Aurelia Butler

As COVID-19 spread globally in 2020, many countries swiftly closed their borders to prevent the disease from entering. However, there was little scientific evidence to support the effectiveness of such measures.
While post-COVID research has extensively focused on the efficacy of internal travel restrictions, cross-border travel has received less attention due to challenges in accessing quality data. In a major multidisciplinary collaboration effort across Finland, Sweden, Norway, and Denmark, a group of researchers — including mathematicians, physicists and computer scientists — have published a pioneering study on the spread of infections across Nordic borders from spring until the end of 2020. The report sheds light on the efficacy of cross-border travel restrictions, helping us better understand which measures actually make a difference.
‘There have been many studies using data and modelling within countries, but this cross-border research is rather unique,’ says Associate Professor of Mathematics Lasse Leskelä from Finland’s Aalto University.
The researchers developed a sophisticated mathematical model relying on a long trail of footwork gathering travel data from the four neighbouring countries. Focus was on the short-term spread of the disease at a stage of the pandemic when infections had already started to spread within each country.
Border closures a blunt tool
The modelling revealed that cross-border closures were only likely to have significant impact in very specific scenarios. For example, a substantial disparity in disease prevalence between two countries would have to be accompanied by a high volume of cross-border traffic for restrictions to notably impact spread. It is notable that even though Sweden’s comparatively loose restrictions in 2020 contributed to the nation having vastly more case numbers than in neighbouring Finland, the overall impact of cross-border travel on the Finnish disease situation was low in absolute terms.
‘The way I see these results is that the closing of borders was mostly not very well justified. This was done out of uncertainty, because countries did not know what else to do. Since it has so many adverse effects, my take on this is that in the future, such drastic measures must be very carefully considered’, says Professor Tapio Ala-Nissilä from Aalto University.

However, the researchers point out that in different stages of a pandemic situation, there can be many layers of complexity. If a government must act, choosing between restricting local populations within its borders versus restricting travel across them, the latter may prove the better option.
‘According to our model, travellers from Sweden were over 10 times more likely to have COVID-19 in the summer of 2020 than the domestic Finnish population. So if you think about when the restrictions should hit and who should be affected, it would make more sense to place restrictions on these travellers at this time,’ Assistant Professor Mikko Kivelä from Aalto University points out.
The model also shows interesting differences between types of travel. Commuters, who may spend half a day in the destination country at a time, played a smaller role in spreading infections than vacationers who possibly spent their entire infectious periods in the country.
Preparing for the next pandemic
Kivelä emphasises that in spring 2020, decision-makers were faced with myriad uncertainties that made it impossible to reliably analyse and estimate the effects of their countermeasures. This is also where the current study makes its most significant contribution — as a predictive model for future use.
‘The really important part is that we have developed different ways of looking at this question: a mathematical machinery to answer questions about what border control interventions are necessary and when to apply,’ says university researcher Mikhail Shubin from the University of Helsinki.
Although the current study pertains to the Nordics, the researchers say that it can be applied to other countries as well. The main concern is getting reliable and comparable data. Often, even if the outward appearance of a particular data set is promising, details like reporting delays will complicate its usage.
‘Access to mobility is not easy to gain, and within the Schengen area in particular there is no detailed tracking for who moves where. You need to have access to lots of data sets, from road crossings to railroads, ferries and aeroplanes. We also used mobile phone data to validate our findings,’ explains Leskelä. ‘Usually, to do this detailed modelling, you need personal contacts and you need to build trust.’
The study is part of the NordicMathCovid project. The project includes teams from Finland, Sweden and Norway and involves a number of universities and public institutions across the Nordics. Supported by NordForsk, the project started in September 2020 and has produced research on pandemic flows and vaccination strategies from varying angles. More

