Page 72 – Physics World

Home » Page 72

Squid use Bragg reflectors in their skin to change colour

Cephalopods such as squid and octopus can rapidly change the colour of their skin, but the way they do it has been something of a mystery – until now. Using a microscopy technique known as holotomography, scientists in the US discovered that the tuneable optical properties of squid skin stem from winding columns of platelets in certain cells. These columns have sinusoidal-wave refractive index profiles, and they function as Bragg reflectors, able to selectively transmit and reflect light at specific wavelengths.

“Our new result not only helps advance our understanding of structural colouration in cephalopods skin cells, it also provides new insights into how such gradient refractive index distributions can be leveraged to manipulate light in both biological and engineered systems,” says Alon Gorodetsky of the University of California, Irvine, who co-led this research study together with then-PhD student Georgii Bogdanov.

Stacked and winding columns of platelets

In their study, Gorodetsky, Bogdanov and colleagues including Roger Hanlon of the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, examined the iridescent cells (iridophores) and cell clusters (splotches) responsible for producing colours in longfin inshore squids (Doryteuthis pealeii). To do this, they used holotomography, which creates three-dimensional images of individual cells and cell clusters by measuring subtle changes in a light beam as it passes through a sample of tissue. From this, they were able to map out changes in the sample’s refractive index across different structures.

The holotomography images revealed that the iridophores comprise stacked and winding columns of platelets made from a protein known as reflectin, which has a high refractive index, alternating with a low-refractive-index extracellular space. These Bragg-reflector-like structures are what allow tissue in the squid’s mantle to switch from nearly transparent to vibrantly coloured and back again.

Other natural Bragg reflectors

Squids aren’t the only animals that use Bragg reflectors for structural colouration, Gorodetsky notes. The scales of Morpho butterflies, for example, get their distinctive blue colouration from nanostructured Bragg gratings made from alternating high-refractive-index lamellae and low-refractive-index air gaps. Another example is the panther chameleon. The skin cells of this famously colour-changing reptile contain reconfigurable photonic lattices consisting of high-refractive-index nanocrystals within a low-refractive-index cytoplasm. These structures allow the animal to regulate its temperature as well as change its colour.

Yet despite these previous findings, and extensive research on cephalopod colouration, Gorodetsky says the question of how squid splotch iridophores can change from transparent to colourful , while maintaining their spectral purity, had not previously been studied in such depth. “In particular, the cells’ morphologies and refractive index distributions in three dimensions had not been previously resolved,” he explains. “Overcoming the existing knowledge gap required the development and application of combined experimental and computational approaches, including advanced imaging, refractive index mapping and optical modelling.”

Extending to infrared wavelengths

After using advanced computational modelling to capture the optical properties of the squid cells, the researchers, who report their work in Science, built on this result by designing artificial nanomaterials inspired by the natural structures they discovered. While the squid iridophores only change their visible appearance in response to neurophysiological stimuli, the researchers’ elastomeric composite materials (which contain both nanocolumnar metal oxide Bragg reflectors and nanostructured metal films) also change at infrared wavelengths.

Hyperdisorder appears in pigment patterns on squid skin

An image of a squid showing black spots on its skin, paired with a zoomed-in photo of the same in which green dotted rings surround eacg dit

Composite materials like the ones the UC Irvine-MBL team developed could have applications in adaptive camouflage or fabrics that adjust to hot and cold temperatures. They might also be used to improve multispectral displays, sensors, lasers, fibre optics and photovoltaics, all of which exploit multilayered Bragg reflectors with sinusoidal-wave refractive index profiles, says Gorodetsky.

The researchers now plan to further explore how gradient refractive index distributions contribute to light manipulation in other biological systems. “We also hope to refine our engineered multispectral composite materials to enhance their performance for specific practical applications, such as advanced camouflage and other wearable optical technologies,” Gorodetsky tells Physics World.

From rabbits and foxes to the human gut microbiome, physics is helping us understand the natural world

This episode of the Physics World Weekly podcast is a conversation with two physicists, Ada Altieri and Silvia De Monte, who are using their expertise in statistical physics to understand the behaviour of ecological communities.

A century ago, pioneering scientists such as Alfred Lotka and Vito Volterra showed that statistical physics techniques could explain – and even predict – patterns that ecologists observe in nature. At first, this work focused on simple ecosystems containing just one or two species (such as rabbits and foxes), which are relatively easy to model.

