Computers Math

The fields of condensed matter physics and material science are intimately linked because new physics is often discovered in materials with special arrangements of atoms. Crystals, which have repeating units of atoms in space, can have special patterns which result in exotic physical properties. Particularly exciting are materials which host multiple types of exotic properties because they give scientists the opportunity to study how those properties interact with and influence each other. The combinations can give rise to unexpected phenomena and fuel years of basic and technological research.
In a new study published in Science Advances this week, an international team of scientists from the USA, Columbia, Czech Republic, England, and led by Dr. Mazhar N. Ali at the Max Planck Institute of Microstructure Physics in Germany, has shown that a new material, KV3Sb5, has a never-seen-before combination of properties that results in one of the largest anomalous Hall effects (AHEs) ever observed; 15,500 siemens per centimeter at 2 Kelvin.
Discovered in the lab of co-author Prof. Tyrel McQueen at Johns Hopkins University, KV3Sb5 combines four properties into one material: Dirac physics, metallic frustrated magnetism, 2D exfoliability (like graphene), and chemical stability.
Dirac physics, in this context, relates to the fact that the electrons in KV3Sb5 aren’t just your normal run-of-the-mill electrons; they are moving extremely fast with very low effective mass. This means that they are acting “light-like”; their velocities are becoming comparable to the speed of light and they are behaving as though they have only a small fraction of the mass which they should have. This results in the material being highly metallic and was first shown in graphene about 15 years ago.
The “frustrated magnetism” arises when the magnetic moments in a material (imagine little bar magnets which try to turn each other and line up North to South when you bring them together) are arranged in special geometries, like triangular nets. This scenario can make it hard for the bar magnets to line up in way that they all cancel each other out and are stable. Materials exhibiting this property are rare, especially metallic ones. Most frustrated magnet materials are electrical insulators, meaning that their electrons are immobile. “Metallic frustrated magnets have been highly sought after for several decades. They have been predicted to house unconventional superconductivity, Majorana fermions, be useful for quantum computing, and more,” commented Dr. Ali.
Structurally, KV3Sb5 has a 2D, layered structure where triangular vanadium and antimony layers loosely stack on top of potassium layers. This allowed the authors to simply use tape to peel off a few layers (a.k.a. flakes) at a time. “This was very important because it allowed us to use electron-beam lithography (like photo-lithography which is used to make computer chips, but using electrons rather than photons) to make tiny devices out of the flakes and measure properties which people can’t easily measure in bulk.” remarked lead author Shuo-Ying Yang, from the Max Planck Institute of Microstructure Physics. “We were excited to find that the flakes were quite stable to the fabrication process, which makes it relatively easy to work with and explore lots of properties.”
Armed with this combination of properties, the team first chose to look for an anomalous Hall effect (AHE) in the material. This phenomenon is where electrons in a material with an applied electric field (but no magnetic field) can get deflected by 90 degrees by various mechanisms. “It had been theorized that metals with triangular spin arrangements could host a significant extrinsic effect, so it was a good place to start,” noted Yang. Using angle resolved photoelectron spectroscopy, microdevice fabrication, and a low temperature electronic property measurement system, Shuo-Ying and co-lead author Yaojia Wang (Max Planck Institute of Microstructure Physics) were able to observe one of the largest AHE’s ever seen.
The AHE can be broken into two general categories: intrinsic and extrinsic. “The intrinsic mechanism is like if a football player made a pass to their teammate by bending the ball, or electron, around some defenders (without it colliding with them),” explained Ali. “Extrinsic is like the ball bouncing off of a defender, or magnetic scattering center, and going to the side after the collision. Many extrinsically dominated materials have a random arrangement of defenders on the field, or magnetic scattering centers randomly diluted throughout the crystal. KV3Sb5 is special in that it has groups of 3 magnetic scattering centers arranged in a triangular net. In this scenario, the ball scatters off of the cluster of defenders, rather than a single one, and is more likely to go to the side than if just one was in the way.” This is essentially the theorized spin-cluster skew scattering AHE mechanism which was demonstrated by the authors in this material. “However the condition with which the incoming ball hits the cluster seems to matter; you or I kicking the ball isn’t the same as if, say, Christiano Ronaldo kicked the ball,” added Ali. “When Ronaldo kicks it, it is moving way faster and bounces off of the cluster with way more velocity, moving to the side faster than if just any average person had kicked it. This is, loosely speaking, the difference between the Dirac quasiparticles (Ronaldo) in this material vs normal electrons (average person) and is related to why we see such a large AHE,” Ali laughingly explained.
These results may also help scientists identify other materials with this combination of ingredients. “Importantly, the same physics governing this AHE could also drive a very large spin Hall effect (SHE) — where instead of generating an orthogonal charge current, an orthogonal spin current is generated,” remarked Wang. “This is important for next-generation computing technologies based on an electron’s spin rather than its charge.”
“This is a new playground material for us: metallic Dirac physics, frustrated magnetism, exfoliatable, and chemically stable all in one. There is a lot of opportunity to explore fun, weird phenomena, like unconventional superconductivity and more,” said Ali, excitedly. More