In an ongoing game of cosmic hide and seek, scientists have a new tool that may give them an edge. Physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have developed a computer program incorporating machine learning that could help identify blobs of plasma in outer space known as plasmoids. In a novel twist, the program has been trained using simulated data.
The program will sift through reams of data gathered by spacecraft in the magnetosphere, the region of outer space strongly affected by Earth’s magnetic field, and flag telltale signs of the elusive blobs. Using this technique, scientists hope to learn more about the processes governing magnetic reconnection, a process that occurs in the magnetosphere and throughout the universe that can damage communications satellites and the electrical grid.
Scientists believe that machine learning could improve plasmoid-finding capability, aid the basic understanding of magnetic reconnection and allow researchers to better prepare for the aftermath of reconnection-caused disturbances.
“As far as we know, this is the first time that anyone has used artificial intelligence trained on simulated data to look for plasmoids,” said Kendra Bergstedt, a graduate student in the Princeton Program in Plasma Physics, which is based at PPPL. Bergstedt was the first author of the paper reporting the results in Earth and Space Science. The work pairs the Lab’s growing expertise in computational sciences with its long history of exploring magnetic reconnection.
Looking for a link
Scientists want to find reliable, accurate methods for detecting plasmoids so they can determine whether they affect magnetic reconnection, a process consisting of magnetic field lines separating, violently reattaching and releasing tremendous amounts of energy. When it occurs near Earth, reconnection can trigger a cascade of charged particles falling into the atmosphere, disrupting satellites, mobile phones and the electrical grid. “Some researchers believe that plasmoids aid fast reconnection in large plasmas,” said Hantao Ji, professor of astrophysical sciences at Princeton University and a distinguished research fellow at PPPL. “But those hypotheses haven’t been proven yet.”
The researchers want to know whether plasmoids can change the rate at which reconnection occurs. They also want to gauge how much energy reconnection imparts to the plasma particles. “But to clarify the relationship between plasmoids and reconnection, we have to know where the plasmoids are,” Bergstedt said. “That’s what machine learning could help us do.”
The scientists used computer-generated training data to ensure the program could recognize a range of plasma signatures. Typically, plasmoids created by computer models are idealized versions based on mathematical formulas with shapes — like perfect circles — that do not often occur in nature. If the program were trained only to recognize these perfect versions, it might miss those with other shapes. To prevent those misses, Bergstedt and Ji decided to use artificial, deliberately imperfect data so the program would have an accurate baseline for future studies. “Compared to mathematical models, the real world is messy,” Bergstedt said. “So we decided to let our program learn using data with fluctuations that you would get in actual observations. For instance, rather than beginning our simulations with a perfectly flat electrical current sheet, we give our sheet some wobbles. We’re hoping that the machine learning approach can allow for more nuance than a strict mathematical model can.” This research builds on past attempts in which Bergstedt and Ji wrote computer programs that incorporated more idealized models of plasmoids.

The use of machine learning will only become more common in astrophysics research, according to the scientists. “It could particularly be helpful when making extrapolations from small numbers of measurements, as we sometimes do when studying reconnection,” said Ji. “And the best way to learn how to use a new tool is to actually use it. We don’t want to stand on the sidelines and miss an opportunity.”
Bergstedt and Ji plan to use the plasmoid-detecting program to examine data being gathered by NASA’s Magnetospheric Multiscale (MMS) mission. Launched in 2015 to study reconnection, MMS consists of four spacecraft flying in formation through plasma in the magnetotail, the area in space pointing away from the sun that is controlled by Earth’s magnetic field.
The magnetotail is an ideal place to study reconnection because it combines accessibility with scale. “If we study reconnection by observing the sun, we can only take measurements from afar,” Bergstedt said. “If we observe reconnection in a laboratory, we can put our instruments directly into the plasma, but the sizes of the plasmas would be smaller than those typically found in space.” Studying reconnection in the magnetotail is an ideal middle option. “It’s a large and naturally occurring plasma that we can measure directly using spacecraft that fly through it,” Bergstedt said.
As Bergstedt and Ji improve the plasmoid-detecting program, they hope to take two significant steps. The first is performing a procedure known as domain adaptation, which will help the program analyze datasets that it has never encountered before. The second step involves using the program to analyze data from the MMS spacecraft. “The methodology we demonstrated is mostly a proof of concept since we haven’t aggressively optimized it,” Bergstedt said. “We want to get the model working even better than it is now, start applying it to real data and then we’ll just go from there!”
This research was supported by the DOE’s Fusion Energy Sciences program under contract DE-AC0209CH11466, by NASA under grants NNH15AB29I and 80HQTR21T0105, and by the National Science Foundation Graduate Research Fellowship under grant DGE-2039656. More

