Memory loss, difficulty finding words, trouble with navigating even in familiar neighbourhoods. These are just a few of the dementia symptoms experienced by an estimated 50 million people worldwide. Of those patients, whose number is projected to triple by 2050, two thirds suffer from Alzheimer’s disease. This neurodegenerative disorder is caused by accumulation and aggregation of amyloid beta and phosphorylated tau proteins in the brain. But how the resulting toxicity to brain cells gives rise to the complex and diverse clinical symptoms of Alzheimer’s is only just beginning to be understood.
Researchers at the Mayo Clinic in Rochester, MN, have now developed a computational model that maps large-scale patterns in brain metabolic activity to clinical data. Their results, published in Nature Communications, indicate that including just a few global correlation features is sufficient to derive an accurate model of clinical status from a three-dimensional image of brain activity.
The researchers used brain scans of over 400 Mayo Clinic patients with Alzheimer’s disease as the backbone of their computational investigation. In preparation for the scans, clinicians inject patients with the radiotracer 18F-fluorodeoxyglucose (FDG), which accumulates in metabolically active cells. They can then employ positron emission tomography (PET) to obtain a three-dimensional map of cellular energy usage. This technique (known as FDG-PET), is commonly applied to image cancers, but also lights up glucose-hungry brain cells.
“FDG-PET is sensitive to neurodegenerative diseases of mental function at the single subject level and is currently used in clinical practice,” explains lead author David Jones. “[This] likely relates to a fundamental relationship between brain function and energy metabolism that reveals the functional neuroanatomy relevant for dementia symptoms.”
FDG-PET scans generate a very high-dimensional data space. Each image voxel carries information that can distinguish different patients from each other, and extracting useful knowledge requires a dimensionality reduction technique. In this work, the scientists employed principle component analysis (PCA), a mathematical tool that can identify the patterns of variation in high-dimensional datasets that explain most of the statistical variance. By applying PCA, they were able to represent 51% of the inter-patient variance (in glucose uptake across the brain) in just 10 patterns, which they called “eigenbrains”.
Because any brain scan can be represented within this reduced coordinate system (as a unique combination of the 10 brain glucose patterns), the authors next constructed a linear model to correlate the scans to patient information, such as age or clinical dementia rating. In unrelated validation datasets, the model could infer a patient’s age just using the FDG-PET image, with a mean error of five years. The model could also predict the presence or absence of clinical dementia with an accuracy of over 90%, even though it was never specifically optimized for such tasks.
An important consideration in the choice of mathematical model is interpretability, that is whether or not one can understand the model’s behaviour at all stages. As Jones emphasizes: “For medical decision making, it is critical to have an interpretable model. In some sense, every patient encounter has some ‘edge case’ features present. Having an interpretable model allows for medical reasoning within the context of the patient that is in front of you, at the time you are making medical decisions.”
While the researchers did not perform a rigorous interpretation of the eigenbrains in the study at this stage, they hypothesized that the most important eigenbrains might correspond to whether the brain is occupied with internal versus external, or abstract versus concrete, information. As a consequence, the model captures general mechanisms of information processing in the brain. By viewing conditions such as Alzheimer’s not merely as a protein folding disease, but as a spectrum of effects on wider brain functions, models like this one can also inform scientists about the workings of the healthy brain.
“This model can inform new taxonomies of degenerative dementias and the mind in general,” Jones concludes. “In addition to making the interpretation of brain scans from patients easier by only needing to look at a few anatomic patterns rather than hundreds, this new model may advance our understanding of how the brain works and breaks down during aging and Alzheimer’s disease, providing new ways to monitor, prevent and treat disorders of the mind.”
A solar cell that incorporates both three-dimensional and two-dimensional forms of perovskite crystals retains its efficiency even after exposure to high heat and humidity – a first for a device made from this promising yet notoriously unstable photovoltaic material. The device, which was developed by researchers in Saudi Arabia, preserves 95% of its initial efficiency after enduring 1000 hours at 85°C temperatures and an 85% relative humidity, thereby meeting a critical industrial stability standard for photovoltaic modules.
Perovskites are crystalline materials with an ABX3 structure, where A is caesium, methylammonium (MA) or formamidinium (FA); B is lead or tin; and X is chlorine, bromine or iodine. They are promising candidates for thin-film solar cells because they can absorb light over a broad range of solar spectrum wavelengths thanks to their tuneable bandgaps. Charge carriers (electrons and holes) can also diffuse through them quickly and over long distances. These properties give perovskite solar cells a power conversion efficiency (PCE) of more than 18%, placing their performance on a par with established solar-cell materials such as silicon, gallium arsenide and cadmium telluride.
The problem is that perovskites suffer from naturally-occurring surface defects and are prone to structural changes known as ion migration. Both factors tend to make perovskite films unstable, and these instabilities becomes even more pronounced with higher temperatures and exposure to water molecules.