A breakthrough in fertility science by researchers from Bristol and Mexico has shattered the universally accepted view of how sperm ‘swim’.
More than three hundred years after Antonie van Leeuwenhoek used one of the earliest microscopes to describe human sperm as having a “tail, which, when swimming, lashes with a snakelike movement, like eels in water,” scientists have revealed this is an optical illusion.
Using state-of-the-art 3D microscopy and mathematics, Dr Hermes Gadelha from the University of Bristol, Dr Gabriel Corkidi and Dr Alberto Darszon from the Universidad Nacional Autonoma de Mexico, have pioneered the reconstruction of the true movement of the sperm tail in 3D.
Using a high-speed camera capable of recording over 55,000 frames in one second, and a microscope stage with a piezoelectric device to move the sample up and down at an incredibly high rate, they were able to scan the sperm swimming freely in 3D.
The ground-breaking study, published in the journal Science Advances, reveals the sperm tail is in fact wonky and only wiggles on one side. While this should mean the sperm’s one-sided stroke would have it swimming in circles, sperm have found a clever way to adapt and swim forwards.
“Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stoke would average itself out, and they would swim forwards,” said Dr Gadelha, head of the Polymaths Laboratory at Bristol’s Department of Engineering Mathematics and an expert in the mathematics of fertility.

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“The sperms’ rapid and highly synchronised spinning causes an illusion when seen from above with 2D microscopes — the tail appears to have a side-to-side symmetric movement, “like eels in water,” as described by Leeuwenhoek in the 17th century.
“However, our discovery shows sperm have developed a swimming technique to compensate for their lop-sidedness and in doing so have ingeniously solved a mathematical puzzle at a microscopic scale: by creating symmetry out of asymmetry,” said Dr Gadelha.
“The otter-like spinning of human sperm is however complex: the sperm head spins at the same time that the sperm tail rotates around the swimming direction. This is known in physics as precession, much like when the orbits of Earth and Mars precess around the sun.”
Computer-assisted semen analysis systems in use today, both in clinics and for research, still use 2D views to look at sperm movement. Therefore, like Leeuwenhoek’s first microscope, they are still prone to this illusion of symmetry while assessing semen quality. This discovery, with its novel use of 3D microscope technology combined with mathematics, may provide fresh hope for unlocking the secrets of human reproduction.
“With over half of infertility caused by male factors, understanding the human sperm tail is fundamental to developing future diagnostic tools to identify unhealthy sperm,” adds Dr Gadelha, whose work has previously revealed the biomechanics of sperm bendiness and the precise rhythmic tendencies that characterise how a sperm moves forward.
Dr Corkidi and Dr Darszon pioneered the 3D microscopy for sperm swimming.
“This was an incredible surprise, and we believe our state-of the-art 3D microscope will unveil many more hidden secrets in nature. One day this technology will become available to clinical centres,” said Dr Corkidi.
“This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization. So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinge on fertilization. These new tools open our eyes to the amazing capabilities sperm have,” said Dr Darszon. More

