A new transition-metal oxide crystal that reversibly and repeatedly absorbs and releases oxygen could be ideal for use in fuel cells and as the active medium in clean energy technologies such as thermal transistors, smart windows and new types of batteries. The “breathing” crystal, discovered by scientists at Pusan National University in Korea and Hokkaido University in Japan, is made from strontium, cobalt and iron and contains oxygen vacancies.

Transition-metal oxides boast a huge range of electrical properties that can be tuned all the way from insulating to superconducting. This means they can find applications in areas as diverse as energy storage, catalysis and electronic devices.

Among the different material parameters that can be tuned are the oxygen vacancies. Indeed, ordering these vacancies can produce new structural phases that show much promise for oxygen-driven programmable devices.

Element-specific behaviours

In the new work, a team of researchers led by physicist Hyoungjeen Jeen of Pusan and materials scientist Hiromichi Ohta in Hokkaido studied SrFe_0.5Co_0.5O_x. The researchers focused on this material, they say, since it belongs to the family of topotactic oxides, which are the main oxides being studied today in solid-state ionics. “However, previous work had not discussed which ion in this compound was catalytically active,” explains Jeen. “What is more, the cobalt-containing topotactic oxides studied so far were fragile and easily fractured during chemical reactions.”

The team succeeded in creating a unique platform from a solid solution of epitaxial SrFe_0.5Co_0.5O_2.5 in which both the cobalt and iron ions bathed in the same chemical environment. “In this way, we were able to test which ion was better for reduction reactions and whether or not it sustained its structural integrity,” Jeen tells Physics World. “We found that our material showed element-specific reduction behaviours and reversible redox reactions.”

The researchers made their material using a pulsed laser deposition technique, ideal for the epitaxial synthesis of multi-element oxides that allowed them to grow SrFe_0.5Co_0.5O_2.5 crystals in which the iron and cobalt ions were randomly located in the crystal. This random arrangement was key to the material’s ability to repeatedly release and absorb oxygen, they say.

“It’s like giving the crystal ‘lungs’ so that it can inhale and exhale oxygen on command,” says Jeen.

Stable and repeatable

This simple breathing picture comes from the difference in the catalytic activity of cobalt and iron in the compound, he explains. Cobalt ions prefer to lose and gain oxygen and these ions are the main sites for the redox activity. However, since iron ions prefer not to lose oxygen during the reduction reaction, they serve as pillars in this architecture. This allows for stable and repeatable oxygen release and uptake.

Until now, most materials that absorb and release oxygen in such a controlled fashion were either too fragile or only functioned at extremely high temperatures. The new material works under more ambient conditions and is stable. “This finding is striking in two ways: only cobalt ions are reduced, and the process leads to the formation of an entirely new and stable crystal structure,” explains Jeen.

The researchers also showed that the material could return to its original form when oxygen was reintroduced, so proving that the process is fully reversible. “This is a major step towards the realization of smart materials that can adjust themselves in real time,” says Ohta. “The potential applications include developing a cathode for intermediate solid oxide fuel cells, an active medium for thermal transistors (devices that can direct heat like electrical switches), smart windows that adjust their heat flow depending on the weather and even new types of batteries.”

Looking ahead, Jeen, Ohta and colleagues aim to investigate the material’s potential for practical applications.

They report their present work in Nature Communications.

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Physicists in Germany say they have measured the correlated behaviour of atoms in molecules prepared in their lowest quantum energy state for the first time. Using a technique known as Coulomb explosion imaging, they showed that the atoms do not simply vibrate individually. Instead, they move in a coupled fashion that displays fixed patterns.

According to classical physics, molecules with no thermal energy – for example, those held at absolute zero – should not move. However, according to quantum theory, the atoms making up these molecules are never completely “frozen”, so they should exhibit some motion even at this chilly temperature. This motion comes from the atoms’ zero-point energy, which is the minimum energy allowed by quantum mechanics for atoms in their ground state at absolute zero. It is therefore known as zero-point motion.

Reconstructing the molecule’s original structure

To study this motion, a team led by Till Jahnke from the Institute for Nuclear Physics at Goethe University Frankfurt and the Max Planck Institute for Nuclear Physics in Heidelberg used the European XFEL in Hamburg to bombard their sample – an iodopyridine molecule consisting of 11 atoms – with ultrashort, high-intensity X-ray pulses. These high-intensity pulses violently eject electrons out of the iodopyridine, causing its constituent atoms to become positively charged (and thus to repel each other) so rapidly that the molecule essentially explodes.

To image the molecular fragments generated by the explosion, the researchers used a customized version of a COLTRIMS reaction microscope. This approach allowed them to reconstruct the molecule’s original structure.

From this reconstruction, the researchers were able to show that the atoms do not simply vibrate individually, but that they do so in correlated, coordinated patterns. “This is known, of course, from quantum chemistry, but it had so far not been measured in a molecule consisting of so many atoms,” Jahnke explains.

Data challenges

One of the biggest challenges Jahnke and colleagues faced was interpreting what the microscope data was telling them. “The dataset we obtained is super-rich in information and we had already recorded it in 2019 when we began our project,” he says. “It took us more than two years to understand that we were seeing something as subtle (and fundamental) as ground-state fluctuations.”

Since the technique provides detailed information that is hidden to other imaging approaches, such as crystallography, the researchers are now using it to perform further time-resolved studies – for example, of photochemical reactions. Indeed, they performed and published the first measurements of this type at the beginning of 2025, while the current study (which is published in Science) was undergoing peer review.

“We have pushed the boundaries of the current state-of-the-art of this measurement approach,” Jahnke tells Physics World, “and it is nice to have seen a fundamental process directly at work.”

For theoretical condensed matter physicist Asaad Sakhel at Balqa Applied University, Jordan, who was not involved in this study, the new work is “an outstanding achievement”. “Being able to actually ‘see’ zero-point motion allows us to delve deeper into the mysteries of quantum mechanics in our quest to a further understanding of its foundations,” he says.

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Physicists at the Chinese Academy of Sciences (CAS) have used diamond-based quantum sensors to uncover what they say is the first unambiguous experimental evidence for the Meissner effect – a hallmark of superconductivity – in bilayer nickelate materials at high pressures. The discovery could spur the development of highly sensitive quantum detectors that can be operated under high-pressure conditions.

Superconductors are materials that conduct electricity without resistance when cooled to below a certain critical transition temperature T_c. Apart from a sharp drop in electrical resistance, another important sign that a material has crossed this threshold is the appearance of the Meissner effect, in which the material expels a magnetic field from its interior (diamagnetism). This expulsion creates such a strong repulsive force that a magnet placed atop the superconducting material will levitate above it.

In “conventional” superconductors such as solid mercury, the T_c is so low that the materials must be cooled with liquid helium to keep them in the superconducting state. In the late 1980s, however, physicists discovered a new class of superconductors that have a T_c above the boiling point of liquid nitrogen (77 K). These “unconventional” or high-temperature superconductors are derived not from metals but from insulators containing copper oxides (cuprates).

Since then, the search has been on for materials that superconduct at still higher temperatures, and perhaps even at room temperature. Discovering such materials would have massive implications for technologies ranging from magnetic resonance imaging machines to electricity transmission lines.

Enter nickel oxides

In 2019 researchers at Stanford University in the US identified nickel oxides (nickelates) as additional high-temperature superconductors. This created a flurry of interest in the superconductivity community because these materials appear to superconduct in a way that differs from their copper-oxide cousins.

