Researchers at Nagoya University have built a graphene nanoribbon that can switch which way it twists using a natural solvent. Graphene nanoribbons are thin ribbon-shaped structures made of fused carbon rings. Twisted or helical versions of these ribbons show promise for advanced light and electronic devices. However, no graphene nanoribbon could switch that twist on demand, until now.

Scientists have worked out how to switch between a right-handed and left-handed twist (chirality) by changing the solvent around it. The discovery ¹ opens opportunities for new optical switches, chemical sensors, and spintronic components that adapt to changing environments.

Tomoyuki Ikai, lead author and professor at Nagoya University’s Graduate School of Engineering, is a polymer chemist inspired by natural helical structures such as DNA and proteins. Polymers are large molecules built from many small, repeating units linked together in a chain. Ikai had been working on ladder polymers, where each repeating unit is locked into place and cannot freely rotate around the bond connecting to its neighbor (unlike a typical flexible linear polymer).

In 2019, Ikai and his collaborators built the first rigid, helix-shaped polymer. In 2021, they perfected a chemical method to fuse these building blocks together without flaws. This method is what let them build the new graphene nanoribbon.

“We used the same fusion approach already used to make predictable helical ladder polymers. But instead of a stable building block, such as [6]helicene, we used [4]helicene, a 4-ring unit written off as too unstable to be useful,” Ikai said.

Soak, twist, repeat

These units are difficult to control on their own because they switch their chirality too fast to hold one handedness. The team’s method fuses these small, restless units into one long chain. Once joined, they found that a neighbor effect took hold: each unit’s twist began to match its neighbors, and long stretches of the ribbon settled into a single, shared spiral.

To lock it in one overall direction, the researchers dissolved the ribbon in a natural chiral liquid, a liquid made of molecules that come in two mirror-image versions. They tested six candidates before they found the right one: beta-pinene, a compound found in pine trees and citrus peel.

Because beta-pinene molecules can be arranged in left- and right-hand mirror-images, these two versions each push the ribbon toward the opposite spiral. This also changed the twist direction of light the ribbon gave off, a property called circularly polarized luminescence, and is the first graphene nanoribbon shown to switch its light this way.

The switching induced a remarkably high degree of helical bias, with almost all of the helicene units aligning in the same direction. The switching effect is strongest at –90°C and grows weaker as the temperature rises toward room temperature.

“For a long time, people saw [4]helicene as unsuitable for chiral materials, because it could not be controlled,” Ikai said. “In this paper, we show that once you link enough of these units together, the instability itself becomes the useful part.”

A chiral solvent’s nudge toward one handedness is usually very weak on its own. Here, the ribbon’s own structure, with each unit mechanically linked to its neighbors, amplifies that weak nudge into a strong, nearly one-handed result.

An adaptive material

The researchers point to several possible future uses: The switchable light could serve optical devices. The ribbon’s chemical sensitivity to beta-pinene suggests it may work as a chiral sensor, and a similar design could one day help detect hazardous or unstable chemicals. Because graphene conducts electricity well, the ribbon may also suit electronic sensors.

The molecule’s twisted shape connects to spintronics, a field that uses the spin of electrons rather than just their charge. Ikai noted that a right-handed or left-handed structure can, in principle, let electrons of one spin pass through more easily than the other, a property helical structures are known for in physics literature.

The current design only holds its twist while it stays in the chiral liquid. If the solvent is removed, the ribbon drifts back to a mixed, unstable state. Ikai’s next goal is a form of “chiral memory,” a way for the ribbon to lock in its twist even after the solvent is gone. This next step builds on memory techniques the researchers have explored in other helical systems and is now applied to graphene for the first time.

