopic pistons push filaments made of cytochromes

Though bacterial filaments were first observed in 2002, scientists initially thought they were made up of so-called pili proteins (“pili” means “hairs” in Latin). Many bacteria do have pili on their surface, and genetic data suggested these hairlike filaments could play a similar role in Geobacter, Malvankar says. In 2021, however, researchers in Malvankar’s lab solved the atomic structure of pili and showed that they instead act as pistons that push filaments made up of cytochromes. In addition, the atomic structures of cytochromes known as OmcS and OmcZ includes a chain of metal-containing heme molecules that carry electrons (red in the image above).

While these atomic structures explained how nanowires transport electrons, the connection between the nanowires and the bacteria’s surface remained a mystery, he adds. This is because most cell surfaces are electrically non-conducting.

“It was thought that another family of proteins embedded in the bacterial membrane, called porin cytochromes, was responsible for this connection despite bacteria being able to transmit electricity even in their absence,” Malvankar explains. “The presence of periplasmic proteins transferring electrons to nanowires eliminates the need for any intermediate electron carriers and explains how cells transmit electrons at a remarkably fast rate (a million electrons per second), even though electrons in proteins can move at rates at least 10 times slower.”

Working out the relationship between PpcA-E and OmcS

The researchers began by measuring the energy of electrons in OmcS. They found it was the same as in PpcA-E, which team member Catharine Shipps says was surprising because the OmcS measurement was expected to differ by 0.1 V. “At the time of the first measurements on OmcS (in 2011), we did not know that OmcS formed nanowires,” says Shipps, who performed this part of the work. “These previous measurements were made by treating the cytochromes as non-filamentous, something that could explain this large discrepancy.”

In 2015, Salgueiro and colleagues at NOVA hypothesised that PpcA-Es could transfer electrons to OmcS. However, testing this hypothesis was not feasible at the time because of the difficulty in obtaining purified OmcS nanowires. Malvankar says that Shipps’ finding added to the picture by suggesting that PpcA-E could donate electrons directly to OmcS – something that another team member, Vishok Srikanth, proposed after noticing that OmcS and PpcA-E stay together when extracted from bacteria. “All these results led us to propose that PpcA-E could pass electrons to nanowires,” he says. The two groups then confirmed their hypothesis using nuclear magnetic resonance spectroscopy.

“Our discovery greatly simplifies the model of how bacteria export electrons by overcoming slow electron flow among individual proteins,” Malvankar tells Physics World. “The discovery by another of our team members, Cong Shen, that this protein family is evolutionary and conserved across many species, not just Geobacter, means that this minimal wiring machinery could be ubiquitous in many bacteria.”

The researchers, who report their work in Nature Communications, are now engineering the newly discovered mechanism into bacteria that are important for the climate or capable of making biofuels. The aim is to help these beneficial organisms grow faster. “We are also working on how another nanowire of cytochrome OmcZ is charged and identifying the role of porin-cytochromes in these processes,” Malvankar says.

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A new imaging technique gives scientists the 3D position of individual atoms within an optical lattice for the first time, surpassing previous methods that provide only 2D images. Developed by a team at the University of Bonn, Germany and the University of Bristol, UK, the technique could improve the precision of atom-based quantum simulators and aid the development of new quantum materials.

“We are now able to take a single snapshot of the atoms in an optical lattice and see exactly where they are in all three dimensions,” explain Carrie Weidner and Andrea Alberti, who co-led the technique’s development. “Previous optical detection techniques were limited to taking ‘flat’ pictures of the atoms, but atoms do not live in a flat world.”

Experiments on atoms in optical lattices typically begin by using laser light to cool the atoms to temperatures just above absolute zero. This slows them almost to a halt and allows them to become trapped in a standing wave of laser light – the lattice. Once trapped, the atoms are exposed to an additional beam of laser light that makes them fluoresce. By imaging this fluorescence, researchers can determine the atoms’ position.

This imaging process is known as quantum gas microscopy, and it was developed more than a decade ago by physicists at Harvard University in the US and at the Max Planck Institute of Quantum Optics in Germany. The standard method, however, provides only the x and y coordinates of each atom. Information on the atoms’ position in the z direction – that is, their distance from the objective within the imaging system – was lacking.

Phase changing

The new method remedies this by taking the light emitted by the fluorescing atoms and modifying it before it reaches a camera. More specifically, the method changes the phase of the emitted light field so that the atom image appears to rotate in space as a function of its position along the imaging system’s line of sight.

“Instead of the typical round specks usually produced in quantum gas microscopy, the deformed wavefront produces a dumbbell shape on the camera that rotates around itself,” Alberti explains. “The direction in which this dumbbell points is dependent on the distance that the light had to travel from the atom to the camera.”