The use of wearable electronics that continuously monitor biosignals has transformed the healthcare and fitness industries. These devices are becoming increasingly common and are projected to reach a market valuation of approximately USD 572.06 billion by 2033. With this rapid growth, there is an escalating demand for high-quality bioelectrodes capable of accurately recording biosignals over extended periods. However, many of the materials currently used for bioelectrodes, such as metals, conductive polymers, and hydrogels, have limitations. They often lack the flexibility to stretch the skin without breaking and have low humidity permeability, leading to sweat buildup and discomfort.
To address these limitations, in a study published in the journal NPG Asia Materialson 20 June 2024, a research team led by Assistant Professor Tatsuhiro Horii and Associate Professor Toshinori Fujie from Tokyo Institute of Technology (Tokyo Tech) has developed a bioelectrode material that is stretchable and permeable to humidity and conforms closely to the skin. This innovative material is composed of layers of conductive fibrous networks consisting of single-wall carbon nanotubes (SWCNTs) on a stretchable poly(styrene-b-butadiene-b-styrene) (SBS) nanosheet. The nanosheet conforms tightly to the skin, allowing for precise biosignal measurements, while the carbon nanotube fibers maintain the material’s stretchability and humidity permeability.
“Self-supporting electrodes that are stretchable, permeable to humidity, and conformable to skin surface bumps are required to allow for the natural deformation of skin without restricting body movements,” says Horii.
The researchers applied SWCNTs as aqueous dispersions onto SBS nanosheets, creating multiple layers reaching a thickness of only 431 nm. Each coating of SWCNTs increased the density and thickness of the fibers, modifying the bioelectrode’s characteristics. While adding more SWCNT layers increased nanosheet stiffness (from an initial 48.5 MPa elastic modulus to 60.8 MPa for a single layer and 104.2 MPa for five layers), the bioelectrode maintained impressive flexibility. Pristine SBS nanosheets and those with one or three layers of SWCNTs (SWCNT 3rd-SBS) stretched elastically by 380% of their original length before permanent deformation. This flexibility surpasses metal electrodes like gold, which have Young’s moduli in the several-hundred-GPa range and can only stretch less than 30% of their original length before breaking.
Another crucial requirement for bioelectrodes is high water vapor permeability to prevent sweat buildup during exercise. Adding SWCNTs is beneficial as its fibrous network structure improves breathability compared to continuous films. In experiments measuring water vapor transmission rate (WVTR), researchers found that SWCNT 3rd-SBS exhibited a WVTR of 28,316 g m-2 (2 h)-1, which is twice that of normal skin.
The bioelectrode material is also highly resilient for extended use. To test the material’s durability, the researchers immersed the bioelectrodes in artificial sweat and subjected them to repeated bending, measuring the change in resistance. In these tests, they found that the resistance increased negligibly, by only 1.1 times in sweat and by 1.3 times over 300 cycles of bending. Furthermore, the SWCNT 3rd-SBS nanosheets showed little to no detachment after being rubbed ten times, indicating its suitability for prolonged use.
To assess its real-world performance, the researchers compared an SBS nanosheet with three layers of SWCNT to commercially available bioelectrode materials such as Ag/AgCl gel electrodes. The bioelectrodes were attached to the forearm, and surface electromyography (sEMG) measurements were taken during gripping motions. In this experiment, the performance of the SWCNT-SBS nanosheet was comparable to that of commercial Ag/AgCl gel electrodes, achieving similar signal-to-noise ratios of 24.6 dB and 33.3 dB, respectively.
“We obtained skin-conformable bioelectrodes with high water vapor permeabilities, which showed comparable performance in sEMG measurements to those of conventional electrodes,” concludes Fujie, highlighting the material’s promising capabilities for healthcare wearables. More