Nowadays, though, researchers such as Altieri and De Monte are turning their attention to far more complex communities. One example is the collection of unicellular organisms known as protists that live among plankton in the ocean. Another, closer to home, is the “microbiome” in the human gut, which may contain hundreds or even thousands of species of bacteria.

Modelling these highly interconnected communities is hugely challenging. But as Altieri and De Monte explain, the potential rewards – from identifying “tipping points” in fragile ecosystems to developing new treatments for gut disorders such as irritable bowel syndrome and Crohn’s disease – are great.

This discussion is based on a Perspective article that Altieri (an associate professor at the Laboratory for Matter and Complex Systems at the Université Paris Cité, France) and De Monte (a senior research scientist at the Institute of Biology in the École Normale Supérieure in Paris and the Max Planck Institute for Evolutionary Biology in Ploen, Germany) wrote for the journal EPL, which sponsors this episode of the podcast.

Scientists decry ‘scientific injustice’ over lack of climate data in developing regions

A shortage of data is hampering efforts to establish the role of climate change in extreme-weather events in the tropics and global south. So say an international team of scientists, who claim the current situation is a “scientific injustice” and call for more investment in climate science and weather monitoring in poorer countries.

The researchers, who are part of World Weather Attribution, have made the call after analysing the role of climate change in an episode of torrential rain in June that triggered a landslide in Colombia. It killed 27 people and triggered devastating floods in Venezuela that displaced thousands.

Their study reported that the Colombian Andes were unusually wet from April to June, while the part of Venezuela where the floods occurred experienced its five wettest days of the year. In the current climate, such weather events would be expected every 10 years in Colombia and every three years in Venezuela.

According to the researchers, there is a high level of uncertainty in the study due to a lack of long-term observational data in the region and high uncertainties in global climate models when assessing the tropics. Colombia and Venezuela have complex tropical climates that are under-researched, with some data even suggesting that rainfall in the region is becoming less intense.

But the group says that the possibility of heavier rainfall linked to climate change should not be ruled out in the region, particularly on shorter, sub-daily timescales, which they could not investigate. They add that Colombia and Venezuela are almost certainly facing increased heatwave, drought and wildfire risk.

Mariam Zachariah at the Centre for Environmental Policy at Imperial College London, who was involved with the work, says that the combination of mountains, coasts, rainforests and complex-weather systems in many tropical countries means “rainfall is varied, intense and challenging to capture in climate models”.

“Many countries with tropical climates have limited capacity to do climate science, meaning we don’t have a good understanding of how they are being affected by climate change,” says Zachariah. “Our recent study on the deadly floods in the Democratic Republic of Congo in May is another example. Once again, our results were inconclusive.”

Climate scientist Paola Andrea Arias Gómez at the Universidad of Antioquia in Colombia, who was also involved in the study, says that extreme weather is “non-stop” in Colombia and Venezuela. “One year we face devastating flash floods; the next, severe droughts and wildfires,” she adds. “Unfortunately, extreme weather is not well understood in northern South America. We urgently need more investment in climate science to understand shifting risks and prepare for what’s ahead. More science will save lives.”

Hints of a 3D quantum spin liquid revealed by neutron scattering

New experimental evidence for a quantum spin liquid – a material with spins that remain in constant fluctuation at extremely low temperatures – has been unveiled by an international team of scientists. The researchers used neutron scattering to reveal photon-like collective spin excitations in a crystal of cerium zirconate.

When most magnetic materials are cooled to nearly absolute zero, their spin magnetic moments will align into an ordered pattern to minimize the system’s energy. Yet in 1973, the future Nobel laureate Philip Anderson proposed an alternative class of magnetic materials in which this low temperature order does not emerge.

Anderson considered the spins of atoms that interact with each other in an antiferromagnetic way. This is when the spin of each atom seeks to point in the opposite direction of its nearest neighbours. If the spins in a lattice are able to adopt this orientation, the lowest energy state is an ordered antiferromagnet with zero overall magnetism.

Geometrical frustration

In a tetrahedral lattice, however, the geometrical arrangement of nearest neighbours means that it is impossible for spins to arrange themselves in this way. This is called frustration, and the result is a material with multiple low-energy spin configurations, which are disordered.

So far, this behaviour has been observed in materials called spin ices – where one of the many possible spin configurations is frozen into place at ultralow temperatures. However, Anderson envisioned that a related class of materials could exist in a more exotic phase that constantly fluctuates between different, equal-energy states, all the way down to absolute zero.