The dumbbell thus acts a bit like the needle on a compass, allowing researchers to read off the z coordinate according to its orientation, adds Dieter Meschede, who leads the Bonn laboratory where the experiments took place.

An idea with a long history

According to Weidner, the original idea for the study came from William Moerner and Rafael Piestun at the universities of Stanford and Colorado, respectively. Alberti adds that it is “fascinating” that no one else had previously thought of using the phase of the light field to obtain information about the z-position of the light-emitting particle. Controlling the phase of the light field is certainly not new, he says.

“It has actually a long history: in fact, to obtain sharp (and not blurry) images, all well-designed imaging systems are constructed to make the phase of all light rays reaching the camera surface (or the retina in our eyes) the same – this is the famous Fermat’s principle,” he explains. “Equalizing all of these phase differences is what corrects optical aberrations. This is essentially what we do when we wear eyeglasses to improve our vision.”

One of the biggest challenges with the technique, Alberti adds, was finding a capable experimenter who could work full-time to bring it to fruition. “We were lucky that Tangi Legrand, a master’s student, decided to take on this challenge,” he says. “Without him, we would not be reporting on our successful results today.”

Precise locations with a single image

Being able to precisely determine the 3D positions of atoms with a single image could be useful in several contexts. It could make it easier to trigger specific interactions between atoms, and it might help scientists develop new quantum materials with special characteristics. “We could investigate the types of quantum mechanical effects that occur when atoms are arranged in a certain order,” Weidner suggests. “This would allow us to simulate the properties of three-dimensional materials to some extent without having to synthesize them.”

A further advantage is that the technique, which is detailed in Physical Review A, is very general. “Our method can be applied to many systems, including molecules, ions, really, any quantum emitter,” Weidner says. “We hope to see this method applied in 3D quantum simulation efforts around the world.”

In the longer term, the researchers say their “dream” is to reconstruct the 3D positions of large arrays containing several thousand atoms. These large arrays require a large field of view, which entails optical aberrations, they explain. “We hope that improved reconstruction methods will be able to deal with these aberrations and therefore extend the field of view over which our technique can be applied,” they say. “They might also help find the 3D positions of atoms located above each other in more densely filled lattices.”

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The first technique capable of measuring the pull of gravity on a particle just microns in diameter could aid the quest for a quantum theory of gravity – a longstanding goal in physics. The new experiment uses a superconducting quantum interference device (SQUID) to detect the force on the particle at ultralow temperatures and suppresses vibrations that might interfere with motion due to gravity.

Gravity differs from the other fundamental forces because it describes a curvature in space-time rather than straightforward interactions between objects. This difference explains, in part, why theoretical physicists have long struggled to reconcile gravity (as described by Einstein’s general theory of relativity) with quantum mechanics. One of the main sticking points is that while the latter assumes space-time is fixed, the former states that it changes in the presence of massive objects. Since experiments to determine which description is correct are extremely difficult to perform, a theory of quantum gravity remains out of reach despite much theoretical effort in areas such as string theory and loop quantum gravity.

Meissner-state field expulsion

In the new work, which is reported in Science Advances, physicist Tjerk Oosterkamp of Leiden University in the Netherlands, together with colleagues at Southampton University, UK and Italy’s Institute for Photonics and Nanotechnologies, probed the boundary between gravity and quantum mechanics by studying the pull of gravity on a magnetic particle with a mass of just 0.43 milligrams – near the limit where quantum effects start to appear. To perform their study, they trapped the particle in a magnetic field generated by passing current through wires that become superconducting at temperatures below 100 millikelvin. The resulting magnetic field “landscape” causes the particle to levitate thanks to a well-known superconducting effect known as Meissner-state field expulsion in which the field arising from currents in the superconductor completely opposes the particle’s own magnetic field.

Once the particle was levitating, the researchers measured very small changes in the magnetic field that arise when it moves around its centre of mass. They did this using an integrated DC SQUID magnetometer while continuously tuning the frequency of the magnetic trapping potential. This enabled them to characterize the amplitude of the particle’s motion as a function of these frequency shifts.

Suppressing vibrations

The researchers then created a gravitational disturbance by rotating a heavy wheel just outside the refrigerator, or cryostat, that contained the experiment. The rotation frequency of the wheel was tuned to excite one of the vibration frequencies of the levitated particle. But before they could measure changes in the particle’s motion due to this gravitational disturbance, Oosterkamp and colleagues first had to make sure that other things that could set the particle moving – such as vibrations coming from the compressor and pumps responsible for cooling the superconductor – were very well suppressed.

“That turned out the be the most pressing challenge in our experiment,” explains Oosterkamp, “but once we had succeeded in doing this, the motion of the particle that remained turned out to be so small that it was disturbed by gravity – and we could actually measure this.”

