Synthesizing New 3D Materials by Twisting

Overlapping two 3D lattices with a relative twist opens the door to synthesizing crystals with diverse symmetries that showcase nontrivial band structures and novel properties.

When two identical periodic lattices overlap in space, with one twisted at an angle relative to the other, they form moiré lattices. The best-known examples are formed from stacked and rotated 2D sheets. These structures can possess fascinating properties not seen in their component layers. Twisted bilayer graphene, for example, can exhibit superconductor and Mott insulator behavior. Ce Wang of Tongji University in China and his colleagues now propose how to construct a 3D moiré lattice using two cubic optical lattices hosting ultracold atoms. The researchers mathematically describe how two simple periodic structures, twisted relative to each other, can lead to 3D optical moiré patterns (Fig. 1). The result is a crystal-like structure with emergent properties that differ from those of the underlying simple lattices. The researchers show that adjusting the twisting axes and angles leads to various crystalline symmetries, enabling the exploration of diverse material properties.

In the past five years, researchers have experimentally demonstrated the moiré lattice concept in optical [4–7] and cold-atom [8] systems. These demonstrations show the possibility of extending the concept from 2D into 3D. One of the most intriguing aspects of 3D moiré patterns is their non-Abelian rotation: The order in which you rotate the lattice significantly affects the final structure. This property contrasts with 2D systems, in which rotating an object by one angle and then another yields the same result regardless of the order.

Non-Abelian rotation can be visualized by rotating a cube. Turning the cube 90° around one axis and then 45° around another axis results in a different orientation than if you had performed those rotations in the reverse order. Now, suppose you have two cubic lattices, one fixed and the other twisted about its axis. Combining the two—creating 3D moiré lattices—will obviously lead to different structures, depending on which rotation happens first. This complexity allows for a richer variety of crystalline arrangements and, consequently, of material properties. Exploiting this non-Abelianism, therefore, offers researchers a powerful tool to “synthesize” new materials by manipulating existing ones and to study how different crystalline symmetries interact with atoms, photons, and electrons.

Wang and colleagues’ mathematical demonstration of the range of 3D moiré crystal structures that can be realized offers a fascinating look into the outcomes that non-Abelian physics makes possible. Notable among these is a feature exclusive to 3D crystals: the presence of Weyl points. Weyl points are the intersections of linearly dispersing energy bands similar to the Dirac points that arise in 2D crystals. Synthesizing a material whose energy band structure contains Weyl points opens doors to exploring a variety of intriguing phenomena such as topologically protected surface states and chiral anomalies. In recent years, researchers have successfully designed various intricate 3D structures to generate Weyl points. Surprisingly, Wang and colleagues’ numerical work reveals that by simply twisting two cubic lattices, the resultant 3D moiré crystals host many Weyl points and nodal lines. This result shows that 3D moiré lattices represent an alternative route to creating topological materials and correspondingly exotic states.

Wang and colleagues propose to implement 3D moiré lattices in the context of ultracold atoms, where these arrangements have the potential to enhance the exploration of diverse crystalline symmetries and their associated nontrivial band structures. However, the concept can be extended to other fields such as optics and acoustics, where it might bring many other new possibilities. One intriguing avenue for further investigation is the study of 3D moiré crystals under nonperiodic, or incommensurate, phases—that is, in crystals that do not possess the property of translational invariance. Note that Wang’s current study only considers the periodic, or commensurate, phase. An immediate question concerning 3D nonperiodic moiré crystals is whether localized wave packets, confined within the 3D space, exist. The existence of such wave packets in 2D is already well established. In a 3D system, because three twisting angles can be tuned separately, one can imagine localizing a wave in one plane while controlling its propagation in another specific, tunable direction. This could offer a unique approach to wave localization (creating a totally novel cavity) and subsequent access to it (novel waveguiding).

Additionally, it would be interesting to explore how nonlinear phenomena that might occur in a lattice with a 3D moiré structure differ from their 2D counterparts. For example, optical Kerr nonlinearity or two-body interactions in Bose-Einstein condensates could bring about new properties like the formation of higher-dimensional solitons controlled by the twisting. Breakthroughs, both experimental and theoretical, in these intriguing directions involving 3D moiré lattices are anticipated in the near future.