Called quantum spin liquids (QSLs), such materials have evaded experimental confirmation – until now. “They behave like a liquid form of magnetism – without any fixed ordering,” explains team member Silke Bühler-Paschen at Austria’s Vienna University of Technology. “That’s exactly why a real breakthrough in this area has remained elusive for decades.” “We studied cerium zirconate, which forms a three-dimensional network of spins and shows no magnetic ordering, even at temperatures as low as 20 mK.”. This material was chosen because it has a pyrochlore lattice, which is based on corner-sharing tetrahedra.

Collective magnetic excitations

The team looked for collective magnetic excitations that are predicted to exist in QSLs. These excitations are expected to have linear energy–momentum relationships, which is similar to how conventional photons propagate. As a result, these particle-like excitations are called emergent photons.

The team used polarized neutron scattering experiments to search for evidence of emergent photons. When neutrons strike a sample, they can exchange energy and momentum with the lattice. This exchange can involve magnetic excitations in the material and the team used scattering experiments to map-out the energy and momenta of these excitations at temperatures in the 33–50 mK range.

“For the first time, we were able to detect signals that strongly indicate a 3D quantum spin liquid – particularly, the presence of so-called emergent photons,” Bühler-Paschen says. “The discovery of these emergent photons in cerium zirconate is a very strong indication that we have indeed found a QSL.”

As well as providing evidence for Anderson’s idea, the research pave the way for the further exploration of other potential QSLs and their applications. “We plan to conduct further experiments, but from our perspective, cerium zirconate is currently the most convincing candidate for a quantum spin liquid,” Bühler-Paschen says.

The research could have important implications for our understanding of high-temperature superconductivity. In his initial theory, Anderson predicted that QSLs could be precursors to high-temperature superconductors.

The research is described in Nature Physics.

Earth-shaking waves from Greenland mega-tsunamis imaged for the first time

In September 2023, seismic detectors around the world began picking up a mysterious signal. Something – it wasn’t clear what – was causing the entire Earth to shake every 90 seconds. After a period of puzzlement, and a second, similar signal in October, theoretical studies proposed an explanation. The tremors, these studies suggested, were caused by standing waves, or seiches, that formed after landslides triggered huge tsunamis in a narrow waterway off the coast of Greenland.

Engineers at the University of Oxford, UK, have now confirmed this hypothesis. Using satellite altimetry data from the Surface Water Ocean Topography (SWOT) mission, the team constructed the first images of the seiches, demonstrating that they did indeed originate from landslide-triggered mega-tsunamis in Dickson Fjord, Greenland. While events of this magnitude are rare, the team say that climate change is likely to increase their frequency, making continued investments in advanced satellite missions essential for monitoring and responding to them.

An unprecedented view into the fjord

Unlike other altimeters, SWOT provides two-dimensional measurements of sea surface height down to the centimetre across the entire globe, including hard-to-reach areas like fjords, rivers and estuaries. For team co-leader Thomas Monahan, who studied the seiches as part of his PhD research at Oxford, this capability was crucial. “It gave us an unprecedented view into Dickson Fjord during the seiche events in September and October 2023,” he says. “By capturing such high-resolution images of sea-surface height at different time points following the two tsunamis, we could estimate how the water surface tilted during the wave – in other words, gauge the ‘slope’ of the seiche.”

The maps revealed clear cross-channel slopes with height differences of up to two metres. Importantly, these slopes pointed in opposite directions, showing that water was moving backwards as well as forwards across the channel. But that wasn’t the end of the investigation. “Finding the ‘seiche in the fjord’ was exciting but it turned out to be the easy part,” Monahan says. “The real challenge was then proving that what we had observed was indeed a seiche and not something else.”

Enough to shake the Earth for days

To do this, the Oxford engineers approached the problem like a game of Cluedo, ruling out other oceanographic “suspects” one by one. They also connected the slope measurements with ground-based seismic data that captured how the Earth’s crust moved as the wave passed through it. “By combining these two very different kinds of observations, we were able to estimate the size of the seiches and their characteristics even during periods in which the satellite was not overhead,” Monahan says.

Although no-one was present in Dickson Fjord during the seiches, the Oxford team’s estimates suggest that the event would have been terrifying to witness. Based on probabilistic (Bayesian) machine-learning analyses, the team say that the September seiche was initially 7.9 m tall, while the October one measured about 3.9 m.

