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Stacking order in a 2D magnet produces Dirac magnons

Three years ago, an international team of researchers observed something unexpected in a sample of chromium triiodide (CrI3): quasiparticles known as magnons appeared to be travelling along its edges, rather than moving through the sample’s bulk. This observation suggested that this two-dimensional layered magnetic material acts as a so-called topological magnon insulator – an unusual property with important applications in the field of dissipationless spintronics. But though the result sparked a flurry of interest in CrI3, the question of why the material behaves in this way remained unresolved.

Thanks to detailed neutron scattering measurements and extensive analyses, the same team has now found that the curious properties of CrI3 stem from the way its layers are stacked. Although single-layer CrI3 and the bulk material are ferromagnetic, two stacked layers are antiferromagnetic, meaning that the material’s magnetic moments point in opposite directions – a superficially simple difference with far-reaching consequences.

2D materials and topological insulators

Two-dimensional (2D) materials like CrI3 are made up of atomically thin layers stacked on top of each other. These layers are held together by weak van der Waals forces and the electrons in them behave very differently from those in bulk materials. For example, electrons in graphene, one of the best-known 2D materials, can move at almost relativistic speeds and behave as if they are massless.

Some 2D materials are also topological insulators – that is, materials in which electrons flow freely along the edges of a 2D sheet but cannot flow along the surface. This effect is related to the spin of the electrons, making these materials promising for spintronic devices, which store and process information using the electrons’ spin states.

Magnons for spintronics

Certain 2D magnetic materials are also predicted to be magnetic and topological insulators. In these materials, which are very rare, quasiparticles known as magnons travel along the edges of a sheet, much like electrons in a conventional 2D topological insulator. Magnons are collective oscillations of the spin magnetic moments of a material and they are expected to be massless (Dirac) particles too, meaning that they can travel over long distances without dissipating. This property would make them interesting for spintronics applications as well.

In the earlier experiments, Lebing Chen, a condensed-matter physicist at Rice University in the US, together with colleagues in the UK and South Korea, found such Dirac magnons in CrI3. The team came to this conclusion by studying the material using inelastic neutron scattering at the Spallation Neutron Source at Oak Ridge National Laboratory. In this technique, neutrons, which have magnetic moments, create magnons when they scatter from a sheet of CrI3, which has a honeycomb lattice structure much like graphene. By measuring the energy lost by the neutrons during the scattering process, Chen and colleagues were able to calculate the properties of these magnons. They found that they exhibit properties consistent with them being topological and having a dissipation-less edge mode.

Spin-orbit coupling

In the new work, described in Physical Review X, the researchers performed further neutron-scattering measurements with much greater precision and resolution. These results showed that the magnons’ topological properties arise thanks to spin-orbit coupling – a relativistic interaction between an electron’s spin and its motion. This coupling induces asymmetric interactions between spin of electrons in the materials, explains Elton Santos of the University of Edinburgh’s Higgs Centre, Jae-Ho Chung of Korea University’s Department of Physics and Rice’s Pengcheng Dai, who led the new study. These interactions make the spin “feel” the magnetic field differently, affecting their topological excitations. “Surprisingly the spins have some chirality, like in a mirror where left and right can work differently,” Santos says. “We observed that without such chiral interactions, or, in more complicated terms, Dzyaloshinskii-Moriya exchange, we can’t describe the data.”

Chung adds that the result has an accompanying magnetic phenomenon too: a stacking-dependent magnetic order in which a single layer of CrI3 is ferromagnetic but two stacked layers are antiferromagnetic. “The reason for this behaviour is that the interaction between stacked layers in CrI3 is a combination of ferromagnetic and antiferromagnetic exchanges – despite apparent ferromagnetic stacking.”

“Our new work also confirms the previously observed topological nature of the spin excitation based on the Dzyaloshinkii-Moriya exchange, and rules out the competing interpretation based on the Kitaev exchange,” Dai tells Physics World. “The latter is known to be an important spin-spin interaction in more complex materials, like spin liquids, but apparently not for CrI3. This came as a surprise to us.”

