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Getting closer to measuring quantum gravity

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.

Heat capacity measurements reveal Majorana fermions

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.

Baryon acoustic oscillations hint that dark energy may have changed over time

Preliminary observations made by the Dark Energy Spectroscopic Instrument (DESI) hint that the acceleration of the expansion of the universe has not been constant – in other words, dark energy has changed over the history of the universe.

At the turn of the millennium, astronomers discovered that the universe has been expanding at an ever increasing rate. This came as a shock to most cosmologists who had assumed that the pull of gravity was slowing the expansion of the universe after the Big Bang.

In 1998 and 1999, two independent teams discovered the acceleration by measuring the distances to supernovae and the speeds that they are receding from Earth. Dark energy – a term that was coined in 1998 – was invoked to provide the vast amount of energy required for this constant acceleration. Three leaders of those teams shared the 2011 Nobel Prize for Physics for the discovery, and in the past quarter century a variety of observations have backed the inclusion of dark energy in the Standard Model of cosmology.

Now, another shock could be coming thanks to DESI, which was designed to study the expansion of the universe.

Robot-controlled optical fibres

DESI is located on the Nicholas U Mayall Telescope at the Kitt Peak National Observatory in Arizona. In comprises thousands of robot-controlled optical fibres that send light to an array of spectrographs. This allowed DESI to make an extensive map of galaxies and quasars in the universe. The spectroscopic data provide a measure of how fast a galaxy is moving away from us, which is determined by a galaxy’s redshift.

Key to DESI’s dark-energy study is that galaxies are not uniformly distributed throughout the universe, but rather are concentrated in bubble-like regions that are surrounded by emptier space. This is a result of how the early universe expanded and cooled. The process started with a hot plasma through which sound waves propagated, creating areas of high and low density called baryon acoustic oscillations (BAO).

Eventually this plasma “froze” to create the gas that would go on to form the earliest stars and galaxies. Bubbles of galaxies tended to form in the dense regions created by BAO, and these expanded along with the universe. Therefore, the size of a galaxy bubble tells astronomers how old it was when it sent us its light. The team also used the light from ancient quasars to illuminate the BAO, allowing them to probe further back in time than was possible with the galaxy measurements.

Tantalizing hints

Putting the spectroscopic and BAO information together, DESI team could determine the expansion rate of the universe at seven different points in time over the past 11 billion years. While their observations are broadly in line with a constant value of dark energy, DESI scientists have reported tantalizing hints of some deviation.

“So far, we’re seeing basic agreement with our best model of the universe, but we’re also seeing some potentially interesting differences that could indicate that dark energy is evolving with time,” explains DESI’s director Michael Levi, who is based at the Lawrence Berkeley National Laboratory in the US. “Those may or may not go away with more data, so we’re excited to start analysing our three-year dataset soon.”

While the team found that its observations are consistent with dark energy varying with time, the statistical significance of the deviation is only about 3σ. This means that there is about a 0.2% chance that the observation is a statistical fluke. In cosmology and some other fields of physics, a significance of 5σ is required for a discovery.

These observations were made in the first year of operation of DESI, which is expected to survey the universe for at least five years.

“It’s astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%,” says Berkeley’s Nathalie Palanque-Delabrouille. “The team put in a tremendous amount of work to account for instrumental and theoretical modelling intricacies, which gives us confidence in the robustness of our first results.”

As well as shedding new light on the expansion of the universe, DESI has also provided new information about the mass of the neutrino.

The BAO observations are described in a preprint on arXiv. Related publications can be found here.

Roll-to-roll-fabricated hybrid perovskite solar cells reach record efficiencies

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.

Tackling England’s physics teacher shortage with a new apprenticeship scheme

Very few world-changing physics breakthroughs happen in a classroom, but many of them started there. However, an increasing number of young people in England simply do not have access to a specialist physics teacher. In 2022 the number of physics teachers recruited in England was only 17% of the government’s target, and the Institute of Physics (IOP) estimates that an additional 3500 teachers are needed to make up the shortage.