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The kind of technology that can deliver sound to a single listener—like something out of a spy film—may soon become a reality. Researchers at Pohang University of Science and Technology (POSTECH) have developed a speaker system that can direct speech to a specific person. ¹

Light and sound behave differently. Light from a flashlight or laser travels in a straight line, whereas sound readily spreads in all directions—that is why voices can be heard over a wall. The question, “Could sound, like light, be sent only where we want it?” led to the team’s highly directional speaker, MiPAL. By modulating ultrasound with audible content, the system can deliver sound only within a targeted area. A representative technology is the parametric array loudspeaker (PAL), which uses the nonlinear interaction of two ultrasonic waves in air to generate and concentrate audible sound in a specific direction.

 

Compared with conventional speakers, PALs can confine sound to a much narrower region. Traditional systems, however, require tens to hundreds of ultrasonic emitters to be precisely arranged and individually controlled, making them expensive and structurally complex. More recent PAL designs have simplified the hardware, but the vibrating diaphragm can also excite unwanted vibration modes, allowing sound to leak sideways and undermining directivity.

 

The team addressed this problem by integrating two types of metamaterials into a single ultrasonic transducer. An acoustic metasurface was mounted at the front. Acting like a lens for sound, it reshapes the irregular ultrasonic wavefronts radiated by the diaphragm into a straight, narrow beam, much like a laser. Elastic meta-units were placed at the rear. Acting more like a sound barrier, they suppress unwanted vibrations at selected frequencies and keep sound from leaking to the sides. In effect, the design uses a double safeguard: it focuses the sound at the front and blocks parasitic noise at the back.

As a result, the single-element device directed sound over a frequency range of 500 Hz to 10 kHz—more than four octaves, covering a large portion of the audible spectrum. The researchers also conducted an experiment simulating aircraft seating, where different announcements or music might need to reach individual passengers without disturbing their neighbors. After playing real broadcast and music signals and measuring sound intensity at different angles, they observed a clear improvement after the elastic meta-units were added.

 

Another key advantage is ease of fabrication and adaptation. The metamaterial structures are modular and can be produced using 3D printing, allowing the device to be readily redesigned for different seat spacing, angles and other operating conditions.

 

Once commercialized, the technology could let passengers on planes or trains listen to personalized announcements or content without disturbing the person next to them. In museums and exhibition halls, explanations could be heard only by visitors standing in front of a particular display. The “sound only I can hear” once seen only in films may soon become part of everyday life.

 

“By co-designing the metamaterials and the ultrasonic device as a single system, we overcame the limitations of cost, complexity and design flexibility at the same time,” said Professor Junsuk Rho. “Research on acoustic and elastic metamaterials has largely remained at the stage of validating principles and demonstrating concepts. This study is the first to integrate metamaterials into functional device and demonstrate their potential for practical use.”

 

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At the center of most galaxies lies a supermassive black hole, with a mass millions to billions of times that of our sun and some of the most extreme gravity in the universe.

Some stars that venture too close to such black holes live to tell the tale. Rather than being completely destroyed, they survive to make repeated close passes, producing a new flare of light each time.

These repeating partial tidal disruption events (rpTDEs) give astronomers the opportunity to watch the same star and black hole interaction unfold again and again, thanks to wide-field time-domain surveys that repeatedly scan large areas of the sky for objects that change in brightness.

But in several cases, researchers have noticed a puzzling pattern: The successive flares grow progressively dimmer. For years, theoretical models couldn’t explain why.

Now, a team of astrophysicists at Syracuse University has shown that the answer may lie in a previously overlooked factor—the star’s spin.¹

When stars meet black holes

In a typical tidal disruption event (TDE), a black hole’s tidal force—the difference in gravitational pull across a nearby star—tears the star completely apart.

As the disrupted stellar debris falls toward, or “accretes” onto, the black hole, it loses energy that is emitted in the form of light over the course of days to months.

While black holes themselves emit no light, TDEs provide a short-lived supply of fuel that lights up the surrounding region, allowing astronomers to study these otherwise invisible objects indirectly.

If a star orbiting a black hole does not come close enough to be completely ripped apart, it can, however, lose a fraction of its mass, resulting in a partial TDE. In a repeating partial TDE, the surviving core continues orbiting the black hole, losing more material with each new close pass, a few months to several years apart.