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Evidence of ‘Negative Time’ Found in Quantum Physics Experiment

Physicists showed that photons can seem to exit a material before entering it, revealing observational evidence of negative time. Quantum physicists are familiar with wonky, seemingly nonsensical phenomena: atoms and molecules sometimes act as particles, sometimes as waves; particles can be connected to one another by a “spooky action at a distance,” even over great distances; and quantum objects can detach themselves from their properties like the Cheshire Cat from Alice’s Adventures in Wonderland detaches itself from its grin. Now researchers led by Daniela Angulo of the University of Toronto have revealed another oddball quantum outcome: photons, wave-particles of light, can spend a negative amount of time zipping through a cloud of chilled atoms. In other words, photons can seem to exit a material before entering it.

The idea for this work emerged in 2017. At the time, Steinberg and a lab colleague, then doctoral student Josiah Sinclair, were interested in the interaction of light and matter, specifically a phenomenon called atomic excitation: when photons pass through a medium and get absorbed, electrons swirling around atoms in that medium jump to higher energy levels. When these excited electrons lapse to their original state, they release that absorbed energy as reemitted photons, introducing a time delay in the light’s observed transit time through the medium.

Sinclair’s team wanted to measure that time delay (which is sometimes technically called a “group delay”) and learn whether it depends on the fate of that photon: Was it scattered and absorbed inside the atomic cloud, or was it transmitted with no interaction whatsoever? “At the time, we weren’t sure what the answer was, and we felt like such a basic question about something so fundamental should be easy to answer,” Sinclair says. “But the more people we talked to, the more we realized that while everyone had their own intuition or guess, there was no expert consensus on what the right answer would be.” Because the nature of these delays can be so strange and counterintuitive, some researchers had written the phenomenon off as effectively meaningless for describing any physical property associated with light.

After three years of planning, his team developed an apparatus to test this question in the lab. Their experiments involved shooting photons through a cloud of ultracold rubidium atoms and measuring the resulting degree of atomic excitation. Two surprises emerged from the experiment: Sometimes photons would pass through unscathed, yet the rubidium atoms would still become excited—and for just as long as if they had absorbed those photons. Stranger still, when photons were absorbed, they would seem to be reemitted almost instantly, well before the rubidium atoms returned to their ground state—as if the photons, on average, were leaving the atoms quicker than expected.

The team then collaborated with Howard Wiseman, a theoretical and quantum physicist at Griffith University in Australia, to devise an explanation. The theoretical framework that emerged showed that the time these transmitted photons spent as an atomic excitation matched perfectly with the expected group delay acquired by the light—even for cases where it seemed as though the photons were reemitted before the atomic excitation had ebbed.

To understand the nonsensical finding, you can think of photons as the fuzzy quantum objects they are, in which any given photon’s absorption and reemission through an atomic excitation is not guaranteed to occur over a certain fixed amount of time; rather, it takes place across a smeared-out, probabilistic range of temporal values. As demonstrated by the team’s experiments, these values can encompass instances when an individual photon’s transit time is instantaneous—or, bizarrely, when it concludes before the atomic excitation has ceased, which gives a negative value.

“I can promise you that we were completely surprised by this prediction,” Sinclair says, referring to the matchup between the group delay and the time that the transmitted photons spent as atomic excitations. “And as soon as we were confident we hadn’t made a mistake, Steinberg and the rest of the team—I had moved on to do a postdoc at [the Massachusetts Institute of Technology] by this point—began planning to do a follow-up experiment to test this crazy prediction of negative dwell time and see if the theory would hold up.”
That follow-up experiment, the one led by Angulo that Steinberg touted on X, can be understood by considering the two ways a photon can be transmitted. In one, the photon wears blinders of sorts and ignores the atom entirely, leaving without even a nod. In the other, it interacts with the atom, boosting it to a higher energy level, before getting reemitted.