Among the nickelates studied, La₃Ni₂O_7-δ (where δ can range from 0 to 0.04) is considered particularly promising because in 2023, researchers led by Meng Wang of China’s Sun Yat-Sen University spotted certain signatures of superconductivity at a temperature of around 80 K. However, these signatures only appeared when crystals of the material were placed in a device called a diamond anvil cell (DAC). This device subjects samples of material to extreme pressures of more than 400 GPa (or 4 × 10⁶ atmospheres) as it squeezes them between the flattened tips of two tiny, gem-grade diamond crystals.

The problem, explains Xiaohui Yu of the CAS’ Institute of Physics, is that it is not easy to spot the Meissner effect under such high pressures. This is because the structure of the DAC limits the available sample volume and hinders the use of highly sensitive magnetic measurement techniques such as SQUID. Another problem is that the sample used in the 2023 study contains several competing phases that could mix and degrade the signal of the La₃Ni₂O_7-δ.

Nitrogen-vacancy centres embedded as in-situ quantum sensors

In the new work, Yu and colleagues used nitrogen-vacancy (NV) centres embedded in the DAC as in-situ quantum sensors to track and image the Meissner effect in pressurized bilayer La₃Ni₂O_7-δ. This newly developed magnetic sensing technique boasts both high sensitivity and high spatial resolution, Yu says. What is more, it fits perfectly into the DAC high-pressure chamber.

Next, they applied a small external magnetic field of around 120 G. Under these conditions, they measured the optically detected magnetic resonance (ODMR) spectra of the NV centres point by point. They could then extract the local magnetic field from the resonance frequencies of these spectra. “We directly mapped the Meissner effect of the bilayer nickelate samples,” Yu says, noting that the team’s image of the magnetic field clearly shows both a diamagnetic region and a region where magnetic flux is concentrated.

Weak demagnetization signal

The researchers began their project in late 2023, shortly after receiving single-crystal samples of La₃Ni₂O_7-δ from Wang. “However, after two months of collecting data, we still had no meaningful results,” Yu recalls. “From these experiments, we learnt that the demagnetization signal in La₃Ni₂O_7-δ crystals was quite weak and that we needed to improve either the nickelate sample or the sensitivity of the quantum sensor.”

To overcome these problems, they switched to using polycrystalline samples, enhancing the quality of the nickelate samples by doping them with praseodymium to make La₂PrNi₂O₇. This produced a sample with an almost pure bilayer structure and thus a much stronger demagnetization signal. They also used shallow NV centres implanted on the DAC cutlet (the smaller face of the two diamond tips).

“Unlike the NV centres in the original experiments, which were randomly distributed in the pressure-transmitting medium and have relatively large ODMR widths, leading to only moderate sensitivity in the measurements, these shallow centres are evenly distributed and well aligned, making it easier for us to perform magnetic imaging with increased sensitivity,” Yu explains.

These improvements enabled the team to obtain a demagnetization signal from the La₂PrNi₂O₇ and La₃Ni₂O_7-δ samples, he tells Physics World. “We found that the diamagnetic signal from the La₂PrNi₂O₇ samples is about five times stronger than that from the La₃Ni₂O_7-δ ones prepared under similar conditions – a result that is consistent with the fact that the Pr-doped samples are of a better quality.”

Physicist Jun Zhao of Fudan University, China, who was not involved in this work, says that Yu and colleagues’ measurement represents “an important step forward” in nickelate research. “Such measurements are technically very challenging, and their success demonstrates both experimental ingenuity and scientific significance,” he says. “More broadly, their result strengthens the case for pressurized nickelates as a new platform to study high-temperature superconductivity beyond the cuprates. It will certainly stimulate further efforts to unravel the microscopic pairing mechanism.”

As well as allowing for the precise sensing of magnetic fields, NV centres can also be used to accurately measure many other physical quantities that are difficult to measure under high pressure, such as strain and temperature distribution. Yu and colleagues say they are therefore looking to further expand the application of these structures for use as quantum sensors in high-pressure sensing.

They report their current work in National Science Review.

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Micron-sized dust particles in the atmosphere could trigger the formation of ice in certain types of clouds in the Northern Hemisphere. This is the finding of researchers in Switzerland and Germany, who used 35 years of satellite data to show that nanoscale defects on the surface of these aerosol particles are responsible for the effect. Their results, which agree with laboratory experiments on droplet freezing, could be used to improve climate models and to advance studies of cloud seeding for geoengineering.

In the study, which was led by environmental scientist Diego Villanueva of ETH Zürich, the researchers focused on clouds in the so-called mixed-phase regime, which form at temperatures of between −39° and 0°C and are commonly found in mid- and high-latitudes, particularly over the North Atlantic, Siberia and Canada. These mixed-phase regime clouds (MPRCs) are often topped by a liquid or ice layer, and their makeup affects how much sunlight they reflect back into space and how much water they can release as rain or snow. Understanding them is therefore important for forecasting weather and making projections of future climate.

Researchers have known for a while that MPRCs are extremely sensitive to the presence of ice-nucleating particles in their environment. Such particles mainly come from mineral dust aerosols (such as K-feldspar, quartz, albite and plagioclase) that get swept up into the upper atmosphere from deserts. The Sahara Desert in northern Africa, for example, is a prime source of such dust in the Northern Hemisphere.

More dust leads to more ice clouds

Using 35 years of satellite data collected as part of the Cloud_cci project and MERRA-2 aerosol reanalyses, Villanueva and colleagues looked for correlations between dust levels and the formation of ice-topped clouds. They found that at temperatures of between -15°C and -30°C, the more dust there was, the more frequent the ice clouds were. What is more, their calculated increase in ice-topped clouds with increasing dust loading agrees well with previous laboratory experiments that predicted how dust triggers droplet freezing.

The new study, which is detailed in Science, shows that there is a connection between aerosols in the micrometre-size range and cloud ice observed over distances of several kilometres, Villanueva says. “We found that it is the nanoscale defects on the surface of dust aerosols that trigger ice clouds, so the process of ice glaciation spans more than 15 orders of magnitude in length,” he explains.

Thanks to this finding, Villaneuva tells Physics World that climate modellers can use the team’s dataset to better constrain aerosol-cloud processes, potentially helping them to construct better estimates of cloud feedback and global temperature projections.

The result also shows how sensitive clouds are to varying aerosol concentrations, he adds. “This could help bring forward the field of cloud seeding and include this in climate geoengineering efforts.”

The researchers say they have successfully replicated their results using a climate model and are now drafting a new manuscript to further explore the implications of dust-driven cloud glaciation for climate, especially for the Arctic.

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Nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in the US have produced and identified molecules containing nobelium for the first time. This element, which has an atomic number of 102, is the heaviest ever to be observed in a directly-identified molecule, and team leader Jennifer Pore says the knowledge gained from such work could lead to a shake-up at the bottom of the periodic table.

“We compared the chemical properties of nobelium side-by-side to simultaneously produced molecules containing actinium (element number 89),” says Pore, a research scientist at LBNL. “The success of these measurements demonstrates the possibility to further improve our understanding of heavy and superheavy-element chemistry and so ensure that these elements are placed correctly on the periodic table.”

The periodic table currently lists 118 elements. As well as vertical “groups” containing elements with similar properties and horizontal “periods” in which the number of protons (atomic number Z) in the nucleus increases from left to right, these elements are arranged in three blocks. The block that contains actinides such as actinium (Ac) and nobelium (No), as well as the slightly lighter lanthanide series, is often shown offset, below the bottom of the main table.

The end of a predictive periodic table?