Pushing the boundaries

Oosterkamp and colleagues originally intended to use their cryostat to cool and excite a mechanical resonator. “We were doing this to try and prove that it could be in two places simultaneously – much in the way that an electron can be when it shows interference effects passing through two slits,” Oosterkamp explains. “From the interference, one infers that the electron is a wave and goes through both slits at once. For our experiment, which has still a long way to go, we have been working on isolating vibrations to cool down a force sensor to observe the same type of effect for a tiny mechanical resonator.”

These initial experiments went so well, he recalls, that they asked themselves: what is the smallest force they could exert on the particle in their set-up to demonstrate the sensitivity of the experiment? “When we realized that gravity measurements were in reach, we were especially motivated,” Oosterkamp recalls.

Experiment needs to be even more sensitive

The next step, Oosterkamp says, is to bring gravitational and quantum effects even closer together. “Being able to measure the gravitational force from a particle that is in two places at once would be very desirable, but we need to make our experiment even more sensitive to do this and make measurements on heavier objects that show quantum effects – like superposition and entanglement, for example,” he says.

To this end, the researchers are working to replace the wheel outside their cryostat with a similar wheel or propeller inside it. “Instead of a wheel with kilogram-sized blocks on it and placed 30 cm away from the sensor, we hope to make milligram masses on a propeller that is just a centimetre away,” Oosterkamp says.

The team is also attempting to isolate external vibrations in their experiment even further and make their system colder. “These measures could improve measurement sensitivities by a 100-fold,” Oosterkamp says.

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Researchers in Japan and Korea claim to have found “conclusive evidence” for the existence of theoretically-proposed particles called Majorana fermions. The evidence for these long-sought-after particles appeared in the thermodynamic behaviour of a so-called Kitaev magnet, and the researchers say their observations cannot be explained by alternative theories.

Majorana fermions are named after the Italian physicist Ettore Majorana, who predicted their existence in 1937. These particles are unusual in that they are their own antiparticles, and in the early 2000s, the theoretical physicist Alexei Kitaev predicted that they could exist in the form of quasiparticles made up of two paired electrons.

These quasiparticles are known as non-Abelian anyons, and one of their main attractions is that they are robust to external perturbations. Specifically, Kitaev showed that, if used as quantum bits (or qubits), certain states would be “topologically protected”, meaning that they can’t be randomly flipped by external noise. This is important because such perturbations are one of the main stumbling blocks to making a practical, error-resistant quantum computer.

Kitaev later proposed that these Majorana states might be engineered as electronic defect states that occur at the ends of quantum nanowires made from a semiconductor located near a superconductor. Much subsequent work has therefore focused on looking for Majorana behaviour in semiconductor-superconductor heterostructures.

A different approach

In the latest study, researchers led by Takasada Shibauchi of the Department of Advanced Materials Science at the University of Tokyo, Japan, together with colleagues at the Korea Advanced Institute of Science and Technology (KAIST), took a different approach. Their work focuses on a material called α-RuCl₃, which is a potential “host” for Majorana fermions because it may belong to a class of materials known as Kitaev spin liquids (KSLs).

These materials are themselves a subtype of quantum spin liquids – solid magnetic materials that cannot arrange their magnetic moments (or spins) into a regular and stable pattern. This “frustrated” behaviour is very different from that of ordinary ferromagnets or antiferromagnets, which have spins that point in the same or alternating directions, respectively. In QSLs, the spins constantly change direction in a fluid-like way, even at ultracold temperatures.

To qualify as a KSL, a material must have a perfect (exactly solvable) two-dimensional honeycomb-shaped lattice, and the spins within this lattice must be coupled via unusual (Ising-type) exchange interactions. Such interactions are responsible for the magnetic properties of everyday materials such as iron, and they occur between pairs of identical particles such as electrons – with the effect of preventing the spins of neighbouring particles from pointing in the same direction. KSLs are thus said to suffer from “exchange-coupling” frustration.

In α-RuCl₃, which has a layered honeycomb structure, each Ru³⁺ ion (with an effective spin of -1/2) has three bonds. Shibauchi and colleagues explain that a cancelation of interactions between the two shortest Ru-Cl-Ru 90° paths leads to Ising interactions with the spin axis perpendicular to the plane that includes these two paths.

“The hallmark of Majorana excitations”

In their experiments, the researchers measured the heat capacity of a single crystal of α-RuCl₃ using a state-of-the-art high-resolution setup. This setup was contained in a dilution refrigerator equipped with a piezo-based two-axis rotator and a superconducting magnet that applies a rotating magnetic field to the sample’s honeycombed plane. These measurements revealed a topological edge mode in the material with a very peculiar dependence on the magnetic field angle. Specifically, the researchers found that at very low temperatures, the material’s heat capacity (a thermodynamic quantity) shows gapless excitations that change to gapped ones when the angle of the magnetic field is tilted by just a few degrees. This dependence on field angle is, they say, is characteristic of Majorana quasiparticle excitations.