More robust to heat and humidity
In the new work, a team of researchers led by Stefaan De Wolf at the King Abdullah University of Science and Technology (KAUST) made a semiconductor heterojunction by sandwiching a photosensitive 3D perovskite, which converts light to electricity by creating pairs of charge carriers, between two 2D perovskite layers (one n-type and one p-type). The electrons and holes then separate, with the electrons migrating towards the n-type 2D perovskite layers and the holes migrating towards the p-type layers, thus generating an electric current.
As 2D perovskites are more robust to heat and humidity than 3D perovskites, growing a layer of the former on a film of the latter produces a heterojunction that can block migrating ions as well as moisture. With the 2D layers essentially forming a passivating “top coat” for the 3D film, the KAUST team’s solar cell boasted a power conversion of 24.7% and an open-circuit voltage of around 1.20 eV.
The researchers also discovered that tailoring the number of 2D sheets (or their dimensionality) was important for effective top-coat passivation. This technique has been employed before to improve the performance and stability of conventional solar cells in which the p-type layer is at the top. Until now, however, it had not worked for inverted devices in which the n-type layer is at the top.
The team, which reports its work in Science, says its members are now designing new molecules for 2D perovskites to develop ultrastable inverted perovskite cells with efficiencies close to those of regular devices, which can exceed 25%. “We will also be performing other IEC [industrial standard] stability tests, including the effects of reverse voltage biasing and testing the devices outdoors to understand – and address – stability-limiting factors,” De Wolf tells Physics World. “We also aim to integrate the developed 2D/3D heterojunctions in other perovskite-based solar cells such as ‘tandem devices’.”
Electrochemistry has long held potential as a tool for constructing a wide variety of organic molecules, and the organic chemistry community is both recognizing and impressively exploiting that potential with increasing regularity. Those efforts have taught us a great deal about how to think about electrochemical reactions and how to use the technique to accomplish new transformations that enrich the synthetic enterprise.
The relationship between organic synthesis and electrochemistry can also be viewed in the opposite direction, with the new synthetic chemistry being developed in order to facilitate exciting opportunities to expand the scope of electrochemical experiments. For example, new synthetic methods that allow for the site-selective generation of chemical reagents on microelectrode arrays have enabled the construction of complex, addressable molecular surfaces. The method offers a previously unknown level of quality control over the surface and enables binding events between molecules to be monitored in “real time”.
Not surprisingly, the exploration of these new electrochemical tools is beginning to “give back” to the synthetic arena. A number of preparative scale synthetic applications have now arisen based on the concept of site-selective reagent generation, and those applications offer new opportunities for controlling the selectivity of a chemical reaction.
This webinar highlights the interplay between organic synthesis and electrochemistry with an emphasis on how key concepts in physical organic chemistry are used to guide the development of new advances in both areas.
Kevin D Moeller joined the chemistry faculty at Washington University in St Louis in 1987 where he is now professor of chemistry. Prof. Moeller’s independent research primarily focuses on how the interplay between electrochemistry, physical organic chemistry, and synthesis can be used to address a variety of topics. His group has explored the chemistry of highly reactive radical cation intermediates in the context of exploiting electrochemistry as a method for triggering unique synthetic transformations. They have examined the use of modern synthetic chemistry as a method for building complex molecular surfaces on microelectrode arrays, and engaged in the design and synthesis of chemical probes for examining and mitigating the reactivity of G-protein signaling pathways. Moeller’s work continues to play a significant role in defining how the synthetic chemistry community can think about and take full advantage of electrochemical methods.
Some high-energy neutrinos detected in the Antarctic ice and thought to have come from outside the solar system may have been produced in Earth’s atmosphere. That is the conclusion of physicists in Poland and Brazil, who have analysed how neutrino detections made by the IceCube Neutrino Observatory may have been shaped by the presence of heavy charm quarks within protons and neutrons. Their result implies that this “intrinsic charm” plays a key role in the generation of high-energy neutrinos when high energy cosmic rays interact with nuclei in the atmosphere.
The IceCube Neutrino Observatory uses one cubic kilometre of the Antarctic ice sheet to detect cosmic neutrinos from outside the solar system. The process involves a neutrino interacting with the ice to create secondary charged particles. These particles are moving faster than the speed of light in ice and therefore produce Cherenkov light, which is detected by some of the hundreds of photomultiplier tubes positioned at regular intervals along a grid of cables suspended within kilometre-deep holes in the ice.
Since 2013 the facility has reported evidence for dozens if not hundreds of cosmic neutrinos. Such particles have energies of at least 100 TeV and are potentially bearers of information about otherwise obscure and distant astrophysical processes. They are distinct from the generally lower-energy neutrinos produced when cosmic rays interact with nuclei in the atmosphere to generate particle showers.
Prompt neutrino flux
In the latest work, Rafał Maciuła and Antoni Szczurek at the Polish Academy of Sciences in Kraków, along with Victor Goncalves at the Federal University of Pelotas in Brazil, argue that some of the supposedly cosmic neutrinos may in fact be of the more humdrum atmospheric variety. These neutrinos would form what is known as the prompt neutrino flux, generated along cosmic rays’ forward trajectory via the production and decay of charmed mesons – particles containing a charm quark or charm antiquark bound to another quark of a different flavour.