Arranging the elements this way is helpful because it gives scientists an intuitive feel for the chemical properties of different elements. It has even made it possible to predict the properties of new elements as they are discovered in nature or, more recently, created in the laboratory.

The problem is that the traditional patterns we’ve come to know and love may start to break down for elements at the bottom of the table, putting an end to the predictive periodic table as we know it. The reason, Pore explains, is that these heavy nuclei have a very large number of protons. In the actinides (Z > 88), for example, the intense charge of these “extra” protons exerts such a strong pull on the inner electrons that relativistic effects come into play, potentially changing the elements’ chemical properties.

“As some of the electrons are sucked towards the centre of the atom, they shield some of the outer electrons from the pull,” Pore explains. “The effect is expected to be even stronger in the superheavy elements, and this is why they might potentially not be in the right place on the periodic table.”

Understanding the full impact of these relativistic effects is difficult because elements heavier than fermium (Z = 100) need to be produced and studied atom by atom. This means resorting to complex equipment such as accelerated ion beams and the FIONA (For the Identification Of Nuclide A) device at LBNL’s 88-Inch Cyclotron Facility.

Producing and directly identifying actinide molecules

The team chose to study Ac and No in part because they represent the extremes of the actinide series. As the first in the series, Ac has no electrons in its 5f shell and is so rare that the crystal structure of an actinium-containing molecule was only determined recently. The chemistry of No, which contains a full complement of 14 electrons in its 5f shell and is the heaviest of the actinides, is even less well known.

In the new work, which is described in Nature, Pore and colleagues produced and directly identified molecular species containing Ac and No ions. To do this, they first had to produce Ac and No. They achieved this by accelerating beams of ⁴⁸Ca with the 88-Inch Cyclotron and directing them onto targets of ¹⁶⁹Tm and ²⁰⁸Pb, respectively. They then used the Berkeley Gas-filled Separator to separate the resulting actinide ions from unreacted beam material and reaction by-products.

The next step was to inject the ions into a chamber in the FIONA spectrometer known as a gas catcher. This chamber was filled with high-purity helium, as well as trace amounts of H₂O and N₂, at a pressure of approximately 150 torr. After interactions with the helium gas reduced the actinide ions to their 2+ charge state, so-called “coordination compounds” were able to form between the 2+ actinide ions and the H₂O and N₂ impurities. This compound-formation step took place either in the gas buffer cell itself or as the gas-ion mixture exited the chamber via a 1.3-mm opening and entered a low-pressure (several torr) environment. This transition caused the gas to expand at supersonic speeds, cooling it rapidly and allowing the molecular species to stabilize.

Once the actinide molecules formed, the researchers transferred them to a radio-frequency quadrupole cooler-buncher ion trap. This trap confined the ions for up to 50 ms, during which time they continued to collide with the helium buffer gas, eventually reaching thermal equilibrium. After they had cooled, the molecules were reaccelerated using FIONA’s mass spectrometer and identified according to their mass-to-charge ratio.

A fast and sensitive instrument

FIONA is much faster than previous such instruments and more sensitive. Both properties are important when studying the chemistry of heavy and superheavy elements, which Pore notes are difficult to make, and which decay quickly. “Previous experiments measured the secondary particles made when a molecule with a superheavy element decayed, but they couldn’t identify the exact original chemical species,” she explains. “Most measurements reported a range of possible molecules and were based on assumptions from better-known elements. Our new approach is the first to directly identify the molecules by measuring their masses, removing the need for such assumptions.”

As well as improving our understanding of heavy and superheavy elements, Pore says the new work might also have applications in radioactive isotopes used in medical treatment. For example, the ²²⁵Ac isotope shows promise for treating certain metastatic cancers, but it is difficult to make and only available in small quantities, which limits access for clinical trials and treatment. “This means that researchers have had to forgo fundamental chemistry experiments to figure out how to get it into patients,” Pore notes. “But if we could understand such radioactive elements better, we might have an easier time producing the specific molecules needed.”

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Researchers in the US have directly imaged a class of extremely low-energy atomic vibrations called moiré phasons for the first time. In doing so, they proved that these vibrations are not just a theoretical concept, but are in fact the main way that atoms vibrate in certain twisted two-dimensional materials. Such vibrations may play a critical role in heat and charge transport and how quantum phases behave in these materials.

“Phasons had only been predicted by theory until now, and no one had ever directly observed them, or even thought that this was possible,” explains Yichao Zhang of the University of Maryland, who co-led the effort with Pinshane Huang of the University of Illinois at Urbana-Champaign. “Our work opens up an entirely new way of understanding lattice vibrations in 2D quantum materials.”

A second class of moiré phonons

When two sheets of a 2D materials are placed on top of each other and slightly twisted, their atoms form a moiré pattern, or superlattice. This superlattice contains quasi-periodic regions of rotationally aligned regions (denoted AA or AB) separated by a network of stacking faults called solitons.

Materials of this type are also known to possess distinctive vibrational modes known as moiré phonons, which arise from vibrations of the material’s crystal lattice. These modes vary with the twist angle between layers and can change the physical properties of the materials.

In addition to moiré phonons, two-dimensional moiré materials are also predicted to host a second class of vibrational mode known as phasons. However, these phasons had never been directly observed experimentally until now.

Imaging phasons at the picometre scale

In the new work, which is published in Science, the researchers used a powerful microscopy technique called electron ptychography that enabled them to image samples with spatial resolutions as fine as 15 picometres (1 pm = 10^-12 m). At this level of precision, explains Zhang, subtle changes in thermally driven atomic vibrations can be detected by analysing the shape and size of individual atoms. “This meant we could map how atoms vibrate across different stacking regions of the moiré superlattice,” she says. “What we found was striking: the vibrations weren’t uniform – atoms showed larger amplitudes in AA-stacked regions and highly anisotropic behaviour at soliton boundaries. These patterns align precisely with theoretical predictions for moiré phasons.”

Zhang has been studying phonons using electron microscopy for years, but limitations on imaging resolutions had largely restricted her previous studies to nanometre (10^-9 m) scales. She recently realized that electron ptychography would resolve atomic vibrations with much higher precision, and therefore detect moiré phasons varying across picometre scales.

She and her colleagues chose to study twisted 2D materials because they can support many exotic electronic phenomena, including superconductivity and correlated insulated states. However, the role of lattice dynamics, including the behaviour of phasons in these structures, remains poorly understood. “The problem,” she explains, “is that phasons are both extremely low in energy and spatially non-uniform, making them undetectable by most experimental techniques. To overcome this, we had to push electron ptychography to its limits and validate our observations through careful modelling and simulations.”

This work opens new possibilities for understanding (and eventually controlling) how vibrations behave in complex 2D systems, she tells Physics World. “Phasons can affect how heat flows, how electrons move, and even how new phases of matter emerge. If we can harness these vibrations, we could design materials with programmable thermal and electronic properties, which would be important for future low-power electronics, quantum computing and nanoscale sensors.”

More broadly, electron ptychography provides a powerful new tool for exploring lattice dynamics in a wide range of advanced materials. The team is now using electron ptychography to study how defects, strain and interfaces affect phason behaviour. These imperfections are common in many real-world materials and devices and can cause their performance to deteriorate significantly. “Ultimately, we hope to capture how phasons respond to external stimuli, like how they evolve with change in temperature or applied fields,” Zhang reveals. “That could give us an even deeper understanding of how they interact with electrons, excitons or other collective excitations in quantum materials.”