Using data from NASA’s Solar Dynamics Observatory, or SDO, scientists have developed a new model that successfully predicted seven of the Sun’s biggest flares from the last solar cycle, out of a set of nine. With more development, the model could be used to one day inform forecasts of these intense bursts of solar radiation.
As it progresses through its natural 11-year cycle, the Sun transitions from periods of high to low activity, and back to high again. The scientists focused on X-class flares, the most powerful kind of these solar fireworks. Compared to smaller flares, big flares like these are relatively infrequent; in the last solar cycle, there were around 50. But they can have big impacts, from disrupting radio communications and power grid operations, to — at their most severe — endangering astronauts in the path of harsh solar radiation. Scientists who work on modeling flares hope that one day their efforts can help mitigate these effects.
Led by Kanya Kusano, the director of the Institute for Space-Earth Environmental Research at Japan’s Nagoya University, a team of scientists built their model on a kind of magnetic map: SDO’s observations of magnetic fields on the Sun’s surface. Their results were published in Science on July 30, 2020.
It’s well-understood that flares erupt from hot spots of magnetic activity on the solar surface, called active regions. (In visible light, they appear as sunspots, dark blotches that freckle the Sun.) The new model works by identifying key characteristics in an active region, characteristics the scientists theorized are necessary to setting off a massive flare.
The first is the initial trigger. Solar flares, especially X-class ones, unleash huge amounts of energy. Before an eruption, that energy is contained in twisting magnetic field lines that form unstable arches over the active region. According to the scientists, highly twisted rope-like lines are a precursor for the Sun’s biggest flares. With enough twisting, two neighboring arches can combine into one big, double-humped arch. This is an example of what’s known as magnetic reconnection, and the result is an unstable magnetic structure — a bit like a rounded “M” — that can trigger the release of a flood of energy, in the form of a flare.
Where the magnetic reconnection happens is important too, and one of the details the scientists built their model to calculate. Within an active region, there are boundaries where the magnetic field is positive on one side and negative on the other, just like a regular refrigerator magnet.
“It’s similar to an avalanche,” Kusano said. “Avalanches start with a small crack. If the crack is up high on a steep slope, a bigger crash is possible.” In this case, the crack that starts the cascade is magnetic reconnection. When reconnection happens near the boundary, there’s potential for a big flare. Far from the boundary, there’s less available energy, and a budding flare can fizzle out — although, Kusano pointed out, the Sun could still unleash a swift cloud of solar material, called a coronal mass ejection.
Kusano and his team looked at the seven active regions from the last solar cycle that produced the strongest flares on the Earth-facing side of the Sun (they also focused on flares from part of the Sun that is closest to Earth, where magnetic field observations are best). SDO’s observations of the active regions helped them locate the right magnetic boundaries, and calculate instabilities in the hot spots. In the end, their model predicted seven out of nine total flares, with three false positives. The two that the model didn’t account for, Kusano explained, were exceptions to the rest: Unlike the others, the active region they exploded from were much larger, and didn’t produce a coronal mass ejection along with the flare.
“Predictions are a main goal of NASA’s Living with a Star program and missions,” said Dean Pesnell, the SDO principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who did not participate in the study. SDO was the first Living with a Star program mission. “Accurate precursors such as this that can anticipate significant solar flares show the progress we have made towards predicting these solar storms that can affect everyone.”
While it takes a lot more work and validation to get models to the point where they can make forecasts that spacecraft or power grid operators can act upon, the scientists have identified conditions they think are necessary for a major flare. Kusano said he is excited to have a promising first result.
“I am glad our new model can contribute to the effort,” he said. More

Researchers at the University of the Witwatersrand in Johannesburg, South Africa, have demonstrated a record setting quantum protocol for sharing a secret amongst many parties. The team created an 11-dimensional quantum state and used it to share a secret amongst 10 parties. By using quantum tricks, the secret can only be unlocked if the parties trust one another. The work sets a new record for the dimension of the state (which impacts on how big the secret can be) and the number of parties with whom it is shared and is an important step towards distributing information securely across many nodes in a quantum network.
Laser & Photonics Reviews published online the research by the Wits team led by Professor Andrew Forbes from the School of Physics at Wits University. In their paper titled: Experimental Demonstration of 11-Dimensional 10-Party Quantum Secret Sharing, the Wits team beat all prior records to share a quantum secret.
“In traditional secure quantum communication, information is sent securely from one party to another, often named Alice and Bob. In the language of networks, this would be considered peer-to-peer communication and by definition has only the two nodes: sender and receiver,” says Forbes.
“Anyone who has sent an email will know that often information must be sent to several people: one sender and many receiving parties. Traditional quantum communication such as quantum key distribution (QKD) does not allow this, and is only of the peer-to-peer form.”
Using structured light as quantum photon states, the Wits team showed how to distribute information from one sender to 10 parties. Then, by using some nifty quantum tricks, they could engineer the protocol so that only if the parties trust one another can the secret be revealed.
“In essence, each party has no useful information, but if they trust one another then the secret can be revealed. The level of trust can be set from just a few of the parties to all of them,” says Forbes. Importantly, at no stage is the secret ever revealed through communication between the parties: they don’t have to reveal any secrets. In this way a secret can be shared in a fundamentally secure manner across many nodes of a network: quantum secret sharing.
“Our work pushes the state-of-the-art and brings quantum communication closer to true network implementation,” says Forbes. “When you think of networks you think of many connections, many parties, who wish to share information and not just two. Now we know how to do this the quantum way.”
The team used structured photons to reach high dimensions. Structured light means ”Patterns of light” and here the team could use many patterns to push the dimension limit. More dimensions mean more information in the light, and translates directly to larger secrets.