The researchers come to this conclusion after using both the IceCube data and results from the Large Hadron Collider (LHC) at the CERN laboratory in Geneva to try and pin down the extent of any intrinsic charm within protons. In contrast to the simple picture of protons being made up of two up quarks and a down quark bound together by gluons, physicists now know that they also contain a sea of quark-antiquark pairs flitting into and out of existence. However, exactly what fraction of those quarks are charm quarks has remained the subject of debate.
The LHC results used by the trio were reported in 2019 and came from experiments that collided protons with a stationary target of helium or argon gas inside the LHCb detector. These collisions would have had enough energy to break free any charm quarks or antiquarks existing within the protons but not so much energy that the collision products would have travelled in a forward direction – where they would have been impossible to measure (as is the case with proton-proton collisions.)
New model
In earlier work, Maciuła and Szczurek compared those results with a new model they developed describing how proton collisions yield pairs of charmed mesons. Although their model could not be used to calculate from first principles the likelihood of a charm quark or antiquark escaping a proton during such collisions, they were able to plug in a variety of different probabilities and then compare the model results with those from LHCb to settle on a best-fit number. Their result was 1.65%, which is the probability of finding intrinsic charm quarks inside a proton (as distinct from charm quarks that are produced by gluons during proton-proton collisions).
What they and Goncalves have now found is that a similar percentage can be obtained by modelling the production and decay of charmed mesons within the Earth’s atmosphere. By again varying the intrinsic charm probability within their model, they closely matched IceCube’s observations when that probability is around 1.5%.
They argue that should that figure turn out to be correct, scientists would be forced to reinterpret some of IceCube’s highest energy neutrinos as being of atmospheric rather than cosmic origin. But they stress that there is still considerable uncertainty in their result. This is because of the different values that can be obtained depending on the exact workings of the strong force – which describes interactions within and between protons and neutrons. One thing that is not clear is whether or not the proliferation of gluons inside protons at high energies tends to self-regulate or “saturate” above a certain energy.
Reducing that uncertainty, they explain, can be achieved by better understanding and measuring both the prompt neutrino flux at IceCube and the production of charm mesons at the LHC. New experiments involving high-energy neutrinos at CERN could also help, they add – namely FASERν and SND@LHC.
Despite the new findings, IceCube principal investigator Francis Halzen of the University of Wisconsin-Madison in the US is confident that intrinsic charm has only a small effect on his collaboration’s astrophysical results. He acknowledges that he and his colleagues have so far been unable to pinpoint the exact contribution of charm-generated neutrinos to their high-energy signals. However, he says that their failure to do so is precisely because the cosmic flux at those energies is so large. “It may be beyond the sensitivity of our measurements, especially because those have been designed to reject atmospheric neutrinos,” he adds.
When exploring an unfamiliar place, many of us default to a rather modern method of navigation: watching a blue dot move around a map, on a screen, and course-correcting to keep it on the right track. We owe this convenient technology to global navigation satellite systems (GNSSs) – such as the Global Positioning System (GPS), Galileo and BeiDou – which regularly broadcast radio-waves from their orbits around the Earth. Our devices use the time difference between receiving signals from different satellites to calculate our location on the Earth’s surface to an impressive degree of accuracy.
But GNSSs have their pitfalls; as soon as you venture underground or into, say, a forest or a building (like an airport or train station), your blue dot might start jumping around erratically, leaving you a bit lost. This is because obstructions between you and the satellites can weaken the signals or reflect them so that they bounce around before reaching your device, causing a time delay that leads to errors in the calculations.
A form of location tracking that we could count on while hiking through the woods or traversing a labyrinthine underground station would be a welcome improvement. This is a reality that physicist Lia Li (@OptoLia on Twitter) is seeking to create with her start-up Zero Point Motion, a company developing sensors that track your position independently of a GNSS.
As it happens, our devices do already have so-called “inertial sensors” designed to do exactly this. The problem is they are not very reliable. These chips consist of tiny silicon structures that move slightly in response to acceleration or rotation of the device, thereby altering the capacitance of the component, which in turn changes its electrical signal. The device is programmed to register that change as an acceleration and double integrate it over time to calculate the displacement of the object from its starting point. But these capacitive signals are easily overwhelmed by electrical noise in the device’s circuitry, Li explains. “This is why, if you’re on your phone or in your car and you switch off the GPS, your pin might hop to a random nearby street and you think ‘I’m definitely not there’.”
Following the dot When underground, in a forest or inside a building, global navigation satellite systems struggle to pinpoint your exact location because the radio waves bounce off the surrounding obstructions. (Courtesy: iStock/svetikd)
Beyond mere convenience, Li points out that better GNSS-independent motion tracking is essential for safety in some applications. Autonomous vehicles, for example, will need extremely reliable and live information about their trajectory relative to their surroundings to ensure that they don’t swerve or change speed in a dangerous way.