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The largest ever survey of low-frequency radio emissions from satellites has detected emissions from the Starlink satellite “mega-constellation” across scientifically important low-frequency bands, including some that are protected for radio astronomy by international regulations. These emissions, which come from onboard electronics and are not intentional transmissions, could mask the weak radio-wave signals that astronomers seek to detect. As well as being damaging for radio astronomy, the researchers at Australia’s Curtin University who conducted the survey say their findings highlight the need for new regulations that cover unintended transmissions, not just deliberate ones.

“It is important to note that Starlink is not violating current regulations, so is doing nothing wrong,” says Steven Tingay, the executive director of the Curtin Institute of Radio Astronomy (CIRA) and a member of the survey team. Discussions with Starlink operator SpaceX on this topic, he adds, have been “constructive”.

The main purpose of Starlink and other mega-constellations is to provide Internet coverage around the world, including in areas that were previously unable to access it. In addition to SpaceX’s Starlink, other mega-constellations include Amazon’s Kuiper (US) and Eutelsat’s OneWeb (UK). This list is likely to expand in the future, with hundreds to tens of thousands of additional satellites planned for launch by China’s Shanghai Spacecom Satellite Technology (operator of the G60 Starlink/Qianfan constellation) and the Russian Federation (operator of the Sfera constellation).

While the effects of mega-constellations on optical astronomy have been widely studied, study leader Dylan Grigg, a PhD student in CIRA’s International Centre for Radio Astronomy Research, says that researchers are just beginning to realize the extent to which they are also adversely affecting radio astronomy. These effects extend to some of the most radio-quiet places on Earth. Indeed, several radio telescopes that were deliberately built in low-radio-noise locations – including the Murchison Widefield Array (MWA) in Western Australia and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, as well as Europe’s Low Frequency Array (LOFAR) – have recently detected interfering satellite signals.

Largest survey of satellite effects on radio astronomy data

To understand the scale of the problem, Tingay, Grigg and colleagues turned to a radio telescope called the Engineering Development Array 2 (EDA2). This is a prototype station for the low-frequency half of the Square Kilometre Array (SKA-Low), which will be the world’s largest and most sensitive radio telescope when it comes online later this decade.

Using the EDA2, the researchers imaged the sky every two seconds at the frequencies that SKA-Low will cover. They did this using a software package Grigg developed that autonomously detects and identifies satellites in the images the EDA2 creates.

Although this was not the first time EDA2 has been deployed to analyse the effects of satellites on radio astronomy data, Grigg says it is the most comprehensive. “Ours is the largest survey looking into Starlink emissions at SKA-Low frequencies, with over 76 million of the images analysed,” he explains. “With the real SKA-Low coming online soon, we need as much information as possible to understand the threat satellite interference poses to radio astronomy.”

Emissions at protected frequencies

During the survey period, the researchers say they detected more than 112 000 radio emissions from over 1800 Starlink satellites. At some frequencies, up to 30% of all survey images contained at least one Starlink detection.

“While Starlink is not the only satellite network, it is the most immediate and frequent source of potential interference for radio astronomy,” Grigg says. “Indeed, it launched 477 satellites during this study’s four-month data collection period alone and has the most satellites in orbit – more than 7000 during the time of this study.”

But it is not only the sheer number of satellites that poses a challenge for astronomers. So, too, does the strength and frequency of their emissions. “Some satellites were detected emitting in bands where no signals are supposed to be present at all,” Grigg says. The list of rogue emitters, he adds, included 703 satellites the team identified at 150.8 MHz – a frequency that is meant to be reserved for radio astronomy under International Telecommunication Union regulations. “Since these emissions may come from components like onboard electronics and they’re not part of an intentional signal, astronomers can’t easily predict them or filter them out,” he says.

Potential for new regulations and mitigations

From a regulatory perspective, the widespread detection of unintended emissions, including within protected frequency bands, demonstrates the need for international regulation and limits on unintended emissions, Grigg tells Physics World. The Curtin team is now working with other radio astronomy research groups around the world with the aim of introducing updated policies that would regulate the impact of satellite constellations on radio astronomy.

In the meantime, Grigg says, “We are in an ongoing dialogue with SpaceX and are hopeful that we can continue to work with them to introduce mitigations to their satellites in the future.”

The survey is described in Astronomy & Astrophysics.

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A team led by researchers at Rutgers University in the US has discovered a new semiconductor that emits bright, deep-blue light. The hybrid copper iodide material is stable, non-toxic, can be processed in solution and has already been integrated into a light-emitting diode (LED). According to its developers, it could find applications in solid-state lighting and display technologies.

Creating white light for solid-state lighting and full-colour displays requires bright, pure sources of red, green and blue light. While stable materials that efficiently emit red or green light are relatively easily to produce, those that generate blue light (especially deep-blue light) are much more challenging. Existing blue-light emitters based on organic materials are unstable, meaning they lose their colour quality over time. Alternatives based on lead-halide perovskites or cadmium-containing colloidal quantum dots are more stable, but also toxic for humans and the environment.

Hybrid copper-halide-based emitters promise the best of both worlds, being both non-toxic and stable. They are also inexpensive, with tuneable optical properties and a high luminescence efficiency, meaning they are good at converting power into visible light.

Researchers have already used a pure inorganic copper iodide material, Cs₃Cu₂I₅, to make deep-blue LEDs. This material emits light at the ideal wavelength of 445 nm, is robust to heat and moisture, and it emits between 87–95% of the excitation photons it absorbs as luminescence photons, giving it a high photoluminescence quantum yield (PLQY).

However, the maximum ratio of photon output to electron input (known as the maximum external quantum efficiency, EQE_max) for this material is very low, at just 1.02%.

Strong deep-blue photoluminescence

In the new work, a team led by Rutgers materials chemist Jing Li developed a hybrid copper iodide with the chemical formula 1D-Cu₄I₈(Hdabco)₄ (CuI(Hda), where Hdabco is 1,4-diazabicyclo-[2.2.2]octane-1-ium. This material emits strong deep-blue light at 449 nm with a PLQY near unity (99.6%).

Li and colleagues opted to use CuI(Hda) as the sole light emitting layer and built a thin-film LED out of it using a solution process. The new device has an EQE_max of 12.6% with colour coordinates (0.147, 0.087) and a peak brightness of around 4000 cd m^-2. It is also relatively stable, with an operational half-lifetime (T₅₀) of approximately 204 hours under ambient conditions. These figures mean that its performance rivals the best existing solution-processed deep-blue LEDs, Li says. The team also fabricated a large-area device measuring 4 cm² to demonstrate that the material could be used in real-world applications.

Interfacial hydrogen-bond passivation strategy

The low PLQY of previous such devices is partly due to the fact that charge carriers (electrons and holes) in these materials rapidly recombine in a non-radiative way, typically due to surface and bulk defects, or traps. The charge carriers also have a low radiative recombination rate, which is associated with a small exciton (electron-hole pair) binding energy.

Li and colleagues overcame this problem in their new device thanks to an interfacial hydrogen-bond passivation (DIHP) strategy that involves introducing hydrogen bonds via an ultrathin sheet of polymethylacrylate (PMMA) and a carbazole-phosphonic acid-based self-assembled monolayer (Ac2PACz) at the two interfaces of the CuI(Hda) emissive layer. This effectively passivates both heterojunctions of the copper-iodide hydride light-emitting layer and optimizes exciton binding energies. “Such a synergistic surface modification dramatically boosts the performance of the deep-blue LED by a factor of fourfold,” explains Li.

According to Li, the study suggests a promising route for developing blue emitters that are both energy-efficient and environmentally benign, without compromising on performance. “Through the fabrication of blue LEDs using a low cost, stable and nontoxic material capable of delivering efficient deep-blue light, we address major energy and ecological limitations found in other types of solution-processable emitters,” she tells Physics World.