“When you see a transmitted photon, you can’t know which of these occurred,” Steinberg says, adding that because photons are quantum particles in the quantum realm, the two outcomes can be in superposition—both things can happen at the same time. “The measuring device ends up in a superposition of measuring zero and measuring some small positive value.” But correspondingly, Steinberg notes, that also means that sometimes “the measuring device ends up in a state that looks not like ‘zero’ plus ‘something positive’ but like ‘zero’ minus ‘something positive,’ resulting in what looks like the wrong sign, a negative value, for this excitation time.”

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Huge neutrino detector sees first hints of particles from exploding stars

Every few seconds, somewhere in the observable Universe, a massive star collapses and unleashes a supernova explosion. Japan’s Super-Kamiokande observatory might now be collecting a steady trickle of neutrinos from those cataclysms, physicists say — amounting to a few detections a year.

These tiny subatomic particles are central to understanding what goes on inside a supernova: because they zip out of the star’s collapsing core and across space, they can provide information about any potentially new physics that occur under extreme conditions.

At last month’s Neutrino 2024 conference in Milan, Italy, Masayuki Harada, a physicist at the University of Tokyo, revealed that the first hints of supernova neutrinos seem to be emerging from the cacophony of particles that the Super-Kamiokande detector collects every day from other sources, such as cosmic rays hitting the atmosphere and nuclear fusion in the Sun’s core. The result “indicates that we started observing a signal”, says Masayuki Nakahata, a physicist at the University of Tokyo and spokesperson for the experiment, which is commonly referred to as Super-K. But Nakahata cautions that the supporting data — collected over 956 days of observation — are still very weak.
Elusive particles

Neutrinos are extremely hard to catch. Most travel through the planet like light through glass, and Super-K catches only a tiny fraction of those that cross it. Even so, the detector has a decent chance of catching neutrinos from supernovae, because the Universe should be awash with them. The collapse of a star releases stupendous numbers of these particles (estimated to be around 1058), producing what astrophysicists call the diffuse supernova neutrino background.


But as yet, no one has been able to detect this background. Neutrinos have only once been conclusively traced back to a collapsing star — Nakahata was among the researchers who spotted the particles using the Kamiokande-II detector, a predecessor of Super-K, in 1987. The detection was possible because the supernova had occurred in the Large Magellanic Cloud, a dwarf galaxy that is close enough that the exploding star’s neutrinos reached Earth in large numbers.

In 2018–20, the Super-K detector, a tank containing 50,000 tonnes of purified water located under one kilometre of rock near Hida in central Honshu Island, underwent a simple but important upgrade aimed at increasing its ability to tell supernova neutrinos apart from other particles.

When a neutrino — or more precisely, its antiparticle, an antineutrino — collides with a proton in the water, that proton can transform into a pair of other particles, a neutron and an antielectron. The antielectron produces a flash of light as it travels at high speed in the water, and that light is captured by the sensors that line the tank’s walls. By itself, that flash of light could be indistinguishable from those produced by neutrinos or antineutrinos from a number of other sources.

But during the upgrade, scientists added a gadolinium-based salt to Super-K’s water. This allows the neutron produced when an antineutrino hits the water to be captured by the gadolinium nucleus, which releases a second, telltale flash of energy. Super-K physicists in search of supernova neutrinos look for a rapid sequence of two flashes, one produced by the antielectron and the second by the neutron being captured.
Solving cosmic mysteries

It will still take several years for genuine supernova signals to clearly emerge, Nakahata says, because double-flash signals can come from other neutrino sources, including those generated by cosmic rays hitting the atmosphere. But by the time Super-K is scheduled to shut down in 2029, he adds, it should have gathered enough data to make a solid claim.

And an even larger experiment called Hyper-Kamiokande, scheduled to be completed around 2027, could massively improve on Super-K’s results. Initially, Hyper-K will be filled with pure water, but “all components of the detector are tested to be compatible with gadolinium”, which could be added at a later time, says Francesca Di Lodovico, a physicist at King’s College London and co-spokesperson for the project.




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How neutron stars 'playing it cool' could unlock exotic physics

Scientists have found that three neutron stars, born in the fires of other exploding stars, have cooled off surprisingly quickly, bringing us closer to understanding the exotic nature of matter within the cores of these extreme objects.