“That amount of water sloshing back and forth over a 10-km-section of fjord walls creates an enormous force,” Monahan says. The September seiche, he adds, produced a force equivalent to 14 Saturn V rockets launching at once, around 500 GN. “[It] was literally enough to shake the entire Earth for days,” he says.

What made these events so powerful was the geometry of the fjord, Monahan says. “A sharp bend near its outlet effectively trapped the seiches, allowing them to reverberate for days,” he explains. “Indeed, the repeated impacts of water against fjord walls acted like a hammer striking the Earth’s crust, creating long-period seismic waves that propagated around the globe and that were strong enough to be detected worldwide.”

Risk of tsunamigenic landslides will likely grow

As for what caused the seiches, Monahan suggests that climate change may have been a contributing factor. As glaciers thin, they undergo a process called de-buttressing wherein the loss of ice removes support from the surrounding rock, leading it to collapse. It was likely this de-buttressing that caused two enormous landslides in Dickson Fjord within a month, and continued global warming will only increase the frequency. “As these events become more common, especially in steep, ice-covered terrain, the risk of tsunamigenic landslides will likely grow,” Monahan says.

The researchers say they would now like to better understand how the seiches dissipated afterwards. “Although previous work successfully simulated how the megatsunamis stabilized into seiches, how they decayed is not well understood,” says Monahan. “Future research could make use of SWOT satellite observations as a benchmark to better constrain the processes behind disputation.”

The findings, which are detailed in Nature Communications, show how top-of-the-line satellites like SWOT can fill these observational gaps, he adds. To fully leverage these capabilities, however, researchers need better processing algorithms tailored to complex fjord environments and new techniques for detecting and interpreting anomalous signals within these vast datasets. “We think scientific machine learning will be extremely useful here,” he says.

Magnetically controlled microrobots show promise for precision drug delivery

Permanent magnetic droplet-derived microrobots — **Multimodal locomotion** Top panel: fabrication and magnetic assembly of permanent magnetic droplet-derived microrobots (PMDMs). Lower panel: magnetic fields direct PMDM chains through complex biological environments such as the intestine. (Courtesy: CC BY 4.0/*Sci. Adv.* 10.1126/sciadv.adw3172)

Microrobots provide a promising vehicle for precision delivery of therapeutics into the body. But there’s a fine balance needed between optimizing multifunctional cargo loading and maintaining efficient locomotion. A research collaboration headed up at the University of Oxford and the University of Michigan has now developed permanent magnetic droplet-derived microrobots (PMDMs) that meet both of these requirements.

The PMDMs are made from a biocompatible hydrogel incorporating permanent magnetic microparticles. The hydrogel – which can be tailored to each clinical scenario – can carry drugs or therapeutic cells, while the particles’ magnetic properties enable them to self-assemble into chains and perform a range of locomotion behaviours under external magnetic control.

“Our motivation was to design a microrobot system with adaptable motion capabilities for potential applications in drug delivery,” explains Molly Stevens from the University of Oxford, experimental lead on this study. “By using self-assembled magnetic particles, we were able to create reconfigurable, modular microrobots that could adapt their shape on demand – allowing them to manoeuvre through complex biological terrains to deliver therapeutic payloads.”

Building the microrobots

To create the PMDMs, Stevens and collaborators used cascade tubing microfluidics to rapidly generate ferromagnetic droplets (around 300 per minute) from the hydrogel and microparticles. Gravitational sedimentation of the 5 µm-diameter microparticles led to the formation of Janus droplets with distinct hydrogel and magnetic phases. The droplets were then polymerized and magnetized to form PMDMs of roughly 0.5 mm in diameter.

The next step involved self-assembly of the PMDMs into chains. The researchers demonstrated that exposure to a precessing magnetic field caused the microrobots to rapidly assemble into dimers and trimers before forming a chain of eight, with their dipole moments aligned. Exposure to various dynamic magnetic fields caused the chains to move via different modalities, including walking, crawling, swinging and lateral movement.

The microrobots were able to ascend and descend stairs, and navigate obstacles including a 3-mm high railing, a 3-mm diameter cylinder and a column array. The reconfigurable PMDM chains could also adapt to confined narrow spaces by disassembling into shorter fragments and overcome tall obstacles by merging into longer chains.