Li adds that the hydrogen-bonding passivation technique is not limited to the material studied in this work. It could also be applied to minimize interfacial energy losses in a wide range of other solution-based, light-emitting optoelectronic systems.

The team is now pursuing strategies for developing other solution-processable, high-performance hybrid copper iodide-based emitter materials similar to CuI(Hda). “Our goal is to further enhance the efficiency and extend the operational lifetime of LEDs utilizing these next-generation materials,” says Li.

The present work is detailed in Nature.

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The first precise mass measurements of an extremely short-lived and proton-rich nucleus, silicon-22, have revealed the “magic” – that is, unusually tightly bound – nature of nuclei containing 14 protons. As well as shedding light on nuclear structure, the discovery could improve our understanding of the strong nuclear force and the mechanisms by which elements form.

At the lighter end of the periodic table, stable nuclei tend to contain similar numbers of neutrons and protons. As the number of protons increases, additional neutrons are needed to balance out the mutual repulsion of the positively-charged protons. As a rule, therefore, an isotope of a given element will be unstable if it contains either too few neutrons or too many.

In 1949, Maria Goeppert Mayer and J Hans D Jensen proposed an explanation for this rule. According to their nuclear shell model, nuclei that contain certain “magic” numbers of nucleons (neutrons and/or protons) are more bound because they have just the right number of nucleons to fully fill their shells. Nuclei that contain magic numbers of both protons and neutrons are even more bound and are said to be “doubly magic”. Subsequent studies showed that for neutrons, these magic numbers are 2, 8, 20, 28, 50, 82 and 126.

While the magic numbers for stable and long-lived nuclei are now well-established, those for exotic, short-lived ones with unusual proton-neutron ratios are comparatively little understood. Do these highly unstable nuclei have the same magic numbers as their more stable counterparts? Or are they different?

In recent years, studies showing that neutron-rich nuclei have magic numbers of 14, 16, 32 and 34 have brought scientists closer to answering this question. But what about protons?

“The hunt for new magic numbers in proton-rich nuclei is just as exciting,” says Yuan-Ming Xing, a physicist at the Institute for Modern Physics (IMP) of the Chinese Academy of Sciences, who led the latest study on silicon-22. “This is because we know much less about the evolution of the shell structure of these nuclei, in which the valence protons are loosely bound.” Protons in these nuclei can even couple to states in the continuum, Xing adds, forming the open quantum systems that have become such a hot topic in quantum research.

Mirror nuclei

After measurements on oxygen-22 (14 neutrons, 8 protons) showed that 14 is a magic number of neutrons for this neutron-rich isotope, the hunt was on for a proton-rich counterpart. An important theory in nuclear physics known as isospin symmetry states that nuclei with interchanged numbers of protons and neutrons will have identical characteristics. The magic numbers for protons and neutrons for these “mirror” nuclei, as they are known, are therefore expected to be the same. “Of all the new neutron-rich doubly-magic nuclei discovered, only one loosely bound mirror nucleus for oxygen-22 exists,” says IMP team member Yuhu Zhang. “This is silicon-22.”

The problem is that silicon-22 (14 protons, 8 neutrons) has a short half-life and is hard to produce in quantities large enough to study. To overcome this, the researchers used an improved version of a technique known as Bρ-defined isochronous mass spectroscopy.

Working at the Cooler-Storage Ring of the Heavy Ion Research Facility in Lanzhou, China, Xing, Zhang and an international team of collaborators began by accelerating a primary beam of stable ³⁶Ar¹⁵⁺ ions to around two thirds the speed of light. They then directed this beam onto a 15-mm-thick beryllium target, causing some of the ³⁶Ar ions to fragment into silicon-22 nuclei. After injecting these nuclei into the storage ring, the researchers could measure their velocity and the time it took them to circle the ring. From this, they could determine their mass. This measurement confirmed that the proton number 14 is indeed magic in silicon-22.

A better understanding of nucleon interactions

“Our work offers an excellent opportunity to test the fundamental theories of nuclear physics for a better understanding of nucleon interactions, of how exotic nuclear structures evolve and of the limit of existence of extremely exotic nuclei,” says team member Giacomo de Angelis, a nuclear physicist affiliated with the National Laboratories of Legnaro in Italy as well as the IMP. “It could also help shed more light on the reaction rates for element formation in stars – something that could help astrophysicists to better model cosmic events and understand how our universe works.”

According to de Angelis, this first mass measurement of the silicon-22 nucleus and the discovery of the magic proton number 14 is “a strong invitation not only for us, but also for other nuclear physicists around the world to investigate further”. He notes that researchers at the Facility for Rare Isotope Beams (FRIB) at Michigan State University, US, recently measured the energy of the first excited state of the silicon-22 nucleus. The new High Intensity Heavy-Ion Accelerator Facility (HIAF) in Huizhou, China, which is due to come online soon, should enable even more detailed studies.

“HIAF will be a powerful accelerator, promising us ideal conditions to explore other loosely bound systems, thereby helping theorists to more deeply understand nucleon-nucleon interactions, quantum mechanics of open quantum systems and the origin of elements in the universe,” he says.

The present study is detailed in Physical Review Letters

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Magnetic fields so strong that they are typically only observed in astrophysical jets and highly magnetized neutron stars could be created in the laboratory, say physicists at the University of Osaka, Japan. Their proposed approach relies on directing extremely short, intense laser pulses into a hollow tube housing sawtooth-like inner blades. The fields created in this improved version of the established “microtube implosion” technique could be used to imitate effects that occur in various high-energy-density processes, including non-linear quantum phenomena and laser fusion as well as astrophysical systems.

Researchers have previously shown that advanced ultra-intense femtosecond (10^-15 s) lasers can generate magnetic fields with strengths of up to several kilotesla. More recently, a suite of techniques that combines advanced laser technologies with complex microstructures promises to push this limit even higher, into the megatesla regime.

Microtube implosion is one such technique. Here, femtosecond laser pulses with intensities between 10²⁰ and 10²²W/cm² are aimed at a hollow cylindrical target with an inner radius of between 1 and 10 mm. This produces a plasma of hot electrons with MeV energies that form a sheath field along the inner wall of the tube. These electrons accelerate ions radially inward, causing the cylinder to implode.

At this point, a “seed” magnetic field deflects the ions and electrons in opposite azimuthal directions via the Lorentz force. The loop currents induced in the same direction ultimately generate a strong axial magnetic field.

Self-generated loop current

Although the microtube implosion technique is effective, it does require a kilotesla-scale seed field. This complicates the apparatus and makes it rather bulky.

 In the latest work, Osaka’s Masakatsu Murakami and colleagues propose a new setup that removes the need for this seed field. It does this by replacing the 1‒10 mm cylinder with a micron-sized one that has a periodically slanted inner surface housing sawtooth-shaped blades. These blades introduce a geometrical asymmetry in the cylinder, causing the imploding plasma to swirl asymmetrically inside it and generating circulating currents near its centre. These self-generated loop currents then produce an intense axial magnetic field with a magnitude in the gigagauss range (1 gigagauss = 100 000 T).

Using “particle-in-cell” simulations running the fully relativistic EPOCH code on Osaka’s SQUID supercomputer, the researchers found that such vortex structures and their associated magnetic field arise from a self-consistent positive feedback mechanism. The initial loop current amplifies the central magnetic field, which in turn constrains the motion of charged particles more tightly via the Lorentz force – and thereby reinforces and intensifies the loop current further.

