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Space & Physics

MIT Physicists uncover key Mechanism behind fractional charge in Graphene

In the decades-long history of studying these phenomena, no one has observed a system that naturally leads to such fractional electron effects, according to the researchers.

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MIT physicists have made a significant breakthrough in understanding the phenomenon where electrons split into fractions of their usual charge, offering new insights into the behaviour of exotic electronic states in graphene and other two-dimensional materials.

This latest research builds on a discovery earlier this year, when a team led by Assistant Professor Long Ju at MIT reported that electrons in pentalayer graphene—a structure composed of five graphene layers stacked on top of boron nitride—exhibited fractional charge. Remarkably, this behaviour was observed without the application of a magnetic field, challenging prior assumptions.

Previously, scientists knew that under a strong magnetic field, electrons could split into fractions as part of the fractional quantum Hall effect. However, Ju’s findings marked the first time such fractional behaviour occurred in graphene without any magnetic influence, which led to the coining of the “fractional quantum anomalous Hall effect.” Since then, researchers have been eager to uncover how fractional charge could emerge in this unusual system.

MIT professor Senthil Todadri, who led the new study published in Physical Review Letters, offers a critical piece of the puzzle. Through detailed quantum mechanical calculations, Todadri and his team discovered that the electrons in pentalayer graphene form a crystal-like structure, which provides the ideal conditions for fractional electron behavior.

“This is a completely new mechanism,” said Todadri. “In the decades-long history of studying these phenomena, no one has observed a system that naturally leads to such fractional electron effects. It opens the door to all kinds of new experimental possibilities.”

The study, which includes contributions from Zhihuan Dong and former postdoc Adarsh Patri, is part of a wider body of research. Two other teams—one from Johns Hopkins University and another from Harvard University, UC Berkeley, and Lawrence Berkeley National Laboratory—have also reported similar findings in the same journal issue.

Building on “Twistronics” and the Magic-Angle Graphene Discovery

This research builds upon the work of MIT physicist Pablo Jarillo-Herrero and his team, who in 2018 were the first to demonstrate that twisting two sheets of graphene could give rise to novel electronic behaviors. This discovery of “magic-angle graphene” spurred a new field known as “twistronics,” focused on understanding how the interactions between twisted two-dimensional materials could lead to unusual quantum phenomena, such as superconductivity and insulating behavior.

“We quickly realized that these twisted systems could provide the right conditions for fractional electron phenomena to emerge,” said Todadri, who collaborated with Jarillo-Herrero on a 2018 study that theorized such systems might exhibit fractional charge without a magnetic field. “We saw these systems as ideal platforms to study these fractional effects.”

A Surprising Discovery and the New Crystal Model

In September 2023, Todadri received an unexpected call from Ju, who was eager to share data showing fractional charge behavior in pentalayer graphene. This discovery caught Todadri by surprise, as it did not align with his earlier predictions. In his 2018 paper, Todadri had theorized that fractional charge would emerge from a specific twisting of the electron wavefunction, and that this twisting would intensify as more graphene layers were added.

“Initially, we expected the wavefunction to wind five times in pentalayer graphene,” Todadri explained. “But Ju’s experiments showed that it only wound once. This raised a big question—how do we explain what we’re seeing?”

Uncovering the Electron “Crystal”

Todadri and his team revisited their hypothesis and discovered they had overlooked an important factor. The conventional approach in the field had been to treat electrons as independent entities and analyze their quantum properties. However, in the confined, two-dimensional space of pentalayer graphene, electrons are forced to interact with each other, behaving according to their quantum correlations in addition to their natural repulsion.

By incorporating these interelectron interactions into their model, the team was able to match their predictions with the experimental data Ju had obtained. This led them to a crucial realization: the moiré pattern formed by the stacked graphene layers induces a weak electrical potential that forces the electrons to interact and form a crystal-like structure. This electron “crystal” creates a complex pattern of quantum correlations, allowing for the formation of fractional charge.

“The crystal has a whole set of unique properties that differentiate it from ordinary crystals,” said Todadri. “This opens up many exciting avenues for future research. In the short term, our work provides a theoretical foundation for understanding the fractional electron observations in pentalayer graphene and predicting similar phenomena in other systems.”

This new insight paves the way for further exploration into how graphene and other two-dimensional materials might be used to engineer new electronic states, with potential applications in quantum computing and other advanced technologies.

Space & Physics

Joint NASA-ISRO radar satellite is the most powerful built to date

NISAR – a portmanteau for the NASA-ISRO synthetic aperture global radar earth observation satellite — will only be the latest collaboration between the two space agencies.