Meteor strike may have destroyed Sodom, collective blob motion, asteroid nuclear impact

According to book of Genesis in the Bible, the city of Sodom was destroyed by God because of the wickedness of its people. While there are several historical sites that could have been Sodom – and some scientists have suggested that the city could have been destroyed by a natural event such as a meteor strike – the story is widely regarded as mythical.

Now, the geologist Sid Mitra at East Carolina University in the US and colleagues have come up with an explanation of where Sodom was and what happened to the city.  They have focused their attention on a Middle Bronze Age city called Tall el-Hammam, which is in the Jordan valley. In 2005 archaeologists discovered a 1.5 m thick layer of debris and destruction in the city that comprises materials that have been subjected to intense heat.

“They found all this evidence of high-temperature burning throughout the entire site,” says Mitra. “And the technology didn’t exist at that time, in the Middle Bronze Age, for people to be able to generate fires of that kind of temperature.”

Ten nuclear bombs

The archaeologists hypothesized that the city was destroyed by an airburst meteor strike like the 1908 Tunguska event, which flattened forests across a swathe of Russia with the energy of 10 Hiroshima-sized nuclear bombs.

To test this idea, scientists in different disciplines have joined forces to study materials found at the site. Mitra is an expert in the analysis of soot and discovered that a large fraction of the organic carbon at Tall el-Hammam is in the form of soot. This, he says, points to a very high temperature fire at the site – something that could have been caused by a meteor.

Evidence studied by other scientists include diamond-like carbon, melted pottery and other materials affected by high temperatures and a pressure shock – which would have been delivered by an airburst meteor.

Mitra and colleagues describe their findings in Scientific Reports and speculate that this catastrophic event 3600 years ago may have been recounted in local oral tradition and then made its way into the Bible. The event could also explain why the region around Tall el-Hammam was abandoned for many years. It could even be the origin of another biblical story: the destruction of the walls of Jericho, which is about 22 km from Tall el-Hammam.

Collective motion

The 1950s science-fiction horror film The Blob features a growing, corrosive, alien entity that envelops everything in its path. Thankfully, researchers haven’t quite managed to recreate that but it is known that blackworms (Lumbriculus variegatus) can aggregate into “blobs” that are capable of collective movement.

Measuring up to 10 cm long, blackworms live in ponds or marshes in Europe and North America. To protect themselves from drought, they merge as entangled, shape-shifting blobs that contain about 100 individuals. The blob then moves to seek out cooler climes.

Researchers have now discovered that the collective movement of the blob can only emerge when there is a fine balance between their individual movement and how well they cling to each other.

The team says the results could be applied to the design of individual soft and flexible robots that entangle and move as a unit, which sounds like the basis for another science-fiction horror film.

And finally, scientists have carried out computer simulations showing that a nuclear bomb could be used against an Earth-threatening asteroid.

Such a scenario – as made famous by the films Armageddon and Deep Impact – could be averted thanks to a 1 megatonne nuclear bomb that if ignited near the surface of a 100 m-long asteroid two months before potential impact would result in up to 99% of the total mass of the asteroid missing Earth.

OzGrav hunts faint signals from continuous gravitational waves

The LIGO–Virgo observatories are three kilometre-scale interferometers that detect tiny spatial displacements (10−18 m) that occur when gravitational waves – ripples in space–time – pass through Earth. The detectors are famous for detecting short, intense pulses of gravitational waves that are emitted in the final moments before pairs of black holes or neutron stars merge. But astrophysicists also want to use these huge facilities to observe much fainter continuous signals from objects such as rotating neutron stars. Meg Millhouse and Lucy Strang at the University of Melbourne and Karl Wette at the Australian National University are members of the Australian Research Council’s Center of Excellence for Gravitational Wave Discovery, or OzGrav, and they talk about their search for continuous gravitational waves.

Who belongs to OzGrav and what are its primary goals?

Karl Wette: OzGrav was funded by the Australian government in 2017 for seven years, with a mission to join with international collaborators in leading the exciting new field of gravitational astronomy, and to inspire the next generation of Australian scientists. It’s made a big difference to the field in Australia. When I did my PhD at Australian National University, well before OzGrav was funded, the number of people working on gravitational waves was a lot smaller, and I was one of the few people working on data analysis. We now have well over 100 students and postdocs working across instrumentation, data analysis, interpretation and astrophysics. We’ve recently submitted a proposal for a new centre to succeed OzGrav, and hopefully that gets funded so that we can continue to grow this new field Down Under.