The discovery was made by a Spanish team led by Alessio Marino of the Institute of Space Sciences (ICE–CSIC) in Barcelona, using European and American space telescopes that work with X-ray light.


A neutron star is the collapsed core of a massive star that has gone supernova, and can contain up to nearly three times the mass of our sun in a spherical volume just about 6.8 miles (11 kilometers) across. All that matter compacted into such a small area means neutron stars are among the densest concentrations of matter in the known universe, second only to black holes. To make that statement more relatable, consider how a tablespoon of neutron-star material would be comparable to the mass of Mount Everest.



This extreme nature also means the physics that governs neutron stars' interiors remains murky. These objects are called neutron stars to begin with because their matter has been crushed to such a degree that negatively charged electrons and positively charged protons get smushed together, overcoming the electrostatic force between them to form an object full of just neutral neutrons. Deeper in the core of a neutron star, matter may be crushed to an even greater extent, forming exotic, never-before-seen particles such as hypothetical hyperons. Perhaps, scientists believe, or neutrons themselves could be popped apart within a neutron star, creating a soup of the universe's most fundamental particles: quarks.

Related: Weird cosmic object identified as the remains of an exploded dead star


What happens inside a neutron star is governed by the neutron star equation of state. Think of this as a playbook that determines a neutron star's internal structure and composition based on things like its mass, temperature, magnetic field and so on. The trouble is, scientists have literally hundreds of options for what this equation of state could be. Since we cannot replicate on Earth the conditions inside a neutron star, testing which model is the right one is highly dependent on matching them to what astronomical observations tell us.

Now, however, the discovery of three neutron stars with substantially lower surface temperatures compared to other neutron stars of similar age has provided a big clue, allowing researchers to rule out three-quarters of the possible models for the neutron star equation of state in one stroke. Two of the neutron stars are pulsars, which are rapidly spinning neutron stars that fire beams of radio jets toward us. The third neutron star, in the Vela Jr supernova remnant, doesn't display pulsar behavior, but that may just be because its radio jets do not point in our direction.

SOURCE: by Keith Cooper

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A black hole made from pure light is impossible, thanks to quantum physics

Black holes can’t be formed from pure light. Quantum physics would curb their creation under any foreseeable conditions, a new study suggests.

Typically, matter is responsible for black holes. They’re often formed when a star’s core implodes at the end of its life. But matter isn’t necessarily required to form a black hole. According to Einstein’s general theory of relativity, black holes could form from concentrated energy alone.

A black hole formed from electromagnetic energy — that is, light — is called a kugelblitz. That concept has been jangling around in physicists’ brains for decades. But actually producing a kugelblitz seems to be a no-go, theoretical physicist Eduardo Martín-Martínez and colleagues report in a paper accepted to Physical Review Letters. “No known source in the current universe would be able to produce it, neither artificial or natural,” says Martín-Martínez, of the University of Waterloo in Canada.

In recent years, science fiction writers have picked up the kugelblitz mystique and run with it. Fans of the Netflix show Umbrella Academy may be familiar with the term, which is German for “ball lightning.” In season 3, a kugelblitz obliterates large swaths of existence.

In general relativity, gravity results from matter curving spacetime. If enough mass is packed into one region, the spacetime can curve so dramatically that it forms a region within which it’s impossible to escape — a black hole. But in general relativity, energy and mass are equivalent. That means energy can curve spacetime just as matter can, suggesting the wild idea that a black hole could form with no matter at all.

That concept is “a very interesting thought,” says theoretical physicist Juan García-Bellido of Universidad Autónoma de Madrid, who was not involved in the new study, “especially if we want to produce something like this in the laboratory.” Scientists have previously considered whether futuristic lasers might one day form a black hole in a lab, and even proposed using a kugelblitz to power a spacecraft.

Alas, calculations suggest that any attempt at a kugelblitz would result in failure, Martín-Martínez says. “You are not going to get even close. You’re not going to get even something that starts attracting you like Earth would.”

That’s because of a quantum effect that occurs when electromagnetic energy is highly concentrated. According to the well-verified theory of quantum electrodynamics, when light reaches those extremes, pairs of particles and antiparticles begin to form. Those particles — electrons and their positively charged antimatter partners, positrons — would escape the region, taking energy with them. That prevents the energy from reaching the levels needed to form a black hole.