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Origami principles can unlock the potential of the smallest robots, enhancing speed, agility and control in machines no more than a centimeter in size. More

Researchers have reported a new form of electronics known as ‘drawn-on-skin electronics,’ allowing multifunctional sensors and circuits to be drawn on the skin with an ink pen. More

Researchers have solved a major problem for optical wireless communications — the process by which light carries information between cell phones and other devices. Light-emitting diodes (LEDs) pulse their light in a coded message that recipient devices can understand.
Now, a team of researchers based in Japan has married the two options into the ideal combination of long lasting and fast LEDs. They published their results on July 22 in Applied Physics Letters.
“A key technology for faster modulation is to decrease the device si earch for Advanced Materials at Tohoku University. “However, this tactic creates a dilemma: although smaller LEDs can be modulated faster, they have lower power.”
Another issue is that both visible and infrared optical wireless communications can have significant solar interference, according to Kojima. To avoid confusion with visible and infrared solar light, the researchers aimed to improve LEDs that specifically communicate via deep ultraviolet light, which can be detected without solar interference.
“Deep ultraviolet LEDs are currently mass produced in factories for applications related to COVID-19,” Kojima said, noting that deep ultraviolet light is used for sterilization processes as well as in solar-blind optical wireless communications. “So, they’re cheap and practical to use.”
The researchers fabricated the deep ultraviolet LEDs on sapphire templates, which are considered an inexpensive substrate, and measured their transmission speed. They found that the deep ultraviolet LEDs were smaller and much quicker in their communications than traditional LEDs at that speed.
“The mechanism underlying this speed is in how a lot of tiny LEDs self-organize in a single deep ultraviolet LED,” Kojima said. “The tiny LED ensemble helps with both power and speed.”
The researchers want to use the deep ultraviolet LEDs in 5G wireless networks. Many technologies are currently under testing to contribute 5G, and Li-Fi, or light fidelity, is one of the candidate technologies.
“Li-Fi’s critical weakness is its solar dependency,” Kojima said. “Our deep ultraviolet LED-based optical wireless technology can compensate for this problem and contribute to society, I hope.”
This work was supported in part by Five-Star Alliance and the Japan Society for the Promotion of Science.

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A way of shrinking the devices used in quantum sensing systems has been developed by researchers at the UK Quantum Technology Hub Sensors and Timing, which is led by the University of Birmingham.
Sensing devices have a huge number of industrial uses, from carrying out ground surveys to monitoring volcanoes. Scientists working on ways to improve the capabilities of these sensors are now using quantum technologies, based on cold atoms, to improve their sensitivity.
Machines developed in laboratories using quantum technology, however, are cumbersome and difficult to transport, making current designs unsuitable for most industrial uses.
The team of researchers has used a new approach that will enable quantum sensors to shrink to a fraction of their current size. The research was conducted by an international team led by University of Birmingham and SUSTech in China in collaboration with Paderborn University in Germany. Their results are published in Science Advances.
The quantum technology currently used in sensing devices works by finely controlling laser beams to engineer and manipulate atoms at super-cold temperatures. To manage this, the atoms have to be contained within a vacuum-sealed chamber where they can be cooled to the desired temperatures.
A key challenge in miniaturising the instruments is in reducing the space required by the laser beams, which typically need to be arranged in three pairs, set at angles. The lasers cool the atoms by firing photons against the moving atom, lowering its momentum and therefore cooling it down.
The new findings show how a new technique can be used to reduce the space needed for the laser delivery system. The method uses devices called optical metasurfaces — manufactured structures that can be used to control light.
A metasurface optical chip can be designed to diffract a single beam into five separate, well-balanced and uniform beams that are used to supercool the atoms. This single chip can replace the complex optical devices that currently make up the cooling system.
Metasurface photonic devices have inspired a range of novel research activities in the past few years and this is the first time researchers have been able to demonstrate its potential in cold atom quantum devices.
Dr Yu-Hung Lien, lead author of the study, says: “The mission of the UK Quantum Technology Hub is to deliver technologies that can be adopted and used by industry. Designing devices that are small enough to be portable or which can fit into industrial processes and practices is vital. This new approach represents a significant step forward in this approach.”
The team have succeeded in producing an optical chip that measures just 0.5mm across, resulting in a platform for future sensing devices measuring about 30cm cubed. The next step will be to optimise the size and the performance of the platform to produce the maximum sensitivity for each application.

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