Why do you expect neutron stars to emit continuous gravitational wave signals?

Meg Millhouse: For a neutron star to emit continuous gravitational waves it must have a time-varying quadrupole moment in its mass. In simple terms the star must be lumpy – not a perfect sphere – and it must be rotating. We believe that neutron stars have rigid crusts and previous research suggests that the crust could be deformed to create “mountains” on the surface. This is actually an overstatement because the deformities are thought to be on the order of millimetres – but that could be enough to create gravitational waves that we can detect. 

We also know that many neutron stars rotate, because we can observe the pulses of radiation that they emit. It is these pulsars that we are studying.

In your latest study, you’ve targeted 15 neutron stars that have recently formed in supernovae. What’s special about these objects?

Lucy Strang: We expect young neutron stars to be lumpier and therefore emit more intense gravitational waves than older stars. Neutron stars are created in supernovae, so we target young supernovae remnants because these are relatively easy to find. 

Aerial view of the Virgo interferometer
Big detector Aerial view of the Virgo interferometer in Italy, which has 3 km long arms. (Courtesy: LIGO–Virgo)

The LIGO–Virgo detectors are kilometre-scale interferometers. How do you point them at neutron stars? 

LS: We can’t point the LIGO–Virgo detectors at specific parts of the sky – they observe the entire sky. Instead, we have a series of mathematical transformations on LIGO–Virgo data to look for signals that could correspond to the neutron stars we are interested in. So in a sense we are pointing a telescope using mathematics.

Are the signals that you are looking for different from the gravitational-wave signals that have already been seen from merging black holes and neutron stars?

KW: Mergers of black holes and neutron stars are violent events that produce short-lived and relatively intense pulses of gravitational waves. The continuous signals that we expect from slightly deformed and rotating neutron stars are much weaker – more like a very low background hum. While the low intensity makes the signals difficult to find, their continuous nature gives us an advantage. The signals are always humming in the background, so if we keep observing and analysing data over a long period of time, we may be able to extract the weak signals. 

Ideally the search would require vast amounts of computing power – but this just isn’t available to us. So, as well as being important from an astrophysical perspective, our research is also a big data challenge – which is very exciting to work on.

Is LIGO–Virgo very sensitive to the gravitational-wave frequencies that you expect from rotating neutron stars?

KW: Most of the pulsars that we know of spin at about 1 Hz or slower. Unfortunately, LIGO–Virgo is not very sensitive to signals at these low frequencies, where seismic interference from things like human activity are a problem. Instead, we focus on neutron stars spinning at hundreds of hertz, where the detectors are much more sensitive. There are some neutron stars that fit the bill and we hope that we can see continuous waves from them

Have you spotted any continuous wave signals so far?

LS: Sadly, no – but that is not surprising because we know that there is currently a low probability of making a detection. However, we have established an upper limit on the strength of signals from neutron stars and that has allowed us to put constraints on some properties of neutron stars.

What sort of constraints?

KW: One thing we are interested in are r-modes. These are like giant ocean waves on the surface of a rotating neutron star, which could broadcast relatively intense gravitational waves. X-ray observations of the pulsar J0537-6910, for example, provide strong evidence that the neutron star is radiating gravitational waves via r-modes. 

However, we haven’t seen such waves and that allows us to exclude certain models of r-mode emissions. This translates directly into a better understanding of the neutron star equation of state, which relates the radii of neutron stars to their masses. Even though we have not made a detection, we can already say something important about neutron-star physics.

Even though we have not made a detection, we can already say something important about neutron-star physics

LS: Another thing we can constrain is the shape of the neutron star. We talked earlier about how we expect lumpy (or, to put it another way, non-spherical) neutron stars to produce continuous waves. The less spherical and more lumpy the neutron star is, the louder we expect the signal to be. By constraining the size of the signal, we’re also constraining how non-spherical the neutron star must be.

As with the r-modes Karl just discussed, this restriction can be translated into constraints on the neutron star equation of state. The equation of state contains the fundamental physics governing the neutron star and determines its mass, radius and so on. The conditions inside a neutron star are impossible to replicate in a laboratory, so astrophysical observations are our only window into physics in these extreme conditions. With each limit we set, we get a step closer to understanding the underlying physics.