The dimming mystery

How much material a star loses during repeated encounters depends partly on its internal structure. Bandopadhyay compares a low-mass star to a fluffy meringue, which can become increasingly vulnerable to the black hole’s tidal forces. By contrast, a higher-mass star has a more centrally concentrated, onion-like star and can shed its outer layers while its dense core remains relatively unaffected, losing decreasing amounts of mass over time.

Those differences can help explain why rpTDEs don’t all behave the same way. But one pattern in particular has mystified researchers. Of the roughly 10 repeating systems identified to date, four have produced flares that grow progressively dimmer.

Decreasing mass loss might seem like an obvious explanation. But previous hydrodynamical simulations showed that, surprisingly, even as the material lost decreased with each encounter, the predicted flares retained roughly the same brightness.

“We were puzzled by this for two years,” Bandopadhyay says.

Their previous work had revealed another effect of the black hole’s tidal forces.In addition to stripping material from the star, they exert a torque that causes the star to spin faster with each close encounter. As a result, although less material falls back toward the black hole, it returns over a shorter period of time, helping to keep the predicted flare at roughly the same brightness.

To reproduce the dimming astronomers were actually observing, the researchers needed what Bandopadhyay called “a new ingredient”—a star that was already spinning rapidly before its first encounter with the black hole.

The new study found that this initial rotation prevents the star from being significantly spun up during each passage. Without that additional spin-up, the timescale over which the stripped material falls back remains relatively constant. As the star loses less material with each encounter, the peak fallback rate—and the predicted brightness of the flare—can finally decline.

Tracing the star’s past

But why would the star already be spinning so rapidly?

“It is also extremely difficult to ‘bind’ a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs,” Coughlin says.

The so-called Hills mechanism may explain both. Under this scenario, a pair of closely orbiting stars passes near a supermassive black hole, which tears the binary apart, ejecting one star and capturing the other.

In a close binary, the stars can become tidally locked, causing each to rotate on its axis at the same rate that the pair orbits each other. The tighter the binary, the shorter that orbital period and the faster a tidally locked star spins. The binaries capable of leaving a captured star on the short orbit observed in rpTDEs would have to be extremely tight—also leaving a tidally locked star spinning rapidly before its capture.

“Ananya’s work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars,” Coughlin says. “From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems.”

Zooming out, Coughlin notes that Hills capture may also have produced some of the stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. The new findings could therefore help explain some of the properties of stars in what he calls “our own cosmological backyard.”

 

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Astronomers at MIT and elsewhere have spotted an extremely bright red spot in the early universe. The object resembles an enormous star, spanning the size of our solar system. But it also is putting out 100 billion times more energy than any known star can physically produce. In fact, such energies are closer to what a black hole might generate. The curious combination suggests that the red spot is an entirely new type of astrophysical source. The astronomers are calling it a “black hole star.”

The team discovered the new object using NASA’s James Webb Space Telescope (JWST). The telescope spotted the bright red dot in the very early universe, just a few hundred million years after the Big Bang. The scientists conclude ¹ that the most likely explanation for the strange red dot is that it is a mashup of a black hole and a star — a combination that has never been observed until now. The object is likely a hugely dense cloud of gas, powered not by standard nuclear fusion, but by a central black hole.

“Our picture of this object is evolving very rapidly,” says lead author Rohan Naidu, a NASA Hubble Fellow and Pappalardo Fellow at MIT’s Kavli Institute for Astrophysics and Space Research (MKI). “We think there is a central black hole that is 100,000 times as massive as the sun. And around this black hole, there would be this very extended envelope of gas that looks like a star the size of the solar system. It’s huge.”

If the bright red dot is indeed a black hole star, it would help to solve the identity of other mysterious “little red dots” that have appeared in nearly every deep space image JWST has taken to date.

“These little red dots seem to be everywhere in the early universe but essentially disappear by the present day,” Naidu says. “What exactly these objects are has been one of the most debated topics of the JWST era.”