As it happens, Li’s scientific background has set her up well to tackle this problem. She was immersed in an academic environment as a child, with both her parents doing PhDs in mechanical engineering at the University of Bristol, UK. The family lived in student halls for several years and Li often spent time in the university labs, surrounded by huge pieces of equipment testing the strength of metals. She knew from a young age that she liked the freedom that research allows for, as well as being at the cutting edge of a field and having access to state-of-the-art equipment.
Best of both worlds
Li went on to study physics at Imperial College London, but it wasn’t until her final-year project, which involved building a laser from scratch, that she discovered her love of photonics. She enjoyed the work so much that she did a summer placement with the same research group following her final exams and even considered doing a PhD with them. However, Li already had a job lined up at BAE Systems’ photonics department for after graduation. Although she questioned whether this was the right decision for her, she decided to give industry a go after her father pointed out that she’d only known the education system so far. It proved a very valuable experience. “I learned so much from these fantastic scientists who had worked there for 20+ years,” she says. “It was a different way of gaining knowledge, and I loved the focus on application and needing something tangible to come out of it.”
After a while, BAE Systems was keen to promote Li to project management, but she wanted to stay in the labs doing hands-on work. So in 2012, after a year and a half in the job, she left to do a PhD in quantum physics at University College London (UCL).
That time in industry has continued to inform her work, however, as it gave her a glimpse into the huge potential of inertial sensors. Many people have the impression that these kinds of sensors are not worth spending time on, largely because of their low quality in everyday devices like phones. But during her spell in the defence industry, Li became aware of quarter-of-a-million-pound inertial sensors that can track your position accurately for hours without a GNSS. “I know there’s technology out there that gives you exactly what you require to enhance safety in cars or to do indoor navigation for long periods,” she says, “so I felt that there was a definitive need to bring some of that high performance to consumers.”
Sensor revolution Lia Li’s prototype optomechanical sensor from her UCL research, which uses the same mechanism as LIGO but on a much smaller scale. Now, she is developing chipscale versions that could revolutionize inertial sensor chips in phones. (Courtesy: Lia Li)
The key to developing this technology is to change the sensing mechanism from the noisy capacitive sensor currently found in our devices to one that uses cavity optomechanics – the interactions between light and the motion of physical objects. Instead of placing the tiny moveable silicon structure so that it influences the capacitance, you put it next to a chipscale optical cavity, which traps laser light by reflecting it internally. Li uses ones with curved boundaries, like rings, spheres and doughnut shapes.
Depending on the cavity’s exact parameters, it will have an optical resonance at certain frequencies of light where the reflections combine to form standing waves. But if the sensor is accelerated, the silicon structure will move closer or further away from the cavity in response, temporarily changing the effective refractive index experienced by the cavity, and therefore causing a slight shift in its resonant frequencies. This shift can be used to calculate the acceleration, which can be double integrated over time, as with the capacitive sensors, to find the displacement.
Unlike capacitive sensors, this mechanism is not affected much by noise. “Because there’s a resonance condition, you get this extra boost in signal-to-noise that makes it super sensitive,” Li explains. “So a tiny amount of motion results in a huge amount of measurable shifting.”
Indeed, a similar mechanism is at the heart of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detected some of the subtlest vibrations scientists have ever looked for. By measuring the shift in resonant frequencies between mirrors that are placed at great distances, LIGO is able to sense gravitational waves as they squeeze and stretch space–time.
LIGO’s historic success in detecting gravitational waves – which won three of its scientists the 2017 Nobel Prize for Physics – strengthened Li’s faith in the revolutionary power of the technology. “LIGO is a giant version of a cavity optomechanics experiment,” she says. “So to me it was really obvious that if it can transform fundamental physics, it would absolutely do the same for normal sensing devices. Lots of academic groups have demonstrated similar effects, but no one had done it at a commercial product level yet. With the power of photonic integrated circuits, which can mass produce these cavities the same way computer chips are made, it feels like the market is ready.”
Finding your feet
Li made her first foray into entrepreneurship straight after finishing her PhD in 2016, when she participated in the Nature/Entrepreneur First Innovation Forum. This was a three-month Dragons’ Den-type scheme where participants were trained in assessing business concepts and pitched their own ideas to a panel of academics and industry experts. She enjoyed being around other entrepreneurs as well as veteran businesspeople who were looking to mentor new founders. “I thought ‘Wow, I really want to be a CEO’,” she says.
Another reason Li wanted to start a company was that she found it difficult to support herself in academia, let alone give others opportunities – a cycle of trying to stay afloat but not having the resources to seek help or grow a team. Her motivation was often dampened by experiencing or witnessing gender and racial inequity in science, but her desire to help younger scientists remained alive due to the support she’s received from strong women in the field.
This can have a particularly negative impact in the already harsh academic environment of short-term contracts and job insecurity. Li experienced the difficult system of constant grant applications when she returned to academia as a postdoctoral researcher at UCL, after finishing the Nature/Entrepreneur First Innovation Forum. She continued to think about commercializing her research, but encountered various dead ends, often because she would burn out trying to do that and her academic work simultaneously.