You are targeting neutron stars created in supernovae, but what if you are lucky enough to observe a nearby supernova?

MM: There are several different gravitational-wave signals that could come from a supernova. The explosion itself would create a short burst of gravitational waves, which would be a very exciting thing to detect. The astronomy community could do multimessenger observations of the event, capturing electromagnetic radiation, and possibly neutrinos, along with gravitational waves. This would give us a wealth of information about supernovae.

If there is a neutron star remnant present after the explosion, it should emit continuous gravitational waves. These could be difficult to detect because we expect that the rotational speed would be decreasing very quickly. If we don’t know what frequency we are looking for or how fast the star is spinning down, it can be a tricky observation. 

LIGO–Virgo scans the entire cosmos. Are you also looking for neutron stars that we don’t already know about?

KW: Yes, that’s another strategy that we are pursuing – eyes wide open surveys that scan the entire sky over a range of signal frequencies. Estimates suggest there are approximately one billion neutron stars in the Milky Way, but we only observe a few thousand of them as pulsars. We hope that maybe a few of the billion are radiating gravitational waves that we can detect, and that would provide new insights into neutron stars. 

We’re still looking at some of the data from the latest LIGO–Virgo observations and we are hoping that we will soon have more results to report. Next year will bring further upgrades to the LIGO–Virgo detectors, which will make them more sensitive to continuous waves. So, this is an ongoing story.

Atoms and molecules make vortex beams

A wave-like property previously only seen in beams of light and electrons has been observed for the first time in atoms and molecules. By passing beams of helium and neon through a grid of specially shaped nanoslits, researchers led by Edvardas Narevicius of Israel’s Weizmann Institute of Science succeeded in giving the beams a non-zero orbital angular momentum (OAM). The resulting structures are known vortex beams, and they could be used for fundamental physics studies such as probing the internal structure of protons.

Many natural systems contain vortices – think of tornadoes and ocean eddies on Earth, the red spot on Jupiter and gravitational vortices around black holes. On all scales, such vortices are characterized by the circulation of a flux around an axis. In the quantum world, these swirling structures are found in ensembles of particles that can be described by a wavefunction, including superfluids and Bose-Einstein condensates.

Twisted wavefronts

When the phase of this wavefunction, or wavefront, varies in a corkscrew fashion around an axis as a beam propagates through space, the wavefront is described as being twisted, and the beam carries OAM. Since elementary particles also carry OAM, researchers have, in recent years, succeeded in generating OAM-bearing vortex beams with laser photons and even electrons. Such beams have led to impressive advances in a range of fields, including optical imaging, optical and electron microscopy, communication and quantum optics.

Until now, however, no vortex beams had ever been created from non-elementary particles. One reason for this is that composite particles are heavier than electrons, meaning that the length scale at which their wave-like nature starts to become evident – their de Broglie wavelength – is smaller. This is a challenge because to generate wave-like beams, particles need to be sent through diffraction gratings with slit dimensions that correspond to their de Broglie wavelength. For atoms and molecules, this wavelength is typically on the order of nanometres – too small for slits created using conventional micromachining techniques.

Transmission gratings with very small periods

Recent advances in nanotechnology have, however, made it possible to fabricate transmission gratings with spacings, or periods, as small as tens of nanometres. These nanoscale gratings can be used to diffract matter waves, and that is what Narevicius and colleagues did.

In a study published in Science, the team describe creating vortex beams by passing a supersonic gas of helium atoms through specially nanofabricated diffraction gratings that contain structures known as fork dislocations. These dislocations induce a circular phase in the diffracted beam, and the researchers confirmed that this was the case using a detector placed behind the gratings.

Like classical vortices and photon vortex beams, the newly-created atom vortex beams show up as a row of doughnut-shaped rings, and the phase of their wavefunction revolves around the axis of the dislocation. Each “doughnut” observed corresponds to a beam with a different OAM.

The researchers repeated their experiment with neon atoms and helium dimers, and they say that the technique could also be applied to other atomic and molecular gases. One particularly exciting possibility, they suggest, would be to make vortex beams with protons and use them to probe the internal structure of this subatomic particle.