This leaves young people at the mercy of a postcode lottery, with schools in socioeconomically disadvantaged areas more likely to suffer

Hari Rentala, head of learning and skills at the IOP

Hari Rentala, head of learning and skills at the IOP, says that in some areas, schools struggle to recruit a single specialist physics teacher. “This leaves young people at the mercy of a postcode lottery, with schools in socioeconomically disadvantaged areas more likely to suffer.” It is no understatement to say that the future of the next generation of physicists is at stake.

The urgent need for more physics teachers is one reason why the Department for Education in England earlier this year launched a new recruitment pathway called “teacher degree apprenticeships” (TDAs). Physicists who want to become teachers currently have to do a degree followed by a postgraduate teaching qualification. TDAs are four-year degrees, but students would spend only 40% of their time in university and the rest working in a school. Prospective teachers will therefore earn money as they study.

The vicious circle of physics teacher shortages

Teaching is often described as a vocation – it is rewarding to instil a passion for physics in young people, but it isn’t as well paid as many pathways available to physics graduates. According to the 2023 Science Teacher Survey, which was supported by the IOP and received more than 3700 responses from teachers and technicians, low pay is one of many barriers to recruitment and retention.

The challenge of recruiting enough physics teachers is made worse by the fact that almost half of new physics teachers leave within their first five years of qualifying. One reason for the high attrition rate is that in England it is common for physics teachers to also cover chemistry and biology despite not having specific expertise in those areas. “This has the potential to decrease job satisfaction and increase workload, especially for newly qualified teachers who may not have studied the other sciences for many years,” says Rentala.

This can leave physics teachers feeling isolated and – especially in their early years – without sufficient access to the informal subject-specific mentoring and support that most of us turn to when we begin our careers

Hari Rentala

The situation is different in Scotland. Physics teachers there generally only teach physics, which leads to them staying far longer in the profession. Rentala says eight times as many physics teachers leave after the first two years in England compared with Scotland. Indeed, many physics graduates who do decide to become teachers in England end up training as maths teachers to avoid teaching subjects in which they do not have a specialism.

However, teacher retention is a UK-wide issue, with more than half of the respondents to the survey reporting that their school had a shortage of physics teachers. “This can leave physics teachers feeling isolated and – especially in their early years – without sufficient access to the informal subject-specific mentoring and support that most of us turn to when we begin our careers,” Rentala says.

As a result of the overall shortage, physics teachers gravitate towards high-achieving schools in more affluent areas, which threatens to entrench regional and economic inequality. In England, 70% of physics A-level students come from only 30% of schools. But will the new TDAs make a difference?

Alternative pathways for physics teachers

Charles Tracy, the IOP’s senior adviser for learning and skills, thinks that TDAs are an efficient way to become a teacher. “Otherwise, they’d have to take a physics degree and learn lots of things that they would never otherwise use in their teaching.” Depending on the course, apprentices might not learn content like, say, relativity and quantum mechanics, which generally do not appear on school syllabuses.

TDAs are due to start accepting applications later this year, with the first cohort of apprentices beginning their training in 2025, although it is not clear which universities will be offering the courses. Tracy, however, thinks that the scheme might particularly appeal to people who are already working in schools, such as teaching assistants or computer or lab technicians.

[There’s a] need for a laser-like focus on improving recruitment and retention – which must involve government action to improve pay, reduce workload and make the accountability systems…less punitive

Geoff Barton, general secretary of the Association of School and College Leaders

But TDAs will only ever be part of the solution. Geoff Barton, general secretary of the Association of School and College Leaders, says that while the principle behind the apprenticeships is good, the pilot scheme of 150 recruits will be a “drop in the ocean”.

“The main event is the challenge that exists right now, and this involves the need for a laser-like focus on improving recruitment and retention – which must involve government action to improve pay, reduce workload and make the accountability system of Ofsted inspections and performance tables less punitive.”