“This is the hallmark of Majorana excitations expected in the spin liquid state, which was theoretically formulated by Kitaev in 2006,” Shibauchi tells Physics World. “We believe that this cannot be explained alternative pictures and thus provides conclusive evidence for these excitations.”

Shibauchi acknowledges that previous results of such measurements have been controversial because researchers found it hard to tell whether a phenomenon known as the half-integer quantum Hall effect – a signature of the Majorana edge mode – appeared or not. While some samples showed the effect, others did not, leading many to believe that a different phenomenon might be responsible. However, Shibauchi says the team’s novel approach, focusing on the angle-dependent gap closing feature specific to Majorana excitations, “addresses these challenges”.

Still a long road ahead

According to the researchers, the new results show that Majorana fermions can be excited in a spin liquid state of a magnetic insulator. “If one can find a way to manipulate these new quasiparticles (which will not be an easy task, that said), fault-tolerant topological quantum computations may be realized in the future,” Shibauchi says.

In their work, which is detailed in Science Advances, the researchers needed to apply a relatively high magnetic field to achieve the Kitaev spin liquid state that hosts the Majorana behaviour. They are now looking for alternative materials in which the Majorana state might appear at lower, or even zero, fields. Emilio Cobanera, a physicist at the SUNY Polytechnic Institute in New York who was not involved in the study, agrees that such materials are possible.

“Thanks to the detective work of Shibauchi and colleagues, we can add to the list the layers of the stable phase of RuCl₃ with confidence, and perhaps we are finally developing the experimental techniques and ingenuity to reveal anyons in many other materials,” he says. “In their work, the team had to differentiate between two exotic scenarios: the physics of the Kitaev honeycomb model on one hand, an exactly solvable model of anyons, and another piece of new physics, magnons associated to topologically non-trivial band structures.”

Cobanera points out that, as Shibauchi and colleagues themselves note, these two scenarios would yield very different predictions for the behaviour of the thermal Hall conductance under changes in direction of an applied, in-plane magnetic field. They therefore followed this observation with state-of-the-art mesoscopic thermal measurements that, Cobanera says, are clearly inconsistent with a magnonic explanation and support semi-quantitatively the scenario with anyons.

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Large-area solar cells made from hybrid perovskite materials have taken a step closer to commercialization thanks to researchers in Australia and the UK who fabricated the cells with industrial methods for the first time. Produced under ambient conditions using a technique known as roll-to-roll printing, the cells show relatively high power conversion efficiencies of up to 15.5% for individual small-area cells and 11% for serially-connected ones in large-area modules. According to the researchers, the cells would also be cheap to produce, with calculated costs dropping to $0.70 per watt once production hits 1 000 000 m² per year.

A perovskite material is termed “hybrid” when it contains both inorganic and organic components. Like all perovskites, hybrids have the chemical formula ABX₃, but in this case A is an organic cation, while B is lead and X can be iodide, bromide or another halide. Structurally, they contain a lead halide framework that is filled with small organic cations, and they show much promise for thin-film solar cells because their tuneable bandgaps allow them to absorb light over a broad range of solar-spectrum wavelengths.

“We have been working on printed organic solar cells for a long time, but the field of organic solar cells was advancing relatively slowly when perovskite solar cells emerged,” says Doojin Vak of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), who led the project together with colleagues from the University of Cambridge, Monash University and the University of New South Wales.

For researchers like Vak, the exciting thing about hybrid perovskite solar cells is that their power conversion efficiencies are, in principle, on par with those of established solar-cell materials such as silicon, gallium arsenide or cadmium telluride. In practice, however, high-efficiency hybrid perovskite solar cells have so far only been demonstrated in the laboratory. Making efficient large-area devices from these materials using industrial processes remains challenging.

An 11% efficiency on 50 cm² panels

In the latest work, Vak and colleagues showed that they could produce hybrid perovskite solar panels with efficiencies of 11% and an area of up to 50 cm² using roll-to-roll printing. This technique produces cells in a continuous process that resembles the way newspapers are printed, with successive coating, printing and drying stages transforming a roll of film at one end to a roll full of the finished product at the other.

Many industrial processes complete all these fabrication steps in a single pass. In this case, however, the researchers used multiple runs to fabricate their devices. They also replaced the vacuum-based metal electrodes conventionally employed in roll-to-roll printing with printed carbon electrodes that are compatible with perovskite materials.