The more we discover about the natural world, the more we find that nature is the greatest engineer. Past research believed that liquids can only be transported in fixed direction on species with specific liquid communication properties and cannot switch the transport direction. Recently, The Hong Kong Polytechnic University (PolyU) researchers have shown that an African plant controls water movement in a previously unknown way — and this could inspire breakthroughs in a range of technologies in fluid dynamics and nature-inspired materials, including applications that require multistep and repeated reactions, such as microassays, medical diagnosis and solar desalination etc. The study has been recently published in the international academic journal Science.
Liquid transport is an unsung miracle of nature. Tall trees, for example, have to lift huge amounts of water every day from their roots to their highest leaves, which they accomplish in perfect silence. Some lizards and plants channel water through capillaries. In the desert, where making the most of scarce moisture is vital, some beetles can capture fog-borne water and direct it along their backs using a chemical gradient.
Scientists have long sought to hone humankind’s ability to move liquids directionally. Applications as diverse as microfluidics, water harvesting, and heat transfer depend on the efficient directional transport of water, or other fluids, at small or large scales. While the above species provide nature-based inspiration, they are limited to moving liquids in a single direction. A research team led by Prof. WANG Liqiu, Otto Poon Charitable Foundation Professor in Smart and Sustainable Energy, Chair Professor of Thermal-Fluid and Energy Engineering, Department of Mechanical Engineering of PolyU, has discovered that the succulent plant Crassula muscosa, native to Namibia and South Africa, can transport liquid in selected directions.
Together with colleagues from the University of Hong Kong and Shandong University, the PolyU researchers noticed that when two separate shoots of the plant were infused with the same liquids, the liquids were transported in opposite directions. In one case, the liquid travelled exclusively towards the tip, whereas the other shoot directed the flow straight to the plant root. Given the arid but foggy conditions in which C. muscosa lives, the ability to trap water and transport it in selected directions is a lifeline for the plant.
As the shoots were held horizontally, gravity can be ruled out as the cause of the selective direction of transport. Instead, the plant’s special properties stem from the tiny leaves packed onto its shoots. Also known as “fins,” they have a unique profile, with a swept-back body (resembling a shark’s fin) tapering to a narrow ending that points to the tip of the plant. The asymmetry of this shape is the secret to C. muscosa’s selective directional liquid transport. It all has to do with manipulating the meniscus — the curved surface on top of a liquid.
Specifically, the key lies in subtle differences between the fin shapes on different shoots. When the rows of fins bend sharply towards the tip, the liquid on the shoot also flows in that direction. However, on a shoot whose fins — although still pointing at the tip — have a more upward profile, the direction of movement is instead to the root. The flow direction depends on the angles between the shoot body and the two sides of the fin, as these control the forces exerted on droplets by the meniscus — blocking flow in one direction and sending it in the other.
Armed with this understanding of how the plant directs liquid flow, the team created an artificial mimic. Dubbed CMIAs, for ‘C. muscosa-inspired arrays’, these 3D-printed fins act like the tilted leaves of C. muscosa, controlling the orientation of liquid flow. Cleverly, while the fins on a natural plant shoot are immobile, the use of a magnetic material for artificial CMIAs allows them to be reoriented at will. Simply by applying a magnetic field, the liquid flow through a CMIA can be reversed. This opens up the possibility of liquid transport along dynamically changing paths in industrial and laboratory settings. Alternatively, flow could be redirected by changing the spacing between fins.
Numerous areas of technology stand to benefit from CMIAs. Prof. Wang said, “There are foresee applications of real-time directional control of fluid flow in microfluidics, chemical synthesis, and biomedical diagnostics. The biology-mimicking CMIA design could also be used not just for transporting liquids but for mixing them, for example in a T-shaped valve. The method is suited to a range of chemicals and overcomes the heating problem found in some other microfluidic technologies.” More

A new way to map the spread and evolution of pathogens, and their responses to vaccines and antibiotics, will provide key insights to help predict and prevent future outbreaks. The approach combines a pathogen’s genomic data with human travel patterns, taken from anonymised mobile phone data.
Researchers from the Wellcome Sanger Institute, University of the Witwatersrand and National Institute for Communicable Diseases in South Africa, the University of Cambridge, and partners across the Global Pneumococcal Sequencing project1, integrated genomic data from nearly 7,000 Streptococcus pneumoniae (pneumococcus)samples collected in South Africa with detailed human mobility data2. This enabled them to see how these bacteria, which cause pneumonia and meningitis3, move between regions and evolve over time.
The findings, published today (3 July) in Nature, suggest initial reductions in antibiotic resistance linked to the 2009 pneumococcal vaccine may be only temporary, as non-targeted strains resistant to antibiotics such as penicillin gained a 68 per cent competitive advantage.
This is the first time researchers have been able to precisely quantify the fitness — their ability to survive and reproduce — of different pneumococcal strains. The insight could inform vaccine development to target the most harmful strains, and may be applicable to other pathogens.
Many infectious diseases such as tuberculosis, HIV, and COVID-19 exist in multiple strains or variants circulating simultaneously, making them difficult to study. Pneumococcus, a bacterium that is a leading cause of pneumonia, meningitis, and sepsis worldwide4, is a prime example with over 100 types and 900 genetic strains globally. Pneumonia alone kills around 740,000 children under the age of five each year5, making it the single largest infectious cause of death in children.
Pneumococcal diversity hampers control efforts, as vaccines targeting major strains leave room for others to fill the vacant niches. How these bacteria spread, how vaccines affect their survival, and their resistance to antibiotics remains poorly understood.
In this new study, researchers analysed genome sequences from 6,910 pneumococcus samples collected in South Africa between 2000 and 2014 to track the distribution of different strains over time. They combined these data with anonymised records of human travel patterns collected by Meta2.

