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Magnets and superconductors go together like oil and water—or so scientists have thought. But a new finding by MIT physicists is challenging this century-old assumption.

In a paper appearing in the journal Nature, the physicists report that they have discovered a “chiral superconductor”—a material that conducts electricity without resistance, and also, paradoxically, is intrinsically magnetic. What’s more, they observed this exotic superconductivity in a surprisingly ordinary material: graphite, the primary material in pencil lead.

Graphite is made from many layers of graphene—atomically thin, lattice-like sheets of carbon atoms—that are stacked together and can easily flake off when pressure is applied, as when pressing down to write on a piece of paper. A single flake of graphite can contain several million sheets of graphene, which are normally stacked such that every other layer aligns. But every so often, graphite contains tiny pockets where graphene is stacked in a different pattern, resembling a staircase of offset layers.

The MIT team has found that when four or five sheets of graphene are stacked in this “rhombohedral” configuration, the resulting structure can exhibit exceptional electronic properties that are not seen in graphite as a whole.

In their new study, the physicists isolated microscopic flakes of rhombohedral graphene from graphite, and subjected the flakes to a battery of electrical tests. They found that when the flakes are cooled to 300 millikelvins (about -273 degrees Celsius), the material turns into a superconductor, meaning that any electrical current passing through the material can flow through without resistance.

They also found that when they swept an external magnetic field up and down, the flakes could be switched between two different superconducting states, just like a magnet. This suggests that the superconductor has some internal, intrinsic magnetism. Such switching behavior is absent in other superconductors.

“The general lore is that superconductors do not like magnetic fields,” says Long Ju, assistant professor of physics at MIT. “But we believe this is the first observation of a superconductor that behaves as a magnet with such direct and simple evidence. And that’s quite a bizarre thing because it is against people’s general impression of superconductivity and magnetism.”

Ju is senior author of the study, which includes MIT co-authors Tonghang Han, Zhengguang Lu, Zach Hadjri, Lihan Shi, Zhenghan Wu, Wei Xu, Yuxuan Yao, Jixiang Yang, Junseok Seo, Shenyong Ye, Muyang Zhou, and Liang Fu, along with collaborators from Florida State University, the University of Basel in Switzerland, and the National Institute for Materials Science in Japan.

Graphene twist
In everyday conductive materials, electrons flow through in a chaotic scramble, whizzing by each other, and pinging off the material’s atomic latticework. Each time an electron scatters off an atom, it has—in essence—met some resistance, and loses some energy as a result, normally in the form of heat. In contrast, when certain materials are cooled to ultracold temperatures, they can become superconducting, meaning that the material can allow electrons to pair up, in what physicists term “Cooper pairs.”

Rather than scattering away, these electron pairs glide through a material without resistance. With a superconductor, then, no energy is lost in translation.

Since superconductivity was first observed in 1911, physicists have shown many times that zero electrical resistance is a hallmark of a superconductor. Another defining property was first observed in 1933, when the physicist Walther Meissner discovered that a superconductor will expel an external magnetic field. This “Meissner effect” is due in part to a superconductor’s electron pairs, which collectively act to push away any magnetic field.

Physicists have assumed that all superconducting materials should exhibit both zero electrical resistance, and a natural magnetic repulsion. Indeed, these two properties are what could enable magnetic levitation (Maglev) trains, whereby a superconducting rail repels and therefore levitates a magnetized car.

Ju and his colleagues had no reason to question this assumption as they carried out their experiments at MIT. In the last few years, the team has been exploring the electrical properties of pentalayer rhombohedral graphene. The researchers have observed surprising properties in the five-layer, staircase-like graphene structure; most recently, that it enables electrons to split into fractions of themselves. This phenomenon occurs when the pentalayer structure is placed atop a sheet of hexagonal boron nitride (a material similar to graphene), and slightly offset by a specific angle, or twist.