Thanks to this adjustment, the team was able to fabricate and analyse more than 10 000 solar cells per day. This “high throughput” experiment allowed the researchers to rapidly identify optimal values for various processing parameters, which increased the efficiency of the final devices.

Prototypes for various applications

“We thought that perovskite solar cells could also be fully printed like organic-based ones and we have made good progress,” Vak says. “The last hurdle was eliminating vacuum-based back electrodes and we managed to achieve that goal in this work.”

The researchers say the modules they produced could be used as prototypes for testing in various applications.“While it is not at the level to be readily adopted in traditional fields where you would normally use mature solar technologies like silicon solar cells, we have identified applications and premium markets in which this technology will have a competitive advantage,” Vak says. “For example, we have been looking into space applications and have installed printed perovskite solar modules on a recently launched satellite.”

In this study, which is published in Nature Communications, the biggest solar modules the researchers fabricated measured 10 cm x 10 cm. While this is considered sizeable within academic research, it is still too small for real-world applications. The next step for the researchers is therefore to scale up their technique. “Fortunately, we have just completed the installation of a state-of-the-art printing facility for perovskite solar cells at CSIRO and we will be able to progress the technology with this new pilot-scale printer,” Vak tells Physics World.

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Roughly one in 12 main-sequence binary stars may have ingested a planet at some point in its past, say astronomers in Australia. This conclusion, which is based on new analyses of the chemical compositions of 91 pairs of such stars, implies that a significant fraction of planetary systems may be unstable – a conclusion that could, in turn, affect the probability of life developing there.

When a star engulfs a planet, its chemical makeup changes. To detect the chemical signatures of these so-called “planetary ingestion events”, astronomers compare the elemental composition of pairs of stars that were born at the same time. Because these “co-natal” stars formed from the same parent molecular core, they should, in theory, have the same chemistry. In reality, about 8% of them don’t – an anomaly the team attribute to one of the co-natal stars ingesting nearby planetary material sometime earlier in its life cycle.

High precision analysis

To reach this conclusion, the astronomers began by identifying 91 pairs of close co-natal stars – that is, those situated less than 10⁶ astronomical units apart – using the European Space Agency’s Gaia satellite. They then used spectral data from three major telescopes (the Magellan Telescope and the Very Large Telescope in Chile, plus the Keck Telescope in Hawaii, US) to examine, with high precision, differences in the pairs’ chemical compositions.

“Thanks to this very high precision analysis, we can see chemical differences between the twins,” says team member Fan Liu of Monash University. “This provides very strong evidence that one of the stars has swallowed planets or planetary material and changed its composition.”

The stars the team studied were all so-called main sequence stars in their prime, not red giants approaching the end of their lives. This is an important distinction because red giants are known to engulf nearby planets as they expand, but ingestion events were thought to be less common for younger stars. “Astronomers think that seeing these kinds of events is possible but they don’t expect us to be able to observe them in such a high occurrence rate,” explains team member Yuan-Sen Ting of the Australian National University (ANU). “But from the observations in our study, we can see that, while the occurrence is not high, it is actually possible. This opens a new window for planet evolution theorists to study.”

While planetary ingestion may seem far-fetched, the astronomers found that it matched their observations better than alternative hypotheses they considered. “As Sherlock Holmes says: when you have eliminated the impossible, whatever remains, however improbable, must be the truth,” Ting observes.

“An unsettling truth”

According to the team, the results from the study, which is detailed in Nature, could provide new constraints on how connections between stellar and planetary chemistry form and evolve. More importantly, though, the team believe the findings could have far-reaching implications for theories of planet formation.

“Another key point of excitement (and perhaps an unsettling truth) is that if a significant fraction of planetary systems are unstable, it suggests that our stable solar system might not be the norm,” Ting tells Physics World. “This gives us a greater appreciation for our unique – and fragile – position in the universe.”

An unlikely pilot study

The new study is part of a larger collaboration called the Complete Census of Co-moving Pairs of Objects (C3PO). The aim of this project, which began when Ting was at Princeton University and the Carnegie Observatories in the US, is to spectroscopically observe a complete sample of all bright co-moving stars. “Although I am mostly a theorist at heart, through work with a student, which I co-supervised with my PhD advisor at Harvard, we unexpectedly found that stars that are co-moving are also born together,” Ting explains. “This led me to think that if this is true, it would greatly expand the candidates we can study, as such studies [of co-natal stars] were mostly done with gravitationally bound binaries, which are much rarer.”

Despite promising results from theory and simulations, investigating this hypothesis observationally was a high-risk endeavour, Ting says, and it came about in an unusual way. “By chance, one of the largest telescopes was undersubscribed, so we were asked to submit some ‘interesting ideas’,” he says. “Within a day, I submitted this idea, with a view to conducting a pilot study. We argued that since this was extra time, it was an opportunity to try something bold.