Towards biomedical applications

By tailoring the hydrogel composition, the researchers showed that the microrobots could deliver different types of cargo with controlled dosage. PMDMs made from rigid polyethylene glycol diacrylate (PEGDA) could deliver fluorescent microspheres, for example, while soft alginate/gelatin hydrogels can be used for cell delivery.

PMDM chains also successfully transported human mesenchymal stem cell (hMSC)-laden Matrigel without compromising cell viability, highlighting their potential to deliver cells to specific sites for in vivo disease treatment.

To evaluate intestinal targeting, the researchers delivered PMDMs to ex vivo porcine intestine. Once inside, the microrobots assembled into chains and exhibited effective locomotion on the intestine surface. Importantly, the viscous and unstructured tissue surface did not affect chain assembly or motion. After navigation to the target site, exposing the PMDMs to the enzyme collagenase instigated controlled cargo release. Even after full degradation of the hydrogel phase, the chains retained integrity and locomotion capabilities.

The team also demonstrated programmable release of different cargoes, using hybrid chains containing rigid PEGDA segments and degradable alginate/gelatin segments. Upon exposure to collagenase, the cargo from the degradable domains exhibited burst release, while the slower degradation of PEGDA delayed the release of cargo in the PEGDA segments.

Delivery of microrobots into a human cartilage model — **Biological environment** Delivery of preassembled PMDM chains into a printed human cartilage model. The procedure consists of injections and assembly, locomotion, drug release and retrieval of PMDMs. Scale bars: 5 mm. (Courtesy: CC BY 4.0/*Sci. Adv.* 10.1126/sciadv.adw3172)

In another potential clinical application, the researchers delivered microrobots to 3D-printed human cartilage with an injury site. This involved catheter-based injection of PMDMs followed by application of an oscillating magnetic field to assemble the PMDMs into a chain. The chains could be navigated by external magnetic fields to the targeted injury site, where the hydrogel degraded and released the drug cargo.

After drug delivery, the team guided the microrobots back to the initial injection site and retrieved them using a magnetic catheter. This feature offers a major advantage over traditional microrobots, which often struggle to retrieve magnetic particles after cargo release, potentially triggering immune responses, tissue damage or other side effects.

“For microrobots to be clinically viable, they must not only perform their intended functions effectively but also do so safely,” explains co-first author Yuanxiong Cao from the University of Oxford. “The ability to retrieve the PMDM chains after they completed the intended therapeutic delivery enhances the biosafety of the system.”

Cao adds that while the focus for the intestine model was to demonstrate navigation and localized delivery, the precise control achieved over the microrobots suggests that “extraction is also feasible in this and other biomedically relevant environments”.

Predicting PMDM performance

Alongside the experiments, the team developed a computational platform, built using molecular dynamics simulations, to provide further insight into the collective behaviour of the PMDMs.

“The computational model was instrumental in predicting how individual microrobot units would self-assemble and respond to dynamic magnetic fields,” says Philipp Schoenhoefer, co-first author from the University of Michigan. “This allowed us to understand and optimize the magnetic interactions between the particles and anticipate how the robots would behave under specific actuation protocols.”

The researchers are now using these simulations to design more advanced microrobot structures with enhanced multifunctionality and mechanical resilience. “The next-generation designs aim to handle the more challenging in vivo conditions, such as high fluid shear and irregular tissue architectures,” Sharon Glotzer from the University of Michigan, simulation lead for the project, tells Physics World.

The microrobots are described in Science Advances.

Entangled expressions: where quantum science and art come together

What happens when you put a visual artist in the middle of a quantum physics lab? This month’s Physics World Stories podcast explores that very question, as host Andrew Glester dives into the artist-in-residence programme at the Yale Quantum Institute in the US.

Serena Scapagnini, 2025. (Credit: Filippo Silvestris)

Each year, the institute welcomes an artist to explore the intersections of art and quantum science, bridging the ever-fuzzy boundary between the humanities and the sciences. You will hear from the current artist-in-residence Serena Scapagnini, a visual artist and art historian from Italy. At Yale, she’s exploring the nature of memory, both human and quantum, through her multidisciplinary projects.

You’ll also hear from Florian Carle, managing director of the institute and the co-ordinator of the residency. Once a rocket scientist, Carle has always held a love of theatre and the arts alongside his scientific work. He believes art–science collaborations open new possibilities for engaging with quantum ideas, and that includes music – which you’ll hear in the episode.