As well as supporting the apprenticeship scheme, last year the IOP submitted evidence to a parliamentary committee that recommended serious reforms to the physics curriculum in England. Rentala believes that ensuring that physics teachers only teach their specialist subject would tackle the high attrition of physics teachers by reducing workload and allowing teachers to focus on the topics that they are passionate about. But he adds that organizations like the IOP cannot do everything on their own. “This is a complex issue and one where the real step-changes can ultimately only be unlocked through government policy.”

Planet-gobbling stars are more common than we thought

Artist's impression of a terrestrial planet being captured by a twin star
Om nom nom: A terrestrial planet being captured by a twin star (Courtesy: untouchable/OPENVERSE)

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.”

REBCO high-temperature superconductors are ideal for tokamak magnets, study suggests

An extensive study done in the US has confirmed that magnets made from rare earth barium copper oxide (REBCO) high-temperature superconductors are ideal for confining plasma in future fusion experiments. The team showed that the magnets are both robust and compact, making them a practical option for future tokamaks such as SPARC, which is being developed by Commonwealth Fusion Systems (CFS) and MIT’s Plasma Science Fusion Centre (PSFC).

The study was done by researchers at CFS and PSFC, who created new diagnostic instruments for studying the magnets.

A tokamak fusion reactor uses very strong magnetic fields to confine a hydrogen plasma inside its doughnut-shaped interior. This allows the plasma to be heated to very high temperatures so that hydrogen nuclei fuse together – releasing large amounts of energy. The ultimate goal of tokamak research is to get much more energy out of fusing plasma than is put in, thus creating a relatively clean source of energy.

These magnetic fields are created by electromagnets and in existing tokamaks, these are wound using wires made from a conventional conductor (copper) or a low-temperature superconductor. Both approaches have advantages and limitations, so fusion researchers are keen on exploring other magnet options. In particular, most existing magnet technologies would be too big and too expensive for use in next-generation devices that will require higher field levels.

Limited fields

“Superconducting magnets with very low power consumption have now been integrated into fusion devices at sufficient scales,” explains Zach Hartwig at MIT, who led the new analysis. “However, they have all used superconductors that were limited to confining magnetic field strengths of approximately 5 T.” Even when confined at these fields, plasma will gradually leak out.

Between 2018 and 2021, a collaboration of researchers at PSFC and CFS developed REBCO magnets with the aim of boosting confining fields – and material’s performance was very promising.

“REBCO is capable of producing extremely high magnetic fields, and can also carry very high electrical current densities at temperatures up to 20 K,” Hartwig explains. “This leads to superior engineering and performance in superconducting magnets.”

Now Hartwig and colleagues report the results of a comprehensive battery of tests of REBCO’s performance as a superconducting magnet, using purpose-built test facilities at MIT.

Nearly double

Through experiments carried out in September 2021, the material demonstrated a peak magnetic field of over 20 T. This is almost double the highest fields achieved previously in other superconducting magnets for similar applications.

Since then, the researchers have carried out further tests that pushed a REBCO magnet to extreme limits of its performance, while doing an analysis of its operation.

The team now present their findings in a series of papers in IEEE Transactions on Applied Superconductivity. They provide an in-depth description of all of the magnet’s components and how they performed at high fields. Hartwig’s team are now confident that REBCO is well suited for its intended purpose.

“Despite the enormous electromechanical loads, the electrical, thermal, and structural performance of the magnet behaved exactly as designed in steady-state operation,” Hartwig says. “This validated the advanced computational modelling developed in the programme, and experimentally confirmed that high-field superconducting magnets are viable for fusion energy,” he adds.

Much smaller volume

Crucially, the experiments demonstrated that REBCO is capable of sustaining a 12 T field suitable for plasma confinement within a volume some 30 to 40 times smaller than previous fusion devices.

“The significant reduction in scale enabled by REBCO will enable lower costs and faster schedules to build magnetic confinement devices, as well as more favourable economics for fusion energy power plants,” Hartwig explains.