Success in lockdown While furloughed from her academic position in 2020 because of the UK’s COVID lockdown, Lia Li used the time to write the business plan for Zero Point Motion. (Courtesy: Joe Burgess)
Then, in 2019, Li won an innovation grant of £50,000 from the UCL Quantum Science and Technology Institute. The award had no strings attached; the only condition was that she had to officially set up a business and have a business bank account. Li found that going through that formal process felt surprisingly significant, and made the idea of Zero Point Motion seem much more real.
When COVID hit the following year, she was furloughed from her academic work, but this gave her the time and freedom she needed to focus on the company. “I wrote the entire business plan during that furlough period,” she recalls. “I actually ended up inventing the two foundation patents for the company during that time.”
The final piece of the puzzle fell into place when Gordon Aspin, a scientist and engineer who has developed four companies relating to smartphone chips in the past, joined as executive chairman. “As a physicist, I have no connection to mass-producing smartphone chips, which he has,” says Li, “so he’s the exact kind of mentor that I needed.”
Last year Li won the Institute of Physics (IOP) Clifford Paterson Medal and Prize, which recognizes “exceptional early-career contributions to the application of physics in an industrial or commercial context”. The IOP cited her pioneering research into commercializing optomechanical sensors, as well as her drive to build a better and more supportive research community. In February this year, she was able to become full-time chief executive of Zero Point Motion.
Silicon vision
Although Li is looking at enhanced navigation as the primary application of her optomechanical sensors, they have potential for many other uses too. The technology is sensitive enough to pick up someone’s heartbeat from underneath a mattress, so it could be used for health monitoring. Since structures like bridges also vibrate in specific ways that change with wear and tear, the sensors could be used to monitor these changes over time and implement predictive maintenance too.
Now, after raising a £2.58m seed round from Foresight Williams Technology, Verve Ventures and u-blox, the next steps for Li are to expand the team and work through early prototype iterations. The aim is to have prototypes that can be tested and demonstrated next year, and to receive feedback from potential end users. “I think I’m slowly learning to be more ambitious,” she says. “It’s hard when you come from a really competitive research area where you feel that you don’t want to dream too big. But in business, you can’t afford to do that because so much of it is how you inspire employees and bring people to the company; hiring is what we’re focussed on right now. So if any readers want to work on this technology and own part of the company, come to Bristol and join us!”
Li’s goal is for Zero Point Motion to become a world-leading high-volume inertial-sensor chip company, but her vision goes beyond her own business. She wants to see the UK grow its semiconductor presence, pointing out that it isn’t currently very strong, with Arm Holdings being the major breakthrough UK-based semiconductor company.
Something needs to galvanize the industry to move towards mass volume, and to embody the Silicon Valley mentality
Lia Li
“There are amazing scientists and engineers in the UK. At BAE, I used to work with people who invented the first silicon gyroscopes inside Segways,” she says. “But I think something needs to galvanize the industry to move towards mass volume, and to embody the Silicon Valley mentality and the ambition as well. Hitting 100 million units per year would be ground-breaking for the UK.”
The Moon’s highlands may be scattered with transparent globules of a glass-like substance, say scientists involved in China’s Chang’e-4 mission. Initial observations from the mission’s Yutu-2 rover suggest that these structures are widespread, and their presence could help us better understand the early history of lunar impacts as well as providing sampling targets for future expeditions to the lunar surface.
The Chang’e-4 mission is exploring the South Pole-Aitken basin on the far side of the Moon. As part of this mission, Yutu-2 has travelled more than 1000 m across the Von Kármán crater since its deployment in early 2019. Reflectance spectra sent back by orbiters and Yutu-2 had previously revealed that the lunar soil, or regolith, in the basin is mainly iron-poor, with substantial amounts of an intrusive igneous rock known as anorthosite as well as coarse-grained rocks. Now, thanks to its panoramic camera, Yutu-2 has obtained images of transparent globules of a type never seen on the Moon before.
Dramatically different
Upon examining the images, a team led by Zhiyong Xiao of the Planetary Environmental and Astrobiological Research Laboratory at Sun Yat-sen University found that the globules ranged from light grey to brown in colour and were either spherical or dumbbell-shaped, measuring 1.5 to 2.5 cm in diameter. Although the Apollo astronauts also collected glassy globules from the lunar surface, their samples were mostly dark and opaque. The Apollo samples were also much bigger and contained a lot of clasts, which are fragments of older rock broken up and embedded in a younger one.
Xiao and colleagues say that the newly-discovered globules are very different from the blocky fragments that surround them. They speculate that they could have been created by high-energy impacts causing iron-poor materials such as pure anorthosites to melt. If this is indeed the case, the glassy deposits are likely to be widespread on the Moon’s far side, since pure anorthosites are a common component of the regolith in the vast lunar highlands and high-energy melts formed by impact events are abundant.
Globules might be extremely young
The shape and surface structure of the globules suggests that they may have formed (or been exposed) relatively recently, since surface materials on the Moon are continually being abraded and impacted. Indeed, growth rates of lunar regolith imply that the top 2-3 cm is completely overturned in less than 100 000 years. However, Xiao explains that the globules’ formation age cannot be calculated exactly based solely on their shape or surface structures.