Discover more about quantum art and science in the free-to-read Physics World Quantum Briefing 2025

This article forms part of Physics World‘s contribution to the 2025 International Year of Quantum Science and Technology (IYQ), which aims to raise global awareness of quantum physics and its applications.

Stayed tuned to Physics World and our international partners throughout the year for more coverage of the IYQ.

Find out more on our quantum channel.

Exographer: a scientific odyssey in pixel form

In an era where video games often prioritize fast-paced action and instant gratification, Exographer offers a refreshing change. With a contemplative journey that intertwines the realms of particle physics and interactive storytelling, this beautifully pixelated game invites players to explore a decaying alien civilization through the lens of scientific discovery while challenging them with dexterity and intellect.

Exographer was developed by particle physicist and science-fiction author Raphaël Granier de Cassagnac and his video-game studio SciFunGames. At its core, it is a puzzle-platformer – where the player’s character has to move around an environment using platforms while solving puzzles. The character in question is Ini, an alien explorer who discovers a multifunctional camera in the opening scenes of the game’s narrative. Stranded on a seemingly deserted planet, Ini is tasked with unlocking the mystery of the world’s fallen civilization.

The camera quickly becomes central to gameplay, allowing for environmental analysis, teleportation to previously visited locations and, most intriguingly, the discovery of subatomic particles through puzzles inspired by Feynman diagrams. These challenges require players to match particle trajectories using various analytical tools, mirroring the investigative processes of real-world physicists.

It is in these games where the particle physics really shines through. Beamlines have to be tracked and redirected to unveil greater understanding of the particles that make up this strange land and, with that, Ini’s abilities to understand the world.

As you crack one puzzle, a door opens and off you pootle to another blockage or locked door. Players will doubtless, as I did, find themselves wandering around areas pondering how to unlock it. A tip for those a little stuck: use the camera wherever a background seems a little different. In most circumstances, clues and cues will be waiting there.

Pixels and particles

The game’s environments are meticulously crafted, drawing inspiration from actual laboratories and observatories. I played the game on Nintendo Switch, but it is also available on several other platforms – including PS5, Xbox and Steam – and it looks pretty much identical on each. The pixel art style is not merely a visual choice but a thematic one, symbolizing the fundamental “pixels” of the universe of elementary particles. As players delve deeper, they encounter representations of particles including electrons, gluons and muons, each unlocking new abilities that alter gameplay and exploration.

Meanwhile, the character of Ini moves in a smooth and – for those gamers among us with a love of physics – realistic way. There is even a hint of lighter gravity as you hold down the button to activate a longer jump.

Computer game pixel art representation of an underwater neutrino observatory — **Game with depth** An undersea puzzle in *Exographer* features a Km3Net-inspired neutrino observatory. (Courtesy: SciFunGames)

What sets Exographer apart is its ability to educate without compromising entertainment. The integration of scientific concepts is seamless, offering players a glimpse into the world of particle physics without overwhelming them. However, it’s worth noting that some puzzles may present a steep learning curve, potentially posing challenges for those less familiar with scientific reasoning.

Complementing the game’s visual and intellectual appeal is its atmospheric soundtrack, composed by Yann Van Der Cruyssen, known for his work on the game Stray. As with Stray – where you take the role of a stray cat with a backpack – the music enhances the sense of wonder and discovery, underscoring the game’s themes of exploration and scientific inquiry.

Exographer is more than just a game; it’s an experience that bridges the gap between science and (pixelated) art. It challenges players to think critically, to explore patiently, and to appreciate the intricate beauty of the universe’s building blocks. For those willing to engage with its depth, Exographer offers a rewarding journey that lingers after the console is turned off.

2024 SciFunGames and Abylight Studios Nintendo Switch £17.99; PS5 £15.99; Xbox £16.74; PC £16.75

Scientists image excitons in carbon nanotubes for the first time

Researchers in Japan have directly visualized the formation and evolution of quasiparticles known as excitons in carbon nanotubes for the first time. The work could aid the development of nanotube-based nanoelectronic and nanophotonic devices.

Carbon nanotubes (CNTs) are rolled-up hexagonal lattices of carbon just one atom thick. When exposed to light, they generate excitons, which are bound pairs of negatively-charged electrons and positively-charged “holes”. The behaviour of these excitons governs processes such as light absorption, emission and charge carrier transport that are crucial for CNT-based devices. However, because excitons are confined to extremely small regions in space and exist for only tens of femtoseconds (fs) before annihilating, they are very difficult to observe directly with conventional imaging techniques.