“Perhaps most importantly, the reduced scale enables a critical transition in fusion energy: moving from multi-national, government-funded science programs into privately funded, mission-driven companies, focused on commercializing a viable new zero carbon energy source,” he adds.

Based on their promising findings, the PSFC and CFS teams now hope their analysis will serve as a valuable guide for future fusion research: perhaps bringing one of the most long-awaited goals in physics a step closer to reality.

Battery cyclers: shedding light on the detail of electrochemical materials and devices

AMETEK SI engineer oversees installation of battery cycler
Made to measure The SI-6200 and SI-9300R battery cyclers from AMETEK SI enable researchers and manufacturers to compare the merits of novel battery chemistries and materials. Above: an AMETEK SI field engineer oversees installation, commissioning and acceptance of the SI-9300R at a customer test facility. (Courtesy: AMETEK SI)

Cutting-edge technologies to cover all the bases versus cost, performance and innovation. That’s the mantra for the product development team at AMETEK Scientific Instruments (AMETEK SI), the US manufacturer of specialist test systems for electrochemical studies and battery characterization. Zoom in a bit and the operational reference points are similarly clear. Front-and-centre is a cosmopolitan customer mix – spanning academic research, industry R&D and volume manufacturing – plus a comprehensive electrochemical portfolio – battery cyclers, potentiostats, galvanostats, frequency response analysers, and plenty more in terms of ancillary hardware and software.

Alignment is all, with AMETEK SI end-users and products mapped synchronously across a diverse range of test and measurement applications – energy-storage systems, physical electrochemistry, corrosion science, biomedical devices and solid-state materials – and building upon a combined 100+ years of technology leadership across the manufacturer’s three core brands: Princeton Applied Research, Solartron Analytical and Signal Recovery.

Performance over time

If that’s the back story, what of the now and next on the development roadmap? Near term, the alignment of technology innovation with evolving end-user requirements underscores AMETEK SI’s latest product offerings: the SI-6200 and SI-9300R battery cyclers for research and production testing of advanced batteries destined for applications in mobile phones, PCs, electric vehicles (EVs) and the like. In brief, these high-performance battery cyclers enable researchers and manufacturers to compare the merits of novel battery chemistries and materials, with repeatable cycling tests running over extended time frames (several weeks to several months) to validate lifetime performance along multiple coordinates.

As such, AMETEK SI battery cyclers enable all manner of test capabilities: constant current, constant voltage (CC-CV); constant power (CP); constant resistance (CR); cyclic voltammetry (CV); as well as a range of fast-pulse and voltage/current ramp schemes. Advanced diagnostic modalities are also in the mix, including electrical impedance spectroscopy (EIS), a non-destructive technique for so-called “state-of-charge” (SOC) analysis and early fault detection inside the battery; urban-drive-profile cycle testing (arbitrary waveforms) to simulate the real-world environment experienced by battery cells in EV applications; and differential capacity analysis to track state-of-health (SOH) and structural changes in battery materials over repeated cycles of charge/discharge.

The SI-6200, for its part, is designed for testing next-generation battery materials, supercapacitors and micro fuel cells at up to 200 mA/10 V per channel. Among the analyser’s key features: stand-alone operation using stackable 10-channel modules (or cabinet-based); fast data acquisition speeds (thousands of measurements per second across all channels); and the incorporation of high-resolution 24-bit converters (to enable early detection of small differences in cell behaviours).

Massimo DeSantis
Massimo DeSantis “It’s all about prioritizing satisfied end-users every time.” (Courtesy: AMETEK SI)

“Because of the relatively low current per channel, the SI-6200 is primarily focused on the testing of prototype coin cells and small-scale energy-storage devices,” explains Massimo DeSantis, Division Vice-President of AMETEK Advanced Measurement Technology (AMT), a broader business unit that includes AMETEK SI. “This initial-stage device development and testing represents the application ‘sweet spot’, though the cycler can be used in a production environment as well.”