“This approach offers a powerful new way to create and study extreme magnetic fields in a compact format,” Murakami says. “It provides an experimental bridge between laboratory plasmas and the astrophysical universe and could enable controlled studies of strongly magnetized plasmas, relativistic particle dynamics and potentially magnetic confinement schemes relevant to both fusion and astrophysics.”

The researchers, who report their work in Physics of Plasmas, are now looking to realize their scheme in an experiment using petawatt-class lasers. “We will also investigate how these magnetic fields can be used to steer particles or compress plasmas,” Murakami tells Physics World.

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Gold can remain solid at temperatures of over 14 times its melting point, far beyond a long-assumed theoretical limit dubbed the “entropy catastrophe”. This finding is based on temperature measurements made using high-resolution inelastic X-ray scattering, and according to team leader Thomas White of the University of Nevada, US, it implies that the question “How hot can you make a solid before it melts?” has no good answer.

“Until now, we thought that solids could not exist above about three times their melting temperatures,” White says. “Our results show that if we heat a material rapidly – that is, before it has time to expand – it is possible to bypass this limit entirely.”

Gold maintains its solid crystalline structure

In their experiments, which are detailed in Nature, White and colleagues heated a 50-nanometre-thick film of gold using intense laser pulses just 50 femtoseconds long (1fs = 10^-15 s). The key is the speed at which heating occurs. “By depositing energy faster than the gold lattice could expand, we created a state in which the gold was incredibly hot, but still maintained its solid crystalline structure,” White explains.

The team’s setup made it possible to achieve heating rates in excess of 10¹⁵ K s ^–1, and ultimately to heat the gold to 14 times its 1064 °C melting temperature. This is far beyond the boundary of the supposed entropy catastrophe, which was previously predicted to strike at 3000 °C.

To measure such extreme temperatures accurately, the researchers used the Linac Coherent Light Source (LCLS) at Stanford University as an ultrabright X-ray thermometer. In this technique, the atoms or molecules in a sample absorb photons from an X-ray laser at one frequency and then re-emit photons of a different frequency. The difference in these frequencies depends on a photon’s Doppler shift, and thus on whether it is propagating towards or away from the detector.

The method works because all the atoms in a material exhibit random thermal motion. The temperature of the sample therefore depends on the average kinetic energy of its atoms. Higher temperatures correspond to faster-moving atoms and a bigger spread in the velocities of the photons moving towards away or from the detector. Hence, the width of the spectrum of light scattered by the sample can be used to estimate its temperature. “This approach bypasses the need for complex computer modelling because we simply measure the velocity distribution of atoms directly,” White explains.

A direct, model-independent method

The team, which also includes researchers from the UK, Germany and Italy, undertook this project because its members wanted to develop a direct, model-independent method to measure atom temperatures in extreme conditions. The technical challenges of doing so were huge, White recalls. “We not only needed a high-resolution X-ray spectrometer capable of resolving energy features of just millielectronvolts (meV) but also an X-ray source bright enough to generate meaningful signals from small, short-lived samples,” he says.

A further challenge is that while pressure and density measurements under extreme conditions are more or less routine, temperature is typically inferred – often with large uncertainties. “In our experiments, these extreme states last just picoseconds or nanoseconds,” he says. “We can’t exactly insert a thermometer.”

White adds that this limitation has slowed progress across plasma and materials physics. “Our work provides the first direct method for measuring ion temperatures in dense, strongly driven matter, unlocking new possibilities in areas like planetary science – where we can now probe conditions inside giant planets – and in fusion energy, where temperature diagnostics are critical.”

Fundamental studies in materials science could also benefit, he adds, pointing out that scientists will now be able to explore the ultimate stability limits of solids experimentally as well as theoretically, studying how materials behave when pushed far beyond conventional thermodynamic boundaries.

The researchers are now applying their method to shock-compressed materials. “Just a few weeks ago, we completed a six-night experiment at the LCLS using the same high-resolution scattering platform to measure both particle velocity and temperature in shock-melted iron,” White says. “This is a major step forward. Not only are we tracking temperature in the solid phase, but now we’re accessing molten states under dynamic compression, that is, at conditions like those found inside planetary interiors.”

White tells Physics World that these experiments also went well, and he and his colleagues are now analysing the results. “Ultimately, our goal is to extend this approach to a wide range of materials and conditions, allowing for a new generation of precise, real-time diagnostics in extreme environments,” he says.

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The global network of Android smartphones makes a useful earthquake early warning system, giving many users precious seconds to act before the shaking starts. These findings, which come from researchers at Android’s parent organization Google, are based on a three-year-long study involving millions of phones in 98 countries. According to the researchers, the network’s capabilities could be especially useful in areas that lack established early warning systems.

“By using Android smartphones, which make up 70% of smartphones worldwide, the Android Earthquake Alert (AEA) system can help provide life-saving warnings in many places around the globe,” says study co-leader Richard Allen, a visiting faculty researcher at Google who directs the Berkeley Seismological Laboratory at the University of California, Berkeley, US.

Traditional earthquake early warning systems use networks of seismic sensors expressly designed for this purpose. First implemented in Mexico and Japan, and now also deployed in Taiwan, South Korea, the US, Israel, Costa Rica and Canada, they rapidly detect earthquakes in areas close to the epicentre and issue warnings across the affected region. Even a few seconds of warning can be useful, Allen explains, because it enables people to take protective actions such as the “drop, cover and hold on” (DCHO) sequence recommended in most countries.

Building such seismic networks is expensive, and many earthquake-prone regions do not have them. What they do have, however, is smartphones. Most such devices contain built-in accelerometers, and as their popularity soared in the 2010s, seismic scientists began exploring ways of using them to detect earthquakes. “Although the accelerometers in these phones are less sensitive than the permanent instruments used in traditional seismic networks, they can still detect tremors during strong earthquakes,” Allen tells Physics World.

A smartphone-based warning system

By the late 2010s, several teams had developed smartphone apps that could sense earthquakes when they happen, with early examples including Mexico’s SkyAlert and Berkeley’s ShakeAlert. The latest study takes this work a step further. “By using the accelerometers in a network of smartphones like a seismic array, we are now able to provide warnings in some parts of the world where they didn’t exist before and are most needed,” Allen explains.

Working with study co-leader Marc Stogaitis, a principal software engineer at Android, Allen and colleagues tested the AEA system between 2021 and 2024. During this period, the app detected an average of 312 earthquakes a month, with magnitudes ranging from 1.9 to 7.8 (corresponding to events in Japan and Türkiye, respectively).

For earthquakes of magnitude 4.5 or higher, the system sent “TakeAction” alerts to users. These alerts are designed to draw users’ attention immediately and prompt them to take protective actions such as DCHO. The system sent alerts of this type on average 60 times per month during the study period, for an average of 18 million individual alerts per month. The system also delivered lesser “BeAware” alerts to regions expected to experience a shaking intensity of 3 or 4.

To assess how effective these alerts were, the researchers used Google Search to collect voluntary feedback via user surveys. Between 5 February 2023 and 30 April 2024, 1 555 006 people responded to a survey after receiving alerts generated from an AEA detection. Their responses indicated that 85% of them did indeed experience shaking, with 36% receiving the alert before the ground began to move, 28% during and 23% after.

Principles of operation

AEA works on the same principles of seismic wave propagation as traditional earthquake detection systems. When an Android smartphone is stationary, the system uses the output of its accelerometer to detect the type of sudden increase in acceleration that P and S waves in an earthquake would trigger. Once a phone detects such a pattern, it sends a message to Google servers with the acceleration information and an approximate location. The servers then search for candidate seismic sources that tally with this information.