“That the telescope time allocation committee put their trust in me, despite the fact that I am a theorist with zero observational experience, was a boon,” Ting adds.

Searching for more planet-eaters

Spurred on by the success of the pilot, Ting moved to Australia. There, he was joined by Liu and another ANU astronomer, David Yong, who took the project to the next level. “We applied for a larger programme, asking for significantly more telescope time,” he says, “but all of this really started with a small spark and a brief discussion with students – could we prove that stars moving together are also co-natal?”

The team now hope to expand the number of planet-ingesting star candidates to analyse – something that might require even more intensive telescope resources. “Theoretically, we also need a better understanding of the conditions under which a planetary system might not be stable, something that is widely speculated but not yet fully understood,” Ting adds. “Some AI tools that I am currently developing might lead to better insights into this problem, so stay tuned.”

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An international team of researchers has identified quasiparticles called merons in a synthetic antiferromagnet for the first time. The result could lead to new concepts for spintronics devices, which use the electron’s magnetic moment, or spin, to store and process information.

Scientists seek to exploit electron spins in this way because spintronics-based computer memory devices would be faster and more compact than today’s purely electronic ones. The question of how best to build such devices has, as yet, no definitive answer, but much recent research has focused on structures called skyrmions as potential building blocks. These structures are quasiparticles made up of numerous electron spins and can be thought of as two-dimensional whirls (or “spin textures”) within a material.

Skyrmions exist in many magnetic materials, including cobalt–iron–silicon and the manganese–silicide thin films in which they were first discovered. They are attractive spintronics candidates because they are robust to external perturbations, making them particularly stable for storing and processing the information they contain. At just tens of nanometres across, they are also much smaller than the magnetic domains used to encode data in today’s disk drives, making them ideal for future data storage technologies such as “racetrack” memories.

Like skyrmions, merons are made up of numerous individual spins. Unlike them, their stray magnetic fields are miniscule, which would facilitate ultrafast operations and even higher information storage densities within a device. Until now, however, merons have only been observed in natural antiferromagnets, where they have proved difficult to analyse and manipulate.

Minimal net magnetic moments

Researchers at Johannes Gutenberg University Mainz (JGU) in Germany; Tohoku University, Japan; and the ALBA Synchrotron Light Facility in Spain have now identified merons in synthetic antiferromagnets made from multilayer stacks of mutually coupled individual ferromagnetic layers. Unlike natural antiferromagnets, these synthetic materials can be prepared in a well-controlled way using established techniques such as sputter deposition.

This exquisite control enabled the team to adjust how the different layers interact, and thereby minimize their net magnetic moments. This gives the system advantages of both antiferromagnets (in which electron spins tend to align antiparallel to each other) and ferromagnets (which have parallel electron spins). Examples include not only low stray magnetic fields, but also stable homochiral textures and fast spin dynamics within a polycrystalline setting, explains Mona Bhukta, a PhD student at JGU and the study’s co-leader.

“In our work, we have successfully stabilized these spin textures in synthetic antiferromagnets with a very small easy-plane anisotropy (so that the preferred orientation of the magnetization lies in the film plane) and imaged their intricate structures by combining several imaging methods,” Bukhta says. The methods they used included magnetic force microscopy and scanning electron microscopy with polarization analysis as well as element-specific photoemission electron microscopy using X-ray magnetic circular dichroism.

Thanks to these imaging techniques, the team identified multiple different spin textures in the stacked material. This was not easy, as the researchers had to image the quasiparticles in a way that resolves all three components of the magnetization vector before they could unequivocally demonstrate the presence of merons. The researchers also developed an analytical model to elucidate the mechanisms that stabilize such structures in their system. The goal in this case was to determine the optimal thickness of each layer and identify the best “host materials” for merons.

Related structures also observed

As well as identifying merons, the team also observed related structures such as antimerons and topologically stabilized bimerons in their synthetic antiferromagnets. Unlike in skyrmions, the direction of the net magnetization and the emergent field produced by bimerons are mutually orthogonal, Bhukta explains.

“This characteristic feature enables us, for example, to directly probe and manipulate the topological Hall effect using the meron spin textures,” she tells Physics World. This effect occurs when electrons flow through a conductor in the presence of a magnetic field. The applied magnetic field exerts a sideways force on the electrons, leading to a voltage difference that is proportional to the strength of the field. If the conductor has an internal magnetic field or magnetic spin texture, this also affects the electrons.

“The Hall signals from bimerons provide a direct means of detecting and quantifying topology, offering us the exciting possibility to develop magnetic-topology-based technologies in which topology serves as the carrier of information,” Bhukta says.