Worth noting that the EIS capability, previously confined to academic research settings, is now transitioning to industrial R&D and production testing. It’s not hard to see why. For starters, EIS enables granular analysis of resistance, capacitance and impedance at different frequencies to track the electrical response of a battery. These frequency-dependent insights shed light on diverse performance issues – side reactions, electrode degradation, electrolyte ageing and SOH degradation among them.

What’s more, EIS is a non-invasive technique that can be used for in-situ or real-time monitoring without interrupting the battery’s normal mode of operation. That’s a big win and means manufacturers can deploy EIS measurements on batteries at different stages of production, ensuring consistency in performance and detecting any defects or anomalies early in the manufacturing process (when workflow efficiency and device reliability are of the essence).

Progressions of power

By extension, the SI-9300R battery cycler – the “big brother” of the SI-6200 – is designed for testing high-power batteries, fuel cells and electrolysers at up to 10 V/200 A. As such, each channel of the SI-9300R performs the same basic functions as the SI-6200 – with built-in EIS capability – though with additional features to target developers of EV batteries and so-called EV “second life” batteries (which have reached the end of their “automotive life” while retaining a residual capacity of about 70–80%).

Like its sister product, the SI-9300R is modular and scalable – with different cabinet options (42U and 24U) that can be controlled and monitored via multiple PCs – while as many as five channels can be connected in parallel to enable tests up to 1000 A. “We’re looking at applications in industrial R&D and indeed any research environment engaged in advanced prototype testing of high-power batteries and fuel cells,” notes DeSantis.

A key aspect of the SI-9300R is the implementation of regenerative power management which, according to the product team, can yield up to a 5x space saving and 90% saving in annual running costs over conventional battery cyclers. Without this sort of active intervention, there’s a lot of heat to be dissipated when the cells are discharging on multiple channels – and that means added real estate for cooling systems inside the analyser. In the SI-9300R, however, power is recirculated internally between channels that are simultaneously discharging and charging.

Meanwhile, system reliability is a major concern for end-users of battery-cycler equipment – the goal being truly autonomous cycling of those battery test sequences. With this in mind, both the SI-6200 and SI-9300R rely on AMETEK SI’s patented Direct-to-Disk technology to ensure a more secure, robust approach to data acquisition and storage. Put simply: data from the battery cycler is stored direct onto a disk drive – no intermediate PC – to minimize the risk of data loss.

All of which is reinforced by Aspire Energy Software – the “soul of AMETEK’s battery analysers” – which provides a fully integrated test environment for management of cyclers, climate chambers, data loggers and ancillary equipment. With product development teams often dispersed globally, Aspire’s shareable libraries enable tests, graphs and data analysis to be preconfigured and shared by all users to ensure consistency of results.

Operationally, it’s significant that AMETEK SI insists on deploying company-trained field engineers to support the installation, commissioning and acceptance of its battery cyclers in the customer’s lab or test facility. “Our motivations are clear,” concludes DeSantis. “It’s all about prioritizing a smoother onboarding process, faster resolution of technical queries, and a training programme that’s geared to deliver satisfied end-users every time.”

AMETEK Scientific Instruments

Science centres inspire scientific literacy and diversity in STEM

In this episode of the Physics World Weekly podcast I am in conversation with Frederic Bertley – who is president and CEO of COSI (Center of Science and Industry) in Columbus, Ohio. Bertley explains how science centres like COSI can boost scientific literacy and talks about the Color of Science initiative, which he founded to highlight and promote diversity in science, technology, engineering, arts and mathematics.

Bertley also talks about his life-long love of ice hockey and how sports can be used to get people interested in science. Indeed, he explains in detail the physics of baseball pitches and the hockey slapshot.

He also talks about how COSI is encouraging Ohioans to observe and understand the total eclipse of the Sun, which will occur in a significant portion of the state on 8 April. He explains how COSI will engage with the public in venues as diverse as libraries and bars to share the science surrounding the eclipse.

Quasiparticles called merons appear in a synthetic antiferromagnet for the first time

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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