The team developed computational models which revealed pneumococcal strains take around 50 years to fully mix throughout South Africa’s population, largely due to localised human movement patterns.
They found that while introduction of a pneumococcal vaccine against certain types of these bacteria in 2009 reduced the number of cases caused by those types6, it also made other non-targeted strains of these bacteria gain a 68 per cent competitive advantage, with an increasing proportion of them becoming resistant to antibiotics such as penicillin. This suggests that the vaccine-linked protection against antibiotic resistance is short-lived.
Dr Sophie Belman, first author of the study, former PhD student at the Wellcome Sanger Institute and now a Schmidt Science Fellow at the Barcelona Supercomputing Centre, Spain, said: “While we found that pneumococcal bacteria generally spread slowly, the use of vaccines and antimicrobials can quickly and significantly change these dynamics. Our models could be applied to other regions and pathogens to better understand and predict pathogen spread, in the context of drug resistance and vaccine effectiveness.”
Dr Anne von Gottberg, author of the study at National Institute for Communicable Diseases, Johannesburg, South Africa, said: “Despite vaccination efforts, pneumonia remains one of the leading causes of death for children under five in South Africa. With continuous genomic surveillance and adaptable vaccination strategies to counter the remarkable adaptability of these pathogens, we may be able to better target interventions to limit the burden of disease.”
Professor Stephen Bentley, senior author of the study at the Wellcome Sanger Institute, said: “The pneumococcus’s diversity has obscured our view on how any given strain spreads from one region to the next. This integrated approach using bacterial genome and human travel data finally allows us to cut through that complexity, uncovering hidden migratory paths in high-definition for the first time. This could allow researchers to anticipate where emerging high-risk strains may take hold next, putting us a step ahead of potential outbreaks.”
Notes Partners from the Global Pneumococcal Sequencing project can be found here: https://www.pneumogen.net/gps/ The human mobility data used in this study are Meta Data for Good baseline data, released during the 2020 SARS-CoV-2 pandemic. These data rely on personal consent for location sharing, and Data for Good ensures individual privacy by preventing re-identification in aggregated datasets. For more information on pneumococcal disease, visit: https://www.cdc.gov/pneumococcal/about/index.html https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5666185/ https://www.who.int/news-room/fact-sheets/detail/pneumonia Before these vaccines, 85 per cent of pneumococcal strains were those targeted by the vaccines. By 2014, this dropped to 33.2 per cent. This change was consistent across all nine provinces in South Africa. More

Researchers have developed a new two-photon polymerization technique that uses two lasers to 3D print complex high-resolution structures. The advance could make this 3D printing process less expensive, helping it find wider use in a variety of applications.
Two-photon polymerization is an advanced additive manufacturing technique that traditionally uses femtosecond lasers to polymerize materials in a precise, 3D manner. Although this process works well for making high-resolution microstructures, it isn’t widely used in manufacturing because femtosecond lasers are expensive and increase the cost of printing parts.
“We combined a relatively low-cost laser emitting visible light with a femtosecond laser emitting infrared pulses to reduce the power requirement of the femtosecond laser,” said research team leader Xianfan Xu from Purdue University. “In this way, with a given femtosecond laser power, the printing throughput can be increased, leading to a lower cost for printing individual parts.”
In the Optica Publishing Group journal Optics Express, the researchers show that the two-laser approach reduces the femtosecond laser 3D printing power needed by as much as 50% compared to using a femtosecond laser alone.
“3D printing with high resolution has many applications, including 3D electronics devices, micro-robots for the biomedical field and 3D structures or scaffolds for tissue engineering,” said Xu. “Our novel, 3D printing approach can be readily implemented in many existing femtosecond laser 3D printing systems.”
Finding the right laser balance
The new work is part of the research team’s effort to continuously improve the printing speed and reduce the printing cost for two-photon polymerization, which uses the phenomenon of two-photo absorption to precisely cure, or solidify, a photosensitive material.