The researchers, who detail their work in Nature Communications, now plan to investigate the interaction between merons and external magnetic fields and electrical currents. “We would also like to study how they interact among themselves,” Bhukta says.

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A team led by microelectronics expert Yongjin Wang of the Nanjing University of Posts and Telecommunications and Suzhou Lighting Chip Monolithic Optoelectronics Technology Co. Ltd. has now done just that by employing four different light sources to establish simultaneous wireless light communications links in any of these environments. “Our new wireless network enables uninterrupted connectivity across environments, facilitating two-way real-time data transmission between the network nodes that carry out communication and data exchange within and between networks,” Wang says. 

Four full-duplex wireless light communication links  

For the underwater portion of their network, the researchers chose blue light because seawater absorbs less in this part of the electromagnetic spectrum, meaning light can travel further. To communicate with airborne device such as drones, they used deep ultraviolet light because it provides “solar-blind” communication with no interference from sunlight. For other air-based applications, they used wireless white light communication, while for point-to-point communications in free space they selected near-infrared laser diodes. These diodes emit light in one direction with high optical power, again allowing the signals to travel further. 

“Our network consists of these four full-duplex wireless light communication links, which are connected in series via Ethernet switches,” explains Wang. “Both wired and wireless access to the all-light communication network is also possible, providing flexible connectivity options.”  

Separating the different light bands also prevents signals from interfering, meaning the network can transmit many signals simultaneously without compromising performance, Wang says. The network can be connected to the Internet via a modem, granting people in remote ocean locations, for example, access to the backbone network for information sharing. It also allows video conferencing and other transmissions via the widely-used TCP/IP (Transition Control Protocol/Internet Protocol) suite, he adds, making it suitable for Internet of Things applications, too. “For example, when a 2560 × 1440-pixel online video at 22 frames per second are fed into the network, users accessing the network from any node can visit this video with little lag,” he tells Physics World.  

From a single communication system to a network  

According to Wang and colleagues, the all-light communication network is a “major breakthrough”, one that should make it possible to transition from single wireless light communication systems to a network of them. Such a network would resist electromagnetic interference (EMI), making it particularly attractive for communicating with underwater equipment and drone clusters. “This is why are we working on integrating mobile nodes in the network, rather than fixed nodes, as is presently the case,” explains Wang. “This will not be easy, however, since it will require tacking the challenge of ‘light alignment’ and network establishment speed.”  

The researchers, who describe the new network in Optics Express, also plan to enhance the throughput of their communication network by using a technique called wavelength division multiplexing. This, they say, will improve the network’s overall efficiency and performance by eliminating the delays associated with using near-infrared laser diodes. 

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Researchers in Germany and the US have found a new explanation for why soft solids stick to surfaces easily but are difficult to remove. Though experts had long hypothesized that various chemical processes and material-specific properties could play a role in this so-called adhesive hysteresis, a team at the universities of Freiburg, Pittsburgh and Akron has now shown that surface roughness alone is enough to account for it. According to the team, this finding could fundamentally change the way we think about the stickiness of soft materials.

If you’ve ever found it easy to stick an object to something, but almost impossible to get it off it once it’s stuck, you’ve observed adhesive hysteresis in action. “Any soft material will show this hysteresis when making contact,” explains team co-leader Lars Pastewka, a physicist in the department of microsystems engineering at Freiburg. “Scotch tape and sticky notes attach easily but are difficult to detach.”

In 1966, scientists seeking to explain this behaviour developed a rule of thumb called the Dahlquist criterion. This criterion states that if a material is very soft – which Pastewka says is sometimes translated as requiring a Young’s modulus of less than 0.1 MPa – it will “bond” when pushed into contact, and it will maintain this “bond” when released.

In the new study, Pastewka says, “We show that there is no real ‘bond’, but that roughness pins the contact line, yielding a physical explanation for the Dahlquist criterion.”

“Stick-slip” instabilities dissipate energy

To reach this conclusion, Pastewka and colleagues at Freiburg and the livMatS Cluster of Excellence developed models that weave together different strands of engineering and physics. These strands include standard contact and fracture mechanics as well as more abstract research on elastic lines in random media (a topic that lies within the branch of physics dealing with complex systems). The results of these models showed discrete “jumps” known as stick-slip instabilities occurring when the perimeters of elastic bodies contact each other.

These stick-slip instabilities dissipate energy and lead to hysteresis, and Pastewka says that his theory and modelling group at Freiburg hypothesized that they could also play a role in adhesion. “To confirm this, we asked our experimental colleagues at Akron to check their measurements,” he says. “They also saw these jumps.”