Technetium-101: from impurity to multipurpose medical tool

Edward Mausolf and Erik Johnstone, partners at Innovative Fuel Solutions LLC, frame their vision as one of necessity. Ten years ago, they were in graduate school becoming technetium chemistry experts, and as they studied how isotopes of technetium could be produced and separated, they realized that there were issues inherent in the technetium supply chain that the industry would continue to face unless substantial changes were made.

Technetium-99m (Tc-99m), the single most-used isotope for nuclear diagnostic imaging procedures, for example, is typically produced from molybdenum-99 (Mo-99) using nuclear reactors. Scientists can’t produce a stockpile of Mo-99/Tc-99m due its relatively short physical half-life (the amount of time it takes an isotope to decay to half of its initial activity); instead, they must produce and distribute it continuously. Other issues, such as ageing nuclear reactor infrastructure and the increasing demand for radioisotopes, have contributed to several major interruptions in the Mo-99/Tc-99m supply chain over the years.

Simply put: there’s a need for new methods that can produce radioisotopes of technetium more efficiently and with less waste.

Mausolf and Johnstone have since devised a method that relies on deuterium–deuterium neutron generators to produce radioisotopes of technetium. Their approach could solve supply chain and infrastructure issues by safely bringing the production of Tc-99m and the previously underutilized radioisotope Tc-101 closer to a patient’s bedside, while also generating less waste and using fewer resources. Results of their initial experiments, reported in the journal Pharmaceuticals, also suggest that Tc-101 – often considered to be an impurity – could be useful in medical imaging and therapy applications.

Producing technetium isotopes with a neutron generator

“We realized that there might be some out-of-the-box thinking to find ways to increase yield. Instead of having a larger pile, use different isotopic ratios of technetium, to be able to make up the difference of the inefficiencies of the current supply chain,” says Mausolf. “And a neutron generator makes sense because you don’t have long-lived fission waste.”

Deuterium–deuterium neutron generators create neutron radiation by colliding isotopes of hydrogen called deuterium. When two deuterium atoms collide in a fusion reaction, they create free neutrons that can collide with a molybdenum target to produce isotopes of technetium. When neutrons bombard a molybdenum target, Tc-101 is produced alongside Tc-99m. And Johnstone and Mausolf want to put Tc-101 to work. Though its half-life is only 14.22 min, Tc-101 produces beta rays (487 keV, 90.3%) and gamma rays (307 keV, 89.4%) when it decays. These could be harnessed for both therapeutic and imaging applications.

“We believe that small-scale radioactive isotope production close to the point of use could be the way forward because it avoids the need to ship fast-decaying products with half-lives measured in hours across the country or even internationally,” Johnstone adds. “Another key feature of our research was pinpointing the production or isolation of Tc-101, which we see as a shorter-lived isotope of technetium that’s not discussed in the literature.”

The scientists say that Tc-101’s short physical half-life should not deter others from considering its medical applications because other isotopes with even shorter half-lives, such as oxygen-15, which has a half-life of just over 2 min, are widely used.

Instead, Mausolf and Johnstone say that the biggest challenge they faced was optimizing the production of Tc-101 from neutrons produced using a deuterium–deuterium neutron generator. Compared with deuterium–tritium neutron generators (which are more powerful, but require increased shielding and regulatory burden due to tritium’s radioactivity), a deuterium–deuterium neutron generator often produces fewer neutrons that also have lower energies, which reduces the amount of a radioisotope that can be produced.

With the goal of increasing Tc-101 yield, the scientists partnered with David Williams at Adelphi Technology. Together, they devised and coupled an optimized separation process to Adelphi’s deuterium–deuterium neutron generators, which produce neutron yields exceeding 10 billion neutrons per second. Neutrons produced in the generator then hit molybdenum, which in turn decays to Tc-99m and Tc-101. The resulting material is contacted with activated carbon to isolate the technetium isotopes from the remaining bulk, low specific activity molybdenum.

With the optimized system, “you always have some equilibrium period in production,” says Williams. “You have a half-life that’s not a half-life because it’s dominated by that of the parent [molybdenum].”