“When a candidate earthquake source satisfies the observed data with a high enough confidence, an earthquake is declared and its magnitude, hypocentre and origin time are estimated based on the arrival time and amplitude of the P and S waves,” explains Stogaitis. “This detection capability is deployed as part of Google Play Services core system software, meaning it is on by default for most Android smartphones. As there are billions of Android phones around the world, this system provides an earthquake detection capability wherever there are people, in both wealthy and less-wealthy nations.”

In the future, Allen says that he and his colleagues hope to use the same information to generate other hazard-reducing tools. Maps of ground shaking, for example, could assist the emergency response after an earthquake.

For now, the researchers, who report their work in Science, are focused on improving the AEA system. “We are learning from earthquakes as they occur around the globe and the Android Earthquake Alerts system is helping to collect information about these natural disasters at a rapid rate,” says Allen. “We think that we can continue to improve both the quality of earthquake detections, and also improve on our strategies to deliver effective alerts.”

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At high temperatures and pressures, molten carbon has two options. It can crystallize into diamond and become one of the world’s most valuable substances. Alternatively, it can crystallize into graphite, which is industrially useful but somewhat less exciting.

Researchers in the US have now discovered what causes molten carbon to “choose” one crystalline form over the other. Their findings, which are based on sophisticated simulations that use machine learning to predict molecular behaviour, have implications for several fields, including geology, nuclear fusion and quantum computing as well as industrial diamond production.

Monitoring crystallization in molten carbon is challenging because the process is rapid and occurs under conditions that are hard to produce in a laboratory. When scientists have tried to study this region of carbon’s phase diagram using high pressure flash heating, their experiments have produced conflicting results.

A better understanding of phase changes near the crystallization point could bring substantial benefits. Liquid-phase carbon is a known intermediate in the synthesis of artificial diamonds, nanodiamonds and the nitrogen-vacancy-doped diamonds used in quantum computing. The presence of diamond in natural minerals can also shed light on tectonic processes in Earth-like planets and the deep-Earth carbon cycle.

Crystallization process can be monitored in detail

In the new work, a team led by chemist Davide Donadio of the University of California, Davis used machine-learning-accelerated, quantum-accurate molecular dynamics simulations to model how diamond and graphite form as liquid carbon cools from 5000 to 3000 K at pressures ranging from 5 to 30 GPa. While such extreme conditions can be created using laser heating, Donadio notes that doing so requires highly specialized equipment. Simulations also provide a level of control over conditions and an ability to monitor the crystallization process at the atomic scale that would be difficult, if not impossible, to achieve experimentally.

The team’s simulations showed that the crystallization behaviour of molten carbon is more complex than previously thought. While it crystallizes into diamond at higher pressures, at lower pressures (up to 15 GPa) it forms graphite instead. This was surprising, the researchers say, because even at these slightly lower pressures, the material’s most thermodynamically stable phase ought to be diamond rather than graphite.

“Nature taking the path of least resistance”

The team attributes this unexpected behaviour to an empirical observation known as Ostwald’s step rule, which states that crystallization often proceeds through intermediate metastable phases rather than directly to the phase that is most thermodynamically stable. In this case, the researchers say that graphite, a nucleating metastable crystal, acts as a stepping stone because its structure more closely resembles that of the parent liquid carbon. For this reason, it hinders the direct formation of the stable diamond phase.

“The liquid carbon essentially finds it easier to become graphite first, even though diamond is ultimately more stable under these conditions,” says co-author Tianshu Li, a professor of civil and environmental engineering at George Washington University. “It’s nature taking the path of least resistance.”

The insights gleaned from this work, which is described in Nature Communications, could help resolve inconsistencies among historical electrical and laser flash-heating experiments, Donadio says. Though these experiments were aimed at resolving the phase diagram of carbon near the graphite-diamond-liquid triple point, various experimental details and recrystallization conditions may have meant that their systems instead became “trapped” in metastable graphitic configurations. Understanding how this happens could prove useful for manufacturing carbon-based materials such as synthetic diamonds and nanodiamonds at high pressure and temperature.

“I have been studying crystal nucleation for 20 years and have always been intrigued by the behaviour of carbon,” Donadio tells Physics World. “Studies based on so-called empirical potentials have been typically unreliable in this context and ab initio density functional theory-based calculations are too slow. Machine learning potentials allow us to overcome these issues, having the right combination of accuracy and computational speed.”

Looking to the future, Donadio says he and his colleagues aim to study more complex chemical compositions. “We will also be focusing on targeted pressures and temperatures, the likes of which are found in the interiors of giant planets in our solar system.”

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Physicists in Austria and Germany have developed a means of controlling quasiparticles known as dark excitons in semiconductor quantum dots for the first time. The new technique could be used to generate single pairs of entangled photons on demand, with potential applications in quantum information storage and communication.

Excitons are bound pairs of negatively charged electrons and positively charged “holes”. When these electrons and holes have opposite spins, they recombine easily, emitting a photon in the process. Excitons of this type are known as “bright” excitons. When the electrons and holes have parallel spins, however, direct recombination by emitting a photon is not possible because it would violate the conservation of spin angular momentum. This type of exciton is therefore known as a “dark” exciton.

Because dark excitons are not optically active, they have much longer lifetimes than their bright cousins. For quantum information specialists, this is an attractive quality, because it means that dark excitons can store quantum states – and thus the information contained within these states – for much longer. “This information can then be released at a later time and used in quantum communication applications, such as optical quantum computing, secure communication via quantum key distribution (QKD) and quantum information distribution in general,” says Gregor Weihs, a quantum photonics expert at the Universität Innsbruck, Austria who led the new study.

The problem is that dark excitons are difficult to create and control. In semiconductor quantum dots, for example, Weihs explains that dark excitons tend to be generated randomly, for example when a quantum dot in a higher-energy state decays into a lower-energy state.

Chirped laser pulses lead to reversible exciton production

In the new work, which is detailed in Science Advances, the researchers showed that they could control the production of dark excitons in quantum dots by using laser pulses that are chirped, meaning that the frequency (or colour) of the laser light varies within the pulse. Such chirped pulses, Weihs explains, can turn one quantum dot state into another.

“We first bring the quantum dot to the (bright) biexciton state using a conventional technique and then apply a (storage) chirped laser pulse that turns this biexciton occupation (adiabatically) into a dark state,” he says. “The storage pulse is negatively chirped – its frequency decreases with time, or in terms of colour, it turns redder.” Importantly, the process is reversible: “To convert the dark exciton back into a bright state, we apply a (positively chirped) retrieval pulse to it,” Weihs says.

One possible application for the new technique would be to generate single pairs of entangled photons on demand – the starting point for many quantum communication protocols. Importantly, Weihs adds that this should be possible with almost any type of quantum dot, whereas an alternative method known as polarization entanglement works for only a few quantum dot types with very special properties. “For example, it could be used to create ‘time-bin’ entangled photon pairs,” he tells Physics World. “Time-bin entanglement is particularly suited to transmitting quantum information through optical fibres because the quantum state stays preserved over very long distances.”

The study’s lead author, Florian Kappe, and his colleague Vikas Remesh describe the project as “a challenging but exciting and rewarding experience” that combined theoretical and experimental tools. “The nice thing, we feel, is that on this journey, we developed a number of optical excitation methods for quantum dots for various applications,” they say via e-mail.