Forming a kugelblitz in a laboratory would require light intensities more than 1050 times that of the state-of-the-art laser pulses, the team calculated. (That’s a mind-bogglingly large factor — a 1 with 50 zeroes after it.) And in nature, the brightest quasars — brilliantly luminous centers of active galaxies — are likewise vastly too dim.

The kibosh on kugelblitzes applies across a huge range of scales. It rules out itty-bitty kugelblitzes with a radius as small as a hundredth of a quintillionth of a nanometer all the way up to 100 million meters. Even outside that range, Martín-Martínez says, a kugelblitz would still be very unlikely.

García-Bellido, however, notes a possible loophole: “It’s much more likely that things like this might have happened in the early universe.”

Just after the Big Bang, the universe is thought to have expanded extremely rapidly, a process known as inflation. That inflation may have imprinted fluctuations in the curvature of spacetime that could cause light to collapse into what’s known as a primordial black hole (SN: 8/7/16). So while light won’t form black holes under its own gravity, that preexisting curvature, García-Bellido says, could have allowed something akin to a kugelblitz.

SOURCE: by Emily Conover

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Demystifying Vortex Rings in Nuclear Fusion, Supernovae

Better understanding the formation of swirling, ring-shaped disturbances—known as vortex rings—could help nuclear fusion researchers compress fuel more efficiently, bringing it closer to becoming a viable energy source.

The model developed by researchers at the University of Michigan (U-M) could aid in the design of the fuel capsule, minimizing the energy lost while trying to ignite the reaction that makes stars shine. In addition, the model could help other engineers who must manage the mixing of fluids after a shock wave passes through, such as those designing supersonic jet engines, as well as physicists trying to understand supernovae.

"These vortex rings move outward from the collapsing star, populating the universe with the materials that will eventually become nebulae, planets, and even new stars—and inward during fusion implosions, disrupting the stability of the burning fusion fuel and reducing the efficiency of the reaction," said Michael Wadas, a doctoral candidate in mechanical engineering at U-M and corresponding author of the study.

"Our research, which elucidates how such vortex rings form, can help scientists understand some of the most extreme events in the universe and bring humanity one step closer to capturing the power of nuclear fusion as an energy source," he said.

Nuclear fusion pushes atoms together until they merge. This process releases several times more energy than breaking atoms apart, or fission, which powers today's nuclear plants. Researchers can create this reaction, merging forms of hydrogen into helium, but at present, much of the energy used in the process is wasted.

Part of the problem is that the fuel can't be neatly compressed. Instabilities cause the formation of jets that penetrate into the hotspot, and the fuel spurts out between them—Wadas compared it to trying to squish an orange with your hands, how juice would leak out between your fingers.

Vortex rings that form at the leading edge of these jets, the researchers have shown, are mathematically similar to smoke rings, the eddies behind jellyfish and the plasma rings that fly off the surface of a supernova.

Perhaps the most famous approach to fusion is a spherical array of lasers all pointing toward a spherical capsule of fuel. This is how experiments are set up at the National Ignition Facility, which has repeatedly broken records for energy output in recent years.

The energy from the lasers vaporizes the layer of material around the fuel—a nearly perfect, lab-grown shell of diamond in the latest record-setter in December 2022. When that shell vaporizes, it drives the fuel inward as the carbon atoms fly outward. This generates a shock wave, which pushes the fuel so hard that the hydrogen fuses.

While the spherical fuel pellets are some of the most perfectly round objects humans have ever made, each has a deliberate flaw: a fill tube, where the fuel enters. Like a straw stuck in that crushed orange, this is the most likely place for a vortex-ring-led jet to form when the compression starts, the researchers explained.

"Fusion experiments happen so fast that we really only have to delay the formation of the jet for a few nanoseconds," said Eric Johnsen, an associate professor of mechanical engineering at U-M, who supervised the study.

The study brought together the fluid mechanics expertise of Wadas and Johnsen as well as the nuclear and plasma physics knowledge in the lab of Carolyn Kuranz, an associate professor of nuclear engineering and radiological sciences.