Additional considerations

The scientists evaluated the yields of Tc-99m and Tc-101 produced with this method and compared yields to those from a deuterium–tritium system as well as natural and enriched molybdenum targets. And though their purpose was not to pinpoint any specific applications of Tc-101, they also considered the impact of producing the isotope from environmental, financial, regulatory and clinical perspectives.

Natalia Mayordomo, a scientist from the HZDR Institute of Resource Ecology who consulted on the study, notes that because of the short physical and biological half-life of Tc-101 and its stable daughter product, ruthenium-101, the amount of radioactive material entering the environment through human waste will be less than that from radioisotopes with longer-lived ground states or decay products.

In addition, says Williams, neutron generator targets have low specific activity, there are no fission products associated with radioisotope production, and shielding requirements wouldn’t be much different to shielding PET isotopes like fluorine-18.

The team’s next steps include raising funding for a pilot production line and scaling up production so that important studies in mice and testing by regulatory agencies such as the US Food and Drug Administration can begin.

Producing isotopes of technetium using a deuterium–deuterium neutron generator and considering new uses for “impurities” like Tc-101 “present a large dynamic change to the argument of how to make technetium work for the medical community,” says Mausolf. “I think it’s got a lot of traction, I think there’s a lot of runway, and I think there’s a lot more to come.”

Why nuclear should be part of our net-zero-carbon future, green jobs for physicists, proton arc therapy

In this episode of the Physics World Weekly podcast, Sophie Zienkiewicz and Henry Preston of the Nuclear Institute’s Young Generation Network argue the case for including nuclear energy in the UK’s net-zero-carbon strategy.

Physics World’s Laura Hiscott and Tami Freeman are also on hand to talk about careers for physicists in the green economy and an emerging cancer treatment called proton arc therapy.

Phase-change memory gets efficiency boost

A new phase-change memory device that uses much less energy than its predecessors could help meet the world’s increasing demand for digital information storage. The device, which was developed by researchers at Stanford University in the US, is made from a so-called superlattice material placed on a flexible/bendable substrate, and it boasts a switching current density of just 0.1 MA/cm2 – making it 100 times more efficient than other memory types of its kind.

The volume of digital information produced worldwide is currently doubling every two years. By 2025, it could reach a staggering 160 zettabytes (a zettabyte is 1021 bytes, or 1 trillion gigabytes). Phase change memories (PCMs), which rely on the ability of certain materials to switch between a crystalline state that conducts electricity well and an amorphous state that does not, could help address this demand. However, switching between the “1” and “0” states in a PCM tends to require relatively high currents, as the material must be heated before it can change phase. This is true regardless of whether the PCM is placed on a rigid substrate such as silicon or a flexible one such as plastic.

Ultralow programming current density

In the latest work, a team led by Eric Pop succeeded in creating a PCM with an ultralow switching current density. The new PCM’s low power use makes it especially attractive for applications related to the Internet of Things (IoT) and mobile devices, both of which often rely on batteries or energy-harvesting systems.

The researchers made their memory devices directly on a flexible polyimide substrate. The phase change material they used was a superlattice made of alternating thin layers of antimony telluride and germanium telluride. They confined this material stack in a “pore” surrounded by insulating aluminium oxide. They then contacted the top and bottom of the structure with titanium nitride metal electrodes.

More energy-efficient on a flexible substrate

As well as its ultralow current density, Pop and colleagues note that their PCM also has another advantage: each step of its fabrication takes place at temperatures below 200 °C, making it compatible with a variety of flexible plastic substrates. This is important because without a good insulating layer, the electrical pulses used to switch the PCM could cause other electronic components in a device to heat up and become less efficient.

“The superlattice material combined with the confined pore-cell design play a key role in this aspect,” Pop explains, “but we found that the very low thermal conductivity of the flexible substrate also helps. In other words, the memory becomes more energy-efficient on a flexible substrate as compared to a conventional silicon substrate.”

Smart applications

The new PCM might be used in any flexible electronic device that requires low-power, non-volatile data storage, Pop tells Physics World. One example is smart sensors for the IoT, which process and store data locally before sending it to the cloud. Such devices could be useful for environmental monitoring, food packaging analysis and electronic skin for robotics as well as biomedical sensing with on-skin or implantable devices. This type of PCM could also be useful in more general-purpose flexible processors, such as those demonstrated recently by ARM and PragmatIC, or for in-memory computing on flexible substrates.