The physicists are now studying the coherence time of the dark exciton states, which is an important property in determining how long they can store quantum information. According to Weihs, the results from this work could make it possible to generate higher-dimensional time-bin entangled photon pairs – for example, pairs of quantum states called qutrits that have three possible values.

“Thinking beyond this, we imagine that the technique could even be applied to multi-excitonic complexes in quantum dot molecules,” he adds. “This could possibly result in multi-photon entanglement, such as so-called GHZ (Greenberger-Horne-Zeilinger) states, which are an important resource in multiparty quantum communication scenarios.”

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A new type of nanostructured lasing system called a metalaser emits light with highly tuneable wavefronts – something that had proved impossible to achieve with conventional semiconductor lasers. According to the researchers in China who developed it, the new metalaser can generate speckle-free laser holograms and could revolutionize the field of laser displays.

The first semiconductor lasers were invented in the 1960s and many variants have since been developed. Their numerous advantages – including small size, long lifetimes and low operating voltages – mean they are routinely employed in applications ranging from optical communications and interconnects to biomedical imaging and optical displays.

To make further progress with this class of lasers, researchers have been exploring ways of creating them at the nanoscale. One route for doing this is to integrate light-scattering arrays called metasurfaces with laser mirrors or insert them inside resonators. However, the wavefronts of the light emitted by these metalasers have proven very difficult to control, and to date only a few simple profiles have been possible without introducing additional optical elements.

Not significantly affected by perturbations

In the new work, a team led by Qinghai Song of the Harbin Institute of Technology, Shenzhen, created a metalaser that consists of silicon nitride nanodisks that have holes in their centres and are arranged in a periodic array. This configuration generates bound states in a continuous medium (BICs). Since the laser energy is concentrated in the centre of each nanodisk, the wavelength of the BIC is not significantly affected by perturbations such as tiny holes in the structure.

“At the same time, the in-plane electric fields of these modes are distributed along the periphery of each nanodisk,” Song explains. “This greatly enhances the light field inside the centre of the hole and induces an effective dipole moment there, which is what produces a geometric phase change to the light emission at each pixel.”

By rotating the holes in the nanodisks, Song says that it is possible to introduce specific geometric phase profiles into the metasurface. The laser emission can then be tailored to create focal spots, focal lines and doughnut shapes as well as holographic images.

And that is not all. Unlike in conventional laser modes, the waves scattered from the new metalaser are too weak to undergo resonant amplification. This means that the speckle noise generated is negligibly small, which resolves the longstanding challenge of reducing speckle noise in holographic displays without reducing image quality.

According to Song, this property could revolutionize laser displays. He adds that the physical concept outlined in the team’s work could be extended to other nanophotonic devices, substantially improving their performance in various optics and photonics applications.

“Controlling laser emission at will has always been a dream of laser researchers,” he tells Physics World. “Researchers have traditionally done this by introducing metasurfaces into structures such as laser oscillators. This approach, while very straightforward, is severely limited by the resonant conditions of this type of laser system. With other types of laser, they had to either integrate a metasurface wave plate outside the laser cavity or use bulky and complicated components to compensate for phase changes.”

With the new metalaser, the laser emission can be changed from fixed profiles such as Hermite-Gaussian modes and Laguerre-Gaussian modes to arbitrarily customized beams, he says. One consequence of this is that the lasers could be fabricated to match the numerical aperture of fibres or waveguides, potentially boosting the performance of optical communications and optical information processing.

Developing a programmable metalaser will be the researchers’ next goal, Song says.

The new metalaser design is described in Nature.

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The most common form of water in the universe appears to be much more complex than was previously thought. While past measurements suggested that this “space ice” is amorphous, researchers in the UK have now discovered that it contains crystals. The result poses a challenge to current models of ice formation and could alter our understanding of ordinary liquid water.

Unlike most other materials, water is denser as a liquid than it is as a solid. It also expands rather than contracts when it cools; becomes less viscous when compressed; and exists in many physical states, including at least 20 polymorphs of ice.

One of these polymorphs is commonly known as space ice. Found in the bulk matter in comets, on icy moons and in the dense molecular clouds where stars and planets form, it is less dense than liquid water (0.94 g cm⁻³ rather than 1 g cm⁻³), and X-ray diffraction images indicate that it is an amorphous solid. These two properties give it its formal name: low-density amorphous ice, or LDA.

While space ice was discovered almost a century ago, Michael Davies, who studied LDA as part of his PhD research at University College London and the University of Cambridge, notes that its exact atomic structure is still being debated. “It is unclear, for example, whether LDA is a ‘true glassy state’ (meaning a frozen liquid with no ordered structure) or a high disordered crystal,” Davies explains.

The memory of ice

In the new work, Davies and colleagues used two separate computational simulations to better understand this atomic structure. In the first simulation, they froze “boxes” of water molecules by cooling them to -150 °C at different rates, which produced crystalline and amorphous ice in varying proportions. They then compared this spectrum of structures to the structure of amorphous ice as measured by X-ray diffraction.

“The best model to match experiments was a ‘goldilocks’ scenario – that is, one that is not too amorphous and not too crystalline,” Davies explains. “Specifically, we found ice that was up to 20% crystalline and 80% amorphous, with the structure containing tiny crystals around 3-nm wide.”

The second simulation began with large “boxes” of ice consisting of many small ice crystals packed together. “Here, we varied the number of crystals in the boxes to again give a range of very crystalline to amorphous models,” Davies says. “We found very close agreement to experiment with models that had very similar structures compared to the first approach with 25% crystalline ice.”

To back up these findings, the UCL/Cambridge researchers performed a series of experiments. “By re-crystallizing different samples of LDA formed via different ‘parent ice phases’ we found that the final crystal structure formed varied depending on the pathway to creation,” Davies tells Physics World. In other words, he adds, “The final structure had a memory of its parent.”

This is important, Davies continues, because if LDA was truly amorphous and contained no crystalline grains at all, this “memory” effect would not be possible.

Impact on our understanding

The discovery that LDA is not completely amorphous has implications for our understanding of ordinary liquid water. The prevailing “two state” model for water is appealing because it accounts for many of water’s thermodynamic anomalies. However, it rests on the assumption that both LDA and high-density amorphous ice have corresponding liquid forms, and that liquid water can be modelled as a mixture of the two.

“Our finding that LDA actually contains many small crystallites presents some challenges to this model,” Davies says. “It is thus of paramount importance for us to now confirm if a truly amorphous version of LDA is achievable in experiments.”

The existence of structure within LDA also has implications for “panspermia” theory, which hypothesizes that the building blocks of life (such as simple amino acids) were carried to Earth within an icy comet.  “Our findings suggest that LDA would be a less efficient transporting material for these organic molecules because a partly crystalline structure has less space in which these ingredients could become embedded,” Davies says.

“The theory could still hold true, though,” he adds, “as there are amorphous regions in the ice where such molecules could be trapped and stored.”

Challenges in determining atomic structure

The study, which is detailed in Physical Review B, highlights the difficulty of determining the exact atomic structure of materials. According to Davies, it could therefore be important for understanding other amorphous materials, including some that are widely used in technologies such as OLEDs and fibre optics.

“Our methodology could be applied to these materials to determining whether they are truly glassy,” he says. “Indeed, glass fibres that transport data along long distances need to be amorphous to function efficiently. If they are found to contain tiny crystals, these could then be removed to improve performance.”

The researchers are now focusing on understanding the structure of other amorphous ices, including high-density amorphous ice. “There is much for us to investigate with regards to the links between amorphous ice phases and liquid water,” Davies concludes.

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