The study’s MIT co-authors are MKI Director Robert Simcoe, the Bruno B. Rossi Professor of Experimental Physics; and Wendy Sun ’26, along with collaborators from multiple other institutions.

A singular source

Naidu and his colleagues didn’t intend to find a black hole star. They were looking for the most distant, earliest galaxies, as part of a survey that they named “Mirage or Miracle” (MoM). The team used the JWST to look into deep space, back when the universe was a few hundred million years old. Their goal was to look for galaxies that actually formed at those early times.

“There’s been this puzzle of many bright galaxies showing up at extremely early times,” Naidu says. “What we found was that what looks like an extremely bright early galaxy, aka a ‘miracle,’ in some cases actually could be a ‘mirage.’”

As they looked through JWST’s images for intriguing sources to target with their survey, they noticed a feature that stood out from the rest: a dot that was very red, and very bright.

“When we see something very red in the universe, we often assume that it is surrounded by dust, like soot or ash,” Simcoe explains. “The same way that the wildfire smoke from Canada recently made the sky in Boston look bright red, astronomical objects can also appear redder than their intrinsic color when you see them through a veil of dust.”

But there were other signatures in the light that didn’t quite match up with what physicists expect from dust. The team also observed another strange pattern: The dot’s light was extremely bright, except below certain wavelengths, where the light completely disappeared.

This spectral drop-off is known as a “Balmer break” — a signature traditionally associated with dense gas soaking up photons in the atmospheres of stars that are a few hundred millions of years old. Vega, one of the brightest stars in the night sky shows exactly this pattern.

“The break we observed in this object is the deepest break we have ever observed in any object, ruling out ‘ordinary’ stars as the source,” Naidu says. “But it made us wonder if we were seeing a new kind of ‘stellar atmosphere,’ but on a spectacular scale.”

What’s more, the red dot’s light contained almost no signature of metals or any elements other than hydrogen and helium. “It was truly singular in so many ways,” Naidu says.

Pure light

To puzzle out what the source of the red dot could be, the team ran simulations of different scenarios to see what combination of astrophysical features could produce the red dot’s distinctive color.

“We started to ask: Could you make something that red using just hydrogen, without any dust?” Simcoe says. “To our surprise, it turns out you can, if you have an extremely dense screen of hydrogen, so dense that it looks more like the surface of an enormous star than a wispy interstellar nebula.”

Their simulations pointed to the red dot possibly being some powerful enshrouded energy source, surrounded by an extremely dense cocoon of hydrogen. If this were the case, it would explain the light-blocking Balmer break and the lack of anything other than hydrogen and helium that the astronomers observed. But it still wouldn’t explain the object’s extreme brightness.

“You have something that looks a bit like a star but is 100 billion times brighter,” Naidu says. “That means you can’t be powering this by nuclear fusion, which is the energy source that sits at the heart of all the stars we have.”

Black holes, however, routinely produce energy at the scales the team observed. Naidu and his colleagues incorporated an active, accreting black hole into their simulations of the hydrogen-cocooned star and varied the black hole’s mass, along with other parameters. They then compared the resulting brightness of the simulated “black hole star” with the brightness that JWST observed from the red dot.

From these simulations, they found the closest match, and concluded that the most likely scenario to explain the red dot, is a black hole star. Specifically, the object likely contains a central black hole that is about 100,000 times as massive as the sun. This powerful core is surrounded by a dense, star-like cocoon of hydrogen that is roughly the size of the solar system.

The team has named the object MoM-BH*-1, after the survey that detected it, as well as the moniker “black hole star – one,” which implies that the object is the first of others. The researchers suspect that black hole stars could explain many of the other little red dots that appear in JWST images. Those objects are not as bright as MoM-BH*-1.

“Every little red dot is consistent with being a black hole star, embedded in a generic early galaxy,” Naidu says. “But what is special about MoM-BH*-1 is, the black hole star is essentially completely outshining its surrounding host galaxy, such that we’re seeing pure black hole star light.”

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