The Stanford team is now exploring other superlattice materials and new designs to further improve the energy-efficiency and thermal stability of their flexible PCM. They say they might achieve this by optimizing the sidewalls of the structure and by reducing the diameters of the pore-like memory cells. “We are also looking into superlattice materials with an even lower thermal conductivity and higher thermal stability to improve the energy-efficiency of our memories,” Pop says.

The new PCM is detailed in Science.

UK announces potential sites for prototype fusion energy plant

Five sites have been shortlisted as a potential home of the UK’s prototype fusion energy plant. Known as the Spherical Tokamak for Energy Production (STEP), it is intended to be a working fusion reactor and have many of the features of a fully operational power station when operational in the 2040s. The five potential sites, announced today, include one in Scotland and four in England, with a final decision on the plant’s location to be made by the end of 2022.

Five potential sites. (Courtesy: UKAEA)

STEP will be based on “spherical” tokamak technology that is currently being pioneered at the UK’s Culham Centre for Fusion Energy (CCFE). The CCFE is owned and managed by the UK Atomic Energy Authority (UKAEA) – which is located at the Culham Science Centre in Oxfordshire. It already houses two world-leading fusion tokamaks – the Mega Amp Spherical Tokamak (MAST-U) and the Joint European Torus.

In 2019 the UK government announced £222m towards a conceptual design report for a fusion power plant based on the spherical tokamak design. This would include prototyping components, carrying out materials research and robotics development, as well as computer modelling. The design effort will involve over 300 people and be complete in 2024. Despite the impact of the COVID-19 pandemic, the design work remains on track with the aim that the plant is fully operational by the 2040s.

After an open call for site proposals between December 2020 and March 2021, 15 sites were assessed. Following that process, five have now been picked to be studied further. They include four sites in England – Goole in East Riding of Yorkshire, Moorside in Cumbria, Ratcliffe-on-Soar in Nottinghamshire and Severn Edge in South Gloucestershire – while the fifth potential site is in Scotland in Ardeer, North Ayrshire.

“The shortlist of sites is a significant step for the programme as it helps bring this challenging, long-term endeavour to life [and] also increases our focus as we push on with design and delivery of what we hope is the world’s first fusion power plant prototype,” says Paul Methven, STEP programme director at UKAEA. “Through the next phase of assessment, we look forward to working with the shortlisted sites and local communities to gain a more in-depth understanding of the socio-economic, commercial and technical conditions associated with each site.”

A significant STEP

Earlier this year, researchers were buoyed by the first results on MAST-U. The spherical tokamak used a novel divertor that will likely be employed on STEP in which the new configuration led to a 10-fold reduction in waste heat load on the reactor walls.

The divertor is the heat exhaust system in the tokamak and if the results from MAST-U can be extrapolated to working fusion reactors, then exhaust material and other components would not need to be regularly changed – making such reactors more cost effective by allowing them to keep operational for longer.

Quantum sensors could revolutionize retinal diagnostic procedures

An electroretinogram (ERG) is a standard clinical method for measuring the function of the human retina. This procedure typically uses either a contact lens electrode or a fibre electrode to record retinal activity, both of which require physical contact with the eye, and therefore cause discomfort for the patient.

Researchers at Aarhus University in Denmark, led by Britta Westner and Sarang Dalal, have tested a potential replacement for these uncomfortable electrodes by using optically pumped magnetometers (OPMs), publishing their results in NeuroImage. OPMs are quantum sensors that detect magnetic fields that are over one billion times smaller than the earth’s magnetic field. Recent advances in quantum technology have enabled the design and commercial production of lightweight and flexible OPM sensors, such as those being incorporated into magnetoencephalography scanners.

While retinal response is typically detected via electrodes, which measure the electrical activity from the eye surface, the OPM sensors instead record the corresponding magnetic field induced by this electrical activity. The resulting magnetoretinograms circumvent the need for direct contact with the patient’s eye.

At Aarhus University, the researchers placed multiple OPMs close to a participant’s eye while performing routine retinal measurements, using a fibre electrode and a flashing light stimulation. During such a diagnostic test, a flash of light stimulates a negative potential in the retina, the “a-wave”, followed by a positive wave, the “b-wave”, which occurs at known times following the stimulus.