Karthik Vinod

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A concept art on NISAR | Photo Credit: NASA

On July 30th, NISAR  — the NASA-ISRO joint space mission — launched to space aboard the GSLV Mark II rocket from Sriharikota, Andhra Pradesh. The satellite, now safely tucked into a sun-synchronous orbit around earth, will enter a commissioning phase over the next three months, to deploy all its instruments.

Perched at an altitude of 750 km, the three ton satellite will complete an orbit around the earth every 12 days, while studying the planet’s diverse geology with unprecedented detail.

NISAR, a portmanteau for the NASA-ISRO synthetic aperture radar mission, marks the culmination of a decade-long effort to build the most powerful earth observation satellite to date.

The story began around 2007, when NASA actively began exploring an ambitious undertaking to build a satellite, which could map the earth and the whole ecosystem. On the agenda were investigations into studying climate change and its role in exacerbating extreme weather events. Greenland and Antarctica, whose disappearing ice sheets have been linked to the global average increase in sea-levels over the years, would be among the vulnerable hotspots to come under surveillance. 

Traditional remote sensing satellites are limited in their need for sunlit conditions and clear skies for imaging. Clouds are transparent to radio and microwaves unlike visible light. As such, a synthetic aperture radar can work across any weather, whether sunlit or not alike.

That said, SAR technology isn’t new. They have been around for about seventy years, since the first proof of principle was proven in the 1950s. In 1978, the US launched the first SAR-equipped earth observation satellite, Seasat, to monitor oceans. Neither Seasat or for that matter any SAR-based successors, could bear resolutions as high as 1 cm, or map terrain across a swath area as wide as about 240 km, as NISAR can.

NASA engaged in a cost-effective strategy, opening doors for international partners to pool resources, and co-develop the satellite and the scientific campaigns.

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(A) Melt pond in Greenland | Photo Credit: Michael Studinger (2008) (B) NASA administrator Charles Bolden and ISRO chairman K. Radhakrishnan sign documents, which included a charter on NISAR, in Toronto | Photo Credit: NASA (2014)

NASA and ISRO share expertise

NASA found an interested party in ISRO, which at the time was developing the Radar Imaging Satellite (RISAT), which had a smaller scope to study India’s geology. India, being especially vulnerable to floods, landslides and cyclones, couldn’t overlook the incentives an extra eye in the sky could provide.

NISAR can track and relay even the minutest of changes on the surface in near real-time. In principle, the satellite should detect a flooded area hidden from view to rescuers on-ground, or even traditional remote sensing satellites which use passive receivers. The satellite can serve a key role in an integrated multi-hazard early warning system.

In 2014, ISRO inked the NISAR agreement with NASA. The mission would only be their latest collaboration between the two space agencies. Previously, they had collaborated on 2008’s Chandrayaan-1. Back then, NASA’s Moon Mineralogy Mapper (M3) instrument and miniSAR radar onboard the Chandrayaan orbiter, led the famous detection of water ice on the moon. 

Although NISAR was originally slated for launch in 2020, innumerable delays followed as they sorted technical challenges, and the abrupt global lockdown amid COVID pandemic.

Upon project completion last year, NISAR had become the most expensive satellite built, with NASA and ISRO pouring some $1.5 billion into development. The costs were unevenly split between them; with NASA spending some $1.3 billion, and ISRO bearing a modest amount at $91 million.

But a white paper details ISRO had contributed an equal value in engineering various components, re-establishing parity. ISRO engineered the spacecraft body, readied tracking stations on-ground, and developed the short wavelength S-band radar. The S-band (at 12 cm) complements NASA’s longer wavelength L-band (24 cm) radar.

The L-band can track changes under thick foliage or leaves, under forests. It can even measure land deformation rates as tiny as 4 mm/year. While the L-band serves as NISAR’s primary means of acquiring radar data, ISRO’s S-band radar will help provide details that concern Indian earth scientists, monitoring coastal erosion for example. Both radars work in tandem with NASA-designed radar receiver and reflector – a 12-meter wide meshed net, resembling a canopy attached to the spacecraft body via a boom. 

Three months from now, once the commissioning phase is complete, NISAR will begin its observational runs, and beam radar data back to earth continuously. The National Remote Sensing Centre in Hyderabad, and Goddard Space Flight Centre in Maryland, will process the respective L & S-band data independently, and archive them online for the world to see, all in a matter of few hours.