The discovery of these globules on the Moon suggest that impacts on other planetary bodies could also form such “tektite-like” glasses. What is more, Xiao suggests that analysing their composition and isotope ages should yield valuable insights into the Moon’s impact history, since such impacts should have been common during the early years of Earth’s satellite. At this stage, however, the researchers are not able to confirm whether these structures are similar to tektites found here on Earth, where they were produced in craters large than 10 km across formed following impacts with water-rich targets.
Valuable sampling targets
Xiao’s team, which reports its work in Science Bulletin, say that the translucent globules on the Moon could provide valuable sampling targets for future lunar surface explorations, as they should be easy to recognize. They add that the globules could also be excellent raw materials from which to manufacture glass in situ, as they have good light-admitting quality.
The researchers are now analysing additional data returned by the Chang’e-4 mission, and hope to confirm the presence of other types of globules. “We are currently investigating the 500 mg of regolith samples returned by the Chang’e-5 mission (allocated to my group), in which hundreds of glass spherules (<500 𝜇m) have been discovered,” Xiao tells Physics World. “From the perspective of comparative planetology, we are combining our study of lunar glass spherules with our ongoing studies of tektites collected in southern China that belong to the vast Australasian strewn field. Our aim is to better understand the mechanisms by which various impact glasses form.”
Your night-mode photos might get a lot crisper thanks to a new device that exploits quantum mechanics to absorb photons more efficiently. Known as a quantum battery, the device stores the energy of the absorbed photons and can be charged simply by shining light on it – great news not just for low-light iPhone photography, but also for solar panels, which could use similar technology to capture the Sun’s energy a lot faster.
In a normal medium for storing energy, like a car battery, the time to charge the battery increases in proportion to the battery’s size. Quantum mechanical effects, however, make it possible to create systems with an energy absorption capacity that increases drastically as they get bigger. This is known as superabsorption, and it enables researchers to create a battery that, somewhat counter-intuitively, charges faster as its size increases.
Capturing light between mirrors
In the latest study, which is described in Science Advances, James Quach and colleagues at the University of Adelaide, Australia; the Politecnico di Milano and the National Research Center (CNR), Italy; and the Universities of St Andrews, Sheffield and Heriot-Watt in the UK created a quantum battery from molecules of an organic dye, trademarked Lumogen-F Orange. Dye molecules of this type can be modelled as an effective two-level system in which the molecule is most likely to be in one of two states: the ground state with minimal energy, or an excited state with higher energy. If laser light is fired at it at the right wavelength, it may absorb a photon and jump to the excited state.
To ensure that these dye molecules absorb photons efficiently, the researchers suspended them in a matrix of a non-reactive polymer and placed them in a cavity consisting of two mirrors. Because everyday mirrors (which are simply sheets of glass coated with reflective metal) are not reflective enough to keep the photons trapped in the cavity, the team used alternating layers of dielectric materials to create a device known as a distributed Bragg reflector.
When a laser beam shines into this highly reflective cavity, the dye molecules absorb photons and jump to the excited state. The amount of energy they absorb can be estimated by sending in a probe beam and monitoring how much of it gets reflected: once excited, molecules cannot absorb any more photons, so they reflect. Therefore, measuring the intensity of reflected light tells you how many dye molecules were excited, and how much energy was absorbed by the dye-battery.
The right concentration
To measure the battery’s charging rates, the group had to develop ultrafast measurement techniques to charge and probe the cavity in rapid succession, at time scales as short as 10 –14 seconds. The researchers also measured the absorption properties of different concentrations of dye while keeping the charging power constant. Remarkably, the time required to charge the cavity went down as the dye concentration went up, meaning that it took less time to charge N molecules in one microcavity than it would to charge N microcavities containing one molecule each. This phenomenon – a signature of superabsorption – occurs because the shared quantum entanglement between dye molecules lets them trap photons better than molecules on their own.
Given this result, one might wonder whether it would be possible to build batteries that charge instantaneously by increasing dye concentrations to some arbitrarily high level. Unfortunately, the way the molecules interact with each other and with light changes at very high concentrations, pushing the battery away from superabsorption. A further drawback is that because the dye molecules absorb energy faster, they also release it faster. While fast discharge may be desirable for some applications, like charging an electric vehicle, it is not good for a battery which should, ideally, store energy for a long time without dissipating it.
In this particular case, noise comes to the rescue. If the noise in the device is just right, the absorption-dissipation cycle can be disrupted as the excited dye molecules steer to something called a “dark mode” where photon emission is much suppressed.
Cavities in cameras
Although the dye molecules in this experiment are good at absorbing energy, and you can store a little bit of energy in a cavity, a useful battery would need to be able to extract the energy from the cavity before it could power, for instance, a mobile phone or a smart watch, or transport energy to a location where it can be stored long-term. To do this, the team would need to add additional components to the cavity that can transport the dye excitations outside, in the form of an electric current. “At the moment, the proof-of-principle devices we made are still quite small, and the charging occurs with light, so immediate applications would need to work with those constraints,” says Quach. Since superabsorption is a general quantum mechanical phenomenon, he adds, it may manifest in other systems as well.