Past hypotheses

Scientists had previously suggested that adhesion hysteresis in soft solids might be caused by the dissipation of viscoelastic energy – that is, energy lost to heat when a material deforms during contact. If a material compresses during contact and expands during release, these energy losses would counteract the movement of the contact surface, increasing the adhesive force during separation.

Another explanation centred on a process called contact ageing, which involves the formation of chemical bonds on the contact surface. Under this hypothesis, the longer the contact exists, the greater the adhesion will be.

Though both explanations sound physically plausible, “Our simulations show that the observed hysteresis can be explained without these specific energy dissipation mechanisms,” says Antoine Sanner, a postdoctoral researcher at Freiburg who did the bulk of the study’s theoretical work. “The only source of energy dissipation in our numerical model is the sudden jumping movement of the edge of the contact, which is induced by the roughness of the surface.”

Simplifying the design of adhesives

Because material systems designed to be sticky are often also designed to be viscoelastic, Pastewka says the new work may simplify the design of (reversible) adhesives. Such adhesives might be employed in the locomotion of soft robots, where there is a need to control the load-bearing capacity of the robots’ contacting limbs. Another application might be pick-and-place systems for manufacturing plants, which are increasingly relying on soft robotics.

The processes described in this study are also affected by interfacial water bridges, and the researchers say they are now exploring the influence of water on adhesion – particularly in the form of capillary adhesions. “Since water is ubiquitous, I believe most adhesive joints are at least to a certain extent mediated by water,” Pastewka says. “We may therefore be able to construct similar (and even simpler) models for capillaries at interfaces.”

All of this is a somewhat surprising outcome for a research project which, according to Pastewka, originally focused on triboelectricity — the phenomenon whereby surfaces in contact with each other become charged. This effect can be exploited for energy harvesting, and it is also related to the processes that charge up clouds during thunderstorms and produce lightning. “Previous research has shown that charge occurs in specific patterns on interfaces, and we thought that thus may be related to how interfaces detach,” Pastewka tells Physics World. “This is why we decided to look into the details of the detachment processes and found the stick-slip instabilities.”

The work is detailed in Science Advances.

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Space weather events known as whistler mode chorus emissions have been observed in the laboratory for the first time. These emissions occur naturally within regions of space dominated by planetary magnetic fields – magnetospheres – and they are related to the aurorae that light up our northern and southern skies every winter. However, their exact origins are poorly understood, and until now, studying them has involved either spacecraft observations or numerical simulations. By recreating the conditions that produce these emissions, researchers at Japan’s National Institute for Fusion Science and the University of Tokyo hope to better understand them and how they affect orbiting satellites as well as ground-based power and communication networks.

Whistler mode chorus emissions are intense, coherent waves that produce and transport high-energy electrons through planetary magnetospheres. They get their name because their frequencies vary repeatedly in a way that reminded early researchers of the “dawn chorus” of birdsong. These plasma waves have been observed in Jupiter’s magnetosphere and in the region affected by Earth’s magnetic field, but never before under controlled conditions in a laboratory.

Recreating magnetosphere-type plasmas

The first task for team leaders Haruhiko Saitoh and Zensho Yoshida was to create a suitably magnetosphere-mimicking magnetic field. The most fundamental type of magnetic field that forms in planetary magnetospheres is a dipole field, and at the University of Tokyo’s Ring Trap 1 (RT-1) facility, this type of field is commonly used to stably confine plasmas for advanced fusion experiments.

In their work, which they describe in Nature Communications, Saitoh and colleagues generated this field using a 110-kg magnetically levitated superconducting coil located within the RT-1’s vacuum vessel. By filling the vacuum vessel with hydrogen gas and exciting the gas with microwaves, they created a high-quality hydrogen plasma containing electrons heated to high temperatures. “Creating an environment similar to the magnetosphere in the laboratory was challenging,” Saitoh tells Physics World, “but RT-1 is able to achieve this thanks to the levitating superconducting coil in a vacuum chamber.”

Chorus emissions could be a universal phenomenon

The researchers used magnetic probes to study how the plasma – including the hot electron component – fluctuates. They found that the plasma spontaneously produced whistler wave chorus emissions whenever it contained a significant proportion of high-temperature electrons. These electrons are responsible for the plasma’s pressure, and the team observed that increasing their number drives the generation of chorus emissions.

According to the researchers, this result suggests that chorus emissions are a universal phenomenon in plasmas that contain high-temperature electrons within a simple dipole magnetic field. Plasmas of this type are common in the geospace, which the team define as “the space around the Earth that is particularly closely linked to human activities”. As such activities intensify, they note, the study of magnetospheric disturbances capable of causing aurorae, as well as power and communication failures, becomes more important. “Chorus emissions are important for understanding and potentially mitigating these effects,” they say.

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