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Space & Physics

New double-slit experiment proves Einstein’s predictions were off the mark

Results from an idealized version of the Young double-slit experiment has upheld key predictions from quantum theory.

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Two individual atoms suspended in a vacuum chamber are illuminated by a laser beam, serving as the two slits. Scattered light interference is captured by a highly sensitive camera shown as a screen. Credit: Courtesy of the researchers/MIT
  • MIT physicists perform the most idealized double-slit experiment to date, using individual atoms as slits.
  • Experiment confirms the quantum duality of light: light behaves as both a particle and a wave, but both behaviors can’t be observed simultaneously.
  • Findings disprove Albert Einstein’s century-old prediction regarding detecting a photon’s path alongside its wave nature.

In a study published in Physical Reviews Letters on July 22, researchers at MIT have realized an idealized version of the famous double-slit experiment in quantum physics yet.

The double-slit experiment—first devised in 1801 by the British physicist Thomas Young—remains a perplexing aspect of reality. Light waves passing through two slits, form interference patterns on a wall placed behind. But this phenomenon is at odds with the fact light also behaves as particles. The contradiction has lent itself to a paradox, which sits at the foundation of quantum mechanics. It has sparked a historic scientific duel nearly a century ago, between physics heavyweights Albert Einstein and Niels Bohr. The study’s findings have now settled the decades-old debate, showing Einstein’s predictions were off the mark.

Einstein had suggested that by detecting the force exerted when a photon passes through a slit—a nudge akin to a bird brushing past a leaf—scientists could witness both light’s wave and particle properties at once. Bohr countered with the argument that observing a photon’s path would inevitably erase its wave-like interference pattern, a tenet since embraced by quantum theory.

The MIT team stripped the experiment to its purest quantum elements. Using arrays of ultracold atoms as their slits and weak light beams to ensure only one photon scattered per atom, they tuned the quantum states of each atom to control the information gained about a photon’s journey. Every increase in “which-path” information reduced the visibility of the light’s interference pattern, flawlessly matching quantum theory and further debunking Einstein’s proposal.

“Einstein and Bohr would have never thought that this is possible, to perform such an experiment with single atoms and single photons,” study senior author and Nobel laureate, Wolfgang Ketterle, stated in a press release. “What we have done is an idealized Gedanken (thought) experiment.”

In a particularly stunning twist, Ketterle’s group also disproved the necessity of a physical “spring”—a fixture in Einstein’s original analogy—by holding their atomic lattice not with springs, but with light. When they briefly released the atoms, effectively making the slits “float” in space, the same quantum results persisted. “In many descriptions, the springs play a major role. But we show, no, the springs do not matter here; what matters is only the fuzziness of the atoms,” commented MIT researcher Vitaly Fedoseev in a media statement. “Therefore, one has to use a more profound description, which uses quantum correlations between photons and atoms.”

The paper arrives as the world prepares for 2025’s International Year of Quantum Science and Technology — marking 100 years since the birth of quantum mechanics. Yoo Kyung Lee, a fellow co-author, noted in a media statement, “It’s a wonderful coincidence that we could help clarify this historic controversy in the same year we celebrate quantum physics.”

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Researchers Uncover New Way to Measure Hidden Quantum Interactions in Materials

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Image credit: Pixabay

A team of MIT scientists has developed a theory-guided strategy to directly measure an elusive quantum property in semiconductors — the electron-phonon interaction — using an often-ignored effect in neutron scattering.

Their approach, published this week in Materials Today Physics, reinterprets an interference effect, typically considered a nuisance in experiments, as a valuable signal. This enables researchers to probe electron-phonon interactions — a key factor influencing a material’s thermal, electrical, and optical behaviour — which until now have been extremely difficult to measure directly.

“Rather than discovering new spectroscopy techniques by pure accident, we can use theory to justify and inform the design of our experiments and our physical equipment,” said Mingda Li, senior author and associate professor at MIT, in a media statement.

By engineering the interference between nuclear and magnetic interactions during neutron scattering, the team demonstrated that the resulting signal is directly proportional to the electron-phonon coupling strength.

“Being able to directly measure the electron-phonon interaction opens the door to many new possibilities,” said MIT graduate student Artittaya Boonkird.

While the current setup produced a weak signal, the findings lay the groundwork for next-generation experiments at more powerful facilities like Oak Ridge National Laboratory’s proposed Second Target Station. The team sees this as a shift in materials science — using theoretical insights to unlock previously “invisible” properties for a range of advanced technologies, from quantum computing to medical devices.

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