"In high-energy-density physics, many studies point out these structures, but haven’t clearly identified them as vortex rings," said Wadas.

Knowing about the deep body of research into the structures seen in fusion experiments and astrophysical observations, Wadas and Johnsen were able to draw on and extend that existing knowledge rather than trying to describe them as completely new features.

Johnsen is particularly interested in the possibility that vortex rings could help drive the mixing between heavy elements and lighter elements when stars explode, as some mixing process must have occurred to produce the composition of planets like Earth.

The model can also help researchers understand the limits of the energy that a vortex ring can carry, and how much fluid can be pushed before the flow becomes turbulent and harder to model as a result. In ongoing work, the team is validating the vortex ring model with experiments.

The research is funded by Lawrence Livermore National Laboratory and the Department of Energy, with computational resources provided by the Extreme Science and Engineering Discovery Environment through the National Science Foundation and the Oak Ridge Leadership Computing Facility.

This press release was provided by the University of Michigan

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A Recent Look at Molecular Spectroscopy

Spectroscopy magazine publishes a column titled, “Molecular Spectroscopy Workbench” that focuses on molecular spectroscopic techniques. Many of these articles are designed to be tutorial in nature, allowing readers an inside look at the interactions between electromagnetic waves and matter.



Below is a compilation of four of our most recent Molecular Spectroscopy Workbench columns. Happy reading!

Monitoring Chemical Changes by Raman Spectroscopy

This article discusses the application of Raman spectroscopy in real-time monitoring of chemical reactions, with a particular focus on polymerization. Polymerization involves the loss of a carbon double bond as the polymer chain lengthens, and the strong signals from π electrons make it easy to track this reaction to completion (1). Despite its usefulness, setting up polymerization experiments for demonstration can be hazardous. The article aims to illustrate the chemical and spectral changes that take place during the curing process of a commercial epoxy (1).

Raman Spectra Used to Understand the Origins of Banding in Spherulites

This article examines the formation and characteristics of spherulites in polymers, which form crystal lamellae radiating from a nucleation site when the polymer is crystallized from the melt. When observed under a microscope with crossed polarizers, these spherulites display a distinctive Maltese cross pattern with banding (2). The lit regions, resulting from crystals growing in directions not parallel to the polarizers, often show banding because of rotations of the crystal lamellae (2). Given the sensitivity of polarized Raman spectra to crystal orientation, the article explores the relationship between the observed banding pattern and Raman polarization/orientation behavior (2). The investigation focuses on spherulites of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHx) with different compositions (2).

Calibrating the Composition of a Copolymer

This article details the use of Raman spectroscopy in collaboration with Isao Noda to study the properties of the bioplastic polyhydroxybutyrate hexanoate (PHBHx). The properties of PHBHx, such as optical clarity, dyeability, flexibility, and thermal characteristics, depend on the percentage of hexanoate, which introduces propyl side branches to the polymer chain (3). These percentages influence the polymer's maximum crystallinity and, consequently, its physical and chemical properties (3). The article highlights the usefulness of Raman spectroscopy in accurately determining the composition of PHBHx, demonstrating a higher-than-expected precision in measuring hexanoate percentages (3).

Measuring the Crystallinity of PHBHx with Varying Amounts of Sidechains on a Benchtop Instrument

This article explores the use of current benchtop instruments with edge filters that provide Raman spectra down to 50 cm-1 for studying the crystallization of polyhydroxybutyrate-hexanoate (PHBHx). These compact instruments simplify the process compared to larger setups. By averaging the signals from spherulites, the orientation effects are mitigated, facilitating the use of multivariate techniques to compare different samples (4). PHBHx polymers are gaining commercial interest due to their biodegradable nature and production via fermentation, positioning them as sustainable alternatives to non-biodegradable, petroleum-derived polymers (4). The article also explains that controlling the polymer's physical properties, such as crystallinity, is achievable by adjusting the molecular weight and the percentage of sidechains. Removing the effects of polymer chain orientation through spectrum averaging is essential for accurate spectral comparison across samples (4).

SOURCE: by Will Wetzel

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