One near-term application of cavity-based quantum batteries would be to improve low-light energy capture in photovoltaic cells used in solar cells and cameras. However, substantial work remains to be done before we can reliably use superabsorption outside of a laboratory. For example, today’s solar cells and cameras can store energy of a wide range of wavelengths, but the quantum battery demonstrated in this experiment can only absorb light at a particular frequency. Quach says he and his colleagues are optimistic that they can scale up the system, and they are looking into ways of storing and transferring energy, with the goal of producing a device that can be easily integrated into existing technologies.
Scientists have successfully used an implanted brain–computer interface (BCI) to enable a person with complete paralysis to communicate – suggesting that verbal communication with such devices may one day be possible for patients in a completely locked-in state.
Electrode arrays
For the trial, a team from the Wyss Center for Bio and Neuroengineering, ALS Voice and the University of Tübingen implanted a type of BCI called an auditory neurofeedback system into the brain of a 34-year-old male patient with amyotrophic lateral sclerosis (ALS) – also known as motor neurone disease. The patient was in a completely locked-in state and had no voluntary muscular control. After implantation, he could form words and phrases and communicate at an average rate of about one character per minute.
Reporting their findings in Nature Communications, the researchers describe how they inserted two tiny 3.2 x 3.2 mm microelectrode arrays, known as Utah arrays, into the surface of the participant’s motor cortex – the part of the brain responsible for movement.
Brain implants: The microelectrode arrays contain 64 needle-like electrodes that record the participant’s neural signals. (Courtesy: The Wyss Center, Geneva)
Each array contains 64 needle-like electrodes that record neural signals, before transferring them to a computer via a wired connection. Software then decodes the data and runs an auditory feedback speller that prompts the user to select letters to form words and sentences. The participant learned how to alter his own brain activity according to the audio feedback received, enabling him to select letters and ultimately spell words and sentences.
As joint lead author Jonas Zimmermann, senior neuroscientist at the Wyss Center, explains, the participant, who lives at home with his family, expressed his wish to take part in the clinical case study before he became completely locked-in.
“A member of the research team was usually present at the patient’s home for experimental sessions,” explains Zimmerman. “Every day the system was in use, there was first a calibration and training session to adapt the software to changes in brain activity. When we were convinced that there was reliable enough control, we ran the speller.”
Based on previous work by the Wyss Center and other groups, Zimmerman says the team had a strong belief that, in this case too, a locked-in patient could communicate using electrodes implanted directly into the brain.
“The patient changes the activity of single neurones. He hears the activity as a sequence of lower pitch or higher pitch tones, depending on how much the neurones are active. We can then ask him to hold the tone high to say ‘yes’ and hold it low to say ‘no’. This enables him to select letters and ultimately spell words and sentences,” he explains.
Wireless device
According to Zimmerman, the study answers a long-standing question about whether people with complete locked-in syndrome (CLIS) – who have lost all voluntary muscle control, including movement of the eyes or mouth – also lose the ability of their brain to generate commands for communication. Successful communication has previously been demonstrated with BCIs in individuals with paralysis but, to Zimmerman’s knowledge, this study is the first to achieve communication by someone who has no remaining voluntary movement and hence for whom the BCI is now the sole means of communication.
“For our participant there are no alternative approaches for communication. He has amyotrophic lateral sclerosis (ALS) – a progressive neurodegenerative disease in which people lose all ability to talk and to move. He cannot move his eyes nor voluntarily move any muscles, so the BCI is his only means of communicating with his family and medical caregivers,” notes Zimmerman.
At present, the BCI system used in the study is for clinical investigations only and is not available outside a research setting. Further demonstrations of its longevity, applicability in other patients, safety and efficacy are needed before it is suitable for widespread clinical use.
“At the Wyss Center, we are currently developing ABILITY, a wireless implantable BCI device for clinical use in people with CLIS,” Zimmerman adds. “Use of a wireless device avoids the potential infection risk associated with the percutaneous cable that connects the electrodes to the computer. We also plan to use this system for speech decoding. The next step is validating the new ABILITY device in pre-clinical and clinical trials.”
Archaeologists have adapted several techniques from physicists and the next one could be the detection of gamma rays in the field. For the first time, a gamma-ray spectrometer has been used in an archaeological setting. The instrument is normally used to identify radioactive contamination on nuclear sites, but now researchers at the University of Reading excavation in Roman Silchester in southern England have used it to find buried buildings and other objects. This was done by detecting gamma radiation emitted during the natural decay of elements in these materials – which can be measured up to one metre under the ground.
The detector was able to find a buried wall, for example, because the wall was made of materials that had a lower level of radioactivity than the surrounding soil. This suggests the wall was made from materials imported from a different geographical area. The technique is also particularly good at identifying materials that have been altered by industrial processes such firing in a kiln – which can change the amount of radioactive elements that an object contains.