Ultrafast and highly sensitive

In the new work, a team led by Jun Nishida and Takashi Kumagai at the Institute for Molecular Science (IMS)/SOKENDAI, together with colleagues at the University of Tokyo and RIKEN, developed a technique for imaging excitons in CNTs. Known as ultrafast infrared scattering-type scanning near-field optical microscopy (IR s-SNOM), it first illuminates the CNTs with a short visible laser pulse to create excitons and then uses a time-delayed mid-infrared pulse to probe how these excitons behave.

“By scanning a sharp gold-coated atomic force microscope (AFM) tip across the surface and detecting the scattered infrared signal with high sensitivity, we can measure local changes in the optical response of the CNTs with 130-nm spatial resolution and around 150-fs precision,” explains Kumagai. “These changes correspond to where and how excitons are formed and annihilated.”

According to the researchers, the main challenge was to develop a measurement that was ultrafast and highly sensitive while also having a spatial resolution high enough to detect a signal from as few as around 10 excitons. “This required not only technical innovations in the pump-probe scheme in IR s-SNOM, but also a theoretical framework to interpret the near-field response from such small systems,” Kumagai says.

The measurements reveal that local strain and interactions between CNTs (especially in complex, bundled nanotube structures) govern how excitons are created and annihilated. Being able to visualize this behaviour in real time and real space makes the new technique a “powerful platform” for investigating ultrafast quantum dynamics at the nanoscale, Kumagai says. It also has applications in device engineering: “The ability to map where excitons are created and how they move and decay in real devices could lead to better design of CNT-based photonic and electronic systems, such as quantum light sources, photodetectors, or energy-harvesting materials,” Kumagai tells Physics World.

Extending to other low-dimensional systems

Kumagai thinks the team’s approach could be extended to other low-dimensional systems, enabling insights into local dynamics that have previously been inaccessible. Indeed, the researchers now plan to apply their technique to other 1D and 2D materials (such as semiconducting nanowires or transition metal dichalcogenides) and to explore how external stimuli like strain, doping, or electric fields affect local exciton dynamics.

“We are also working on enhancing the spatial resolution and sensitivity further, possibly toward single-exciton detection,” Kumagai says. “Ultimately, we aim to combine this capability with in operando device measurements to directly observe nanoscale exciton behaviour under realistic operating conditions.”

The technique is detailed in Science Advances.

A new path to robust edge states using heat and disorder

Topological insulators are materials that behave as insulators in their interior but support the flow of electrons along their edges or surfaces. These edge states are protected against weak disorder, such as impurities, but can be disrupted by strong disorder. Recently, researchers have explored a new class of materials known as topological Anderson insulators. In these systems, strong disorder leads to Anderson localization, which prevents wave propagation in the bulk while still allowing robust edge conduction.

The Fermi energy is the highest energy an electron can have in a material at absolute zero temperature. If the Fermi energy lies in a conductive region, the material will conduct; if it lies in a ‘gap’, the material will be insulating. In a conventional topological insulator, the Fermi energy sits within the band gap. In topological Anderson insulators, it sits within the mobility gap rather than the conventional band gap, making the edge states highly stable. Electrons can exist in the mobility gap (unlike in the band gap), but they are localized and trapped. Until now, the transition from a topological insulator to a topological Anderson insulator has only been achieved through structural modifications, which limits the ability to tune the material’s properties.

In this study, the authors present both theoretical and experimental evidence that this phase transition can be induced by applying heat. Heating introduces energy exchange with the environment, making the system non-Hermitian. This approach provides a new way to control the topological state of a material without altering its structure. Further heating prompts a second phase transition, from a topological Anderson insulator to an Anderson insulator, where all electronic states become localized, and the material becomes fully insulating with no edge conduction.

This research deepens our understanding of how disorder influences topological phases and introduces a novel method for engineering and tuning these phases using thermal effects. It also provides a powerful tool for modulating electron conductivity through a simple, non-invasive technique.

Read the full article

Topological Anderson phases in heat transport

He Gao et al 2024 Rep. Prog. Phys. 87 090501

Do you want to learn more about this topic?

Interacting topological insulators: a review by Stephan Rachel (2018)