“In a conventional two-photon polymerization printing process, the femtosecond laser is first used to initiate a photochemical process that reduces the inhibition species in the material before printing starts,” said Xu. “We used a low-cost laser for this purpose instead.”
The new approach combines single-photon absorption from a 532 nm nanosecond laser with two-photon absorption from an 800-nm femtosecond laser. For this to work, the researchers had to find the right balance between the printing and inhibition caused by the two lasers. They did this by creating a new mathematical model to help them understand the photochemical processes involved and to compute the combined effect of two-photon and single-photon excitation processes. They also used the model to identify the dominant processes controlling how much the femtosecond laser’s power could be reduced while still achieving desirable printing results.
Printing detailed structures
After finetuning the new approach, they used it to print various 2D and 3D structures using reduced femtosecond laser power. These included detailed woodpiles measuring just 25 × 25 × 10 μm as well as a micron-scale buckyball, chiral structure and trefoil knot. Experimental results showed that the new method reduced the power required from the femtosecond laser by up to 80 percent for 2D structures and up to around 50 percent for 3D structures.
“This new printing approach could impact manufacturing technologies, influencing the development of devices across consumer electronics and healthcare sectors both now and in the future,” said Xu. The researchers are now working to further improve the printing speed and reduce the cost of 3D printing. More

A research effort led by scientists at the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) has made advances that could enable a broader range of currently unimagined optoelectronic devices.
The researchers, whose previous innovation included incorporating a perovskite layer that allowed the creation of a new type of polarized light-emitting diode (LED) that emits spin-controlled photons at room temperature without the use of magnetic fields or ferromagnetic contacts, now have gone a step further by integrating a III-V semiconductor optoelectronic structure with a chiral halide perovskite semiconductor. That is, they transformed an existing commercialized LED into one that also controls the spin of electrons. The results provide a pathway toward transforming modern optoelectronics, a field that relies on the control of light and encompasses LEDs, solar cells, and telecommunications lasers, among other devices.
“It’s up to one’s imagination where this might go or where this might end up,” said Matthew Beard, a senior research fellow at NREL and coauthor of the newly published Nature article, “Room temperature spin injection across a chiral-perovskite/III-V interface.”
Beard also serves as director of the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Science Basic Energy Sciences within DOE. The reported research was funded by CHOISE and relied on a broad range of scientific expertise drawn from NREL, the Colorado School of Mines, University of Utah, University of Colorado Boulder, and the Universite de Lorraine in France.
The goal of CHOISE is to understand control over the interconversion of charge, spin, and light using carefully designed chemical systems. In particular, the work focuses on control over the electron spin that can be either “up” or “down.” Most current-day optoelectronic devices rely on the interconversion between charge and light. However, spin is another property of electrons, and control over the spin could enable a wide plethora of new effects and functionality. The researchers published a paper in 2021 in which they reported how by using two different perovskite layers they were able to control the spin by creating a filter that blocks electrons “spinning” in the wrong direction.
They hypothesized at the time that advancements could be made in optoelectronics if they could successfully incorporate the two semiconductors, and then went on to do just that. The breakthroughs made, which include eliminating the need for subzero Celsius temperatures, can be used to increase data processing speeds and decrease the amount of power needed.
“Most current-day technologies are all based on controlling charge,” Beard said. “Most people just forget about the electron spin, but spin is very important, and it’s also another parameter that one can control and utilize.”
Manipulating the spin of electrons in a semiconductor has previously required the use of ferromagnetic contacts under an applied magnetic field. Using chiral perovskites, the researchers were able to transform an LED to one that emits polarized light at room temperature and without a magnetic field. Chirality refers to the material’s structure that cannot be superimposed on its mirror image, such as a hand. For example, a “left-handed” oriented chiral system may allow transport of electrons with “up” spins but block electrons with “down” spins, and vice versa. The spin of the electron is then converted to the “spin,” or polarization, of the emitted light. The degree of polarization, which measures the intensity of light that is polarized in one direction, reached about 2.6% in the previous research. The addition of the III-V semiconductor — which is made of materials in the third and fifth columns of the periodic table — boosted the polarization to about 15%. The degree of polarization serves as a direct measure of spin accumulation in the LED.
“This work is particularly exciting to me, as it combines spin functionality with a traditional LED platform,” said the first author of the work, Matthew Hautzinger. “You can buy an LED analogous to what we used for 14 cents, but with the chiral perovskite incorporated, we’ve transformed an already robust (and well understood) technology into a futuristic spin-control device.” More