Our brains have a remarkable capacity for storing and processing information in very efficient ways. That is why researchers are keen on developing neuromorphic computers that mimic how the brain works. The memristor is a component that is used in many neuromorphic architectures. It “remembers” the amount of charge that has flowed through it, with the information being stored in terms of the device’s resistance. Or at least it does this in principle, because researchers have not yet been able to create a device that operates as an ideal memristor.
Now, Feng Zhao and Brandon Sueoka at Washington State University have created a memristor that takes advantage of the material properties of honey. They sandwiched a layer of honey between a copper electrode and a copper oxide electron to create their memristor. They then looked at how the memristor performs in a circuit that mimics how information is passed between two neurons.
They found that the device’s performance was on par with other comparable memristors. They also showed that the device could be dissolved in water. This means that the device is partly biodegradable and that it would be easy to recycle.
Exotic materials known as kagome superconductors can play host to a rare state of matter in which electric currents form “loops” around unit cells in the material’s crystalline lattice. This discovery, made by researchers at Switzerland’s Paul Scherrer Institute (PSI) together with international collaborators, could reveal new information about how superconductivity emerges in materials where complex effects such as frustrated magnetism and intertwined orders play a major role.
Kagome metals are named after a traditional Japanese basket-weaving technique that produces a lattice of interlaced, corner-sharing symmetrical triangles. When the atoms of a metal or other conductor are arranged in this so-called kagome pattern, their electrons behave in unusual ways, giving rise to interaction-driven electronic phases of matter that can be identified by studying symmetries of the material.
In one such electronic phase, electrons organize themselves into a looped pattern along the kagome bonds between regions of the lattice with high and low concentrations of electric charge. Researchers have long debated whether such unusual loop currents could be a precursor to important condensed-matter phenomena such as high-temperature superconductivity (the ability to conduct electricity without resistance below critical temperatures of 77 K or higher) or the quantum anomalous Hall effect, in which a material exhibits a quantized voltage drop in a direction transverse to the flow of an electric current even in the absence of an external magnetic field.
A rare state of matter
A multinational team led by scientists at the PSI’s Laboratory for Muon Spin Spectroscopy has now identified a signature of such a phase in a kagome superlattice material with the chemical formula KV3Sb5, which becomes a superconductor at a critical temperature of 1.2 K. After spotting the weak internal magnetic fields that indicate the presence of charge ordering, the researchers also found evidence that these magnetic fields break “time-reversal symmetry”, which requires the laws of physics to look the same irrespective of whether time moves forwards or backwards.
To detect the tell-tale signs of time-reversal-symmetry breaking, the physicists used a technique called muon spin rotation/relaxation spectroscopy (uSR). Muons are elementary particles that are similar to electrons but have more than 200 times the electron’s mass. They have a finite lifetime, and at the end of it, they decay into lighter particles such as positrons, which are the antiparticles of electrons.
In the current study, which is described in Nature, the physicists implanted muons into their sample of KV3Sb5. The muons interact with their local environment via their internal angular momentum, which is sensitive to magnetic fields produced by electronic current loops. This means that as the muons decay into positrons, the way the polarization of the angular momentum evolves with time changes in a manner that signifies the presence of such loops.
Quantum anomalous Hall effect could come from orbital currents
The team observed a systematic shift in the magnetic signal and concluded that time-reversal symmetry breaks in KV3Sb5 at 80 K, the temperature at which the current loops form. Interestingly, the material also exhibits a giant quantum anomalous Hall effect at this temperature. This effect was previously unexplained, but the PSI researchers say their result provides the best evidence yet that it stems from orbital currents. This hypothesis, long debated for Kagome superlattices, could also apply to other unconventional superconductors that exhibit a large quantum anomalous Hall effect, such as graphene.
“Normally, these loop currents are hard to detect since their signals are usually too weak,” explains team leader Zurab Guguchia. “We overcame this problem by applying a high magnetic field of up to 9.5 Tesla to amplify the electronic response.”
The researchers say that their discovery of time-reversal symmetry-breaking fields – which implies both orbital currents and the peculiar charge ordering that gives rise to them – opens doors to “exotic avenues of physics and next-generation device research”. They add that the concept of orbital currents also forms the basis of “orbitronics”, in which the orbital degree of freedom is used as an information carrier in solid-state devices.
The researchers acknowledge that there is as yet no consensus on the superconducting gap structure in kagome materials with the formula AV3Sb5 (where A can be K, Rb or Cs). They attribute this lack of consensus to “the challenges of performing spectroscopic studies under extreme conditions including ultra-low temperatures and large pressures”. They plan to tackle these barriers by performing zero-field, high-field and high-pressure muon spin relaxation experiments to directly probe the interplay between chiral charge order and superconductivity across the temperature-pressure phase diagram of AV3Sb5. “This will allow us to assess not only the time-reversal symmetry-breaking nature of these two states, but also the evolution of the low-energy superconducting excitations as charge order is suppressed under pressure,” Guguchia tells Physics World.