ERG and MRG signals
Close match: The OPM sensors measure the signature a- and b-waves of healthy retinal activity without any physical contact with the eye. Here, we see the ERG signal and OPM MRG signals overlaid. (Courtesy: CC BY 4.0/NeuroImage 10.1016/j.neuroimage.2021.118528)

These features were clearly visible in the data recorded by the OPMs and subsequent comparison to the fibre electrode measurements showed a close match. To ensure the OPMs were not simply detecting signals from the electrode, the scientists repeated the experiment without the electrode on the eye and found very similar results.

Interestingly, artefacts due to eye blinks were reduced without the fibre electrode, as the OPMs offered a more comfortable scanning environment. However, the OPMs still suffered from a lower signal-to-noise ratio than the fibre electrode, likely due to a noisy magnetic environment.

As the magnetic fields originating from the retina are much smaller than the residual environmental magnetic field, the researchers operated the OPMs in a magnetically shielded room, to screen out some of this magnetic noise. They note that additional cost-effective active magnetic shielding could reduce this interference, resulting in a comparable signal-to-noise ratio to the ERG.

The team concludes that OPMs have the potential to replace fibre electrodes and contact lens electrodes to provide a contactless and comfortable clinical retinal scanning system. Furthermore, the flexibility of these magnetic field sensors will benefit neuroscientific research into human vision and the pathology of vision impairment.

 

Inferno-like exoplanet has ionized calcium in its atmosphere

Signatures of ionized calcium in the upper atmosphere of an ultra-hot, Jupiter-like exoplanet have been found by international team of astronomers led by Emily Deibert at the University of Toronto. The researchers say that the ions could have only formed if the upper atmosphere of the exoplanet WASP-76b is either far hotter or far windier than previously thought. The discovery is the first result of the Exoplanets with Gemini Spectroscopy (ExoGemS) survey. Further insights into the origins of the calcium ions could soon be gained through future observations of the atmospheres of other exoplanets – which are planets orbiting stars other than the Sun.

First discovered in 2016, WASP-76b has about the same mass as Jupiter and has a radius almost twice as large. It completes a full orbit of its star in just 1.8 days and this stellar transit is visible from Earth, making WASP-76b an ideal candidate for atmospheric analysis. Previous studies have revealed that the exoplanet has one of the most exotic atmospheres known to astronomers.

Through several years of close observation, astronomers now widely agree that the planet is tidally locked with its star: leaving one side in permanent daytime, with temperatures reaching 2400 °C. Such an environment would vaporize iron, which would then condense in the upper atmosphere and fall as liquid rain. Meanwhile, the other side exists in permanent night, at temperatures of roughly 1300 °C.

Now, WASP-76b has become the first subject of the ExoGemS survey, which uses the Gemini North Telescope on Mauna Kea in Hawaii. ExoGemS aims to probe the atmospheres of over 40 exoplanets, spanning a wide range of masses and temperatures.

Triplet of peaks

As they analysed the absorption spectrum of light from WASP-76b, Deibert and colleagues found the signatures of several different ionized elements in the exoplanet’s atmosphere: including sodium, lithium, and potassium. The most interesting result was a particular triplet of absorption peaks, which suggests an abundance of ionized calcium in WASP-76b’s upper atmosphere. Intriguingly, these ions can only form in very extreme planetary environments.

Through their analysis, the astronomers narrowed down the possible origins of these ions to two possible causes: either temperatures in the exoplanet’s upper atmosphere are far higher than astronomers previously thought; or the atmosphere harbours extremely strong winds. To gain further insights into these possible origins, Deibert and colleagues will now search for similar signatures of ionized calcium on other exoplanets that will be studied by the ExoGemS survey.

By analysing a large sample of alien worlds, the astronomers aim to learn far more about the chemical compositions of their atmospheres. Other factors including wind and rotation patterns; cloud formation mechanisms; and the rates at which their atmospheric gases escape will also be analysed. By studying these features on planets with widely varying masses and temperatures, the team hopes to gain a better understanding of how exotic planetary atmospheres form and evolve.

The observations are described in The Astrophysical Journal Letters.

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