Curious as to how electron fractions might change with changing conditions, the researchers followed up their initial discovery with similar tests, this time by misaligning the graphene and hexagonal boron nitride structures. To their surprise, they found that when they misaligned the two materials and sent an electrical current through, at temperatures less than 300 millikelvins, they measured zero resistance. It seemed that the phenomenon of electron fractions disappeared, and what emerged instead was superconductivity.

The researchers went a step further to see how this new superconducting state would respond to an external magnetic field. They applied a magnet to the material, along with a voltage, and measured the electrical current coming out of the material. As they dialed the magnetic field from negative to positive (similar to a north and south polarity) and back again, they observed that the material maintained its superconducting, zero-resistance state, except in two instances, once at either magnetic polarity.

In these instances, the resistance briefly spiked, before switching back to zero, and returning to a superconducting state.

“If this were a conventional superconductor, it would just remain at zero resistance, until the magnetic field reaches a critical point, where superconductivity would be killed,” says Zach Hadjri, a first-year student in the group. “Instead, this material seems to switch between two superconducting states, like a magnet that starts out pointing upward, and can flip downwards when you apply a magnetic field. So it looks like this is a superconductor that also acts like a magnet, which doesn’t make any sense.”

‘One of a kind’
As counterintuitive as the discovery may seem, the team observed the same phenomenon in six similar samples. They suspect that the unique configuration of rhombohedral graphene is the key. The material has a very simple arrangement of carbon atoms. When cooled to ultracold temperatures, the thermal fluctuation is minimized, allowing any electrons flowing through the material to slow down, sense each other, and interact.

Such quantum interactions can lead electrons to pair up and superconduct. These interactions can also encourage electrons to coordinate. Namely, electrons can collectively occupy one of two opposite momentum states, or “valleys.” When all electrons are in one valley, they effectively spin in one direction, versus the opposite direction. In conventional superconductors, electrons can occupy either valley, and any pair of electrons is typically made from electrons of opposite valleys that cancel each other out. The pair overall then has zero momentum, and does not spin.

In the team’s material structure, however, they suspect that all electrons interact such that they share the same valley, or momentum state. When electrons then pair up, the superconducting pair overall has a “non-zero” momentum, and spinning, that—along with many other pairs—can amount to an internal, superconducting magnetism.

“You can think of the two electrons in a pair spinning clockwise, or counterclockwise, which corresponds to a magnet pointing up, or down,” Tonghang Han, a fifth-year student in the group, explains. “So we think this is the first observation of a superconductor that behaves as a magnet due to the electrons’ orbital motion, which is known as a chiral superconductor. It’s one of a kind. It is also a candidate for a topological superconductor, which could enable robust quantum computation.”

“Everything we’ve discovered in this material has been completely out of the blue,” says Zhengguang Lu, a former postdoc in the group and now an assistant professor at Florida State University. “But because this is a simple system, we think we have a good chance of understanding what is going on, and could demonstrate some very profound and deep physics principles.”

“It is truly remarkable that such an exotic chiral superconductor emerges from such simple ingredients,” adds Liang Fu, professor of physics at MIT. “Superconductivity in rhombodedral graphene will surely have a lot to offer.”

Improving energy conversion efficiency in power electronics is vital for a sustainable society, with wide-bandgap semiconductors like GaN and SiC power devices offering advantages due to their high-frequency capabilities. However, energy losses in passive components at high frequencies hinder efficiency and miniaturization. This underscores the need for advanced soft magnetic materials with lower energy losses.

In a study published in Communications Materials, a research team led by Professor Mutsuko Hatano from the School of Engineering, Institute of Science, Tokyo, Japan, has developed a novel method for analyzing such losses by simultaneously imaging the amplitude and phase of alternating current (AC) stray fields, which are key to understanding hysteresis losses.

Using a diamond quantum sensor with nitrogen-vacancy (NV) centers and developing two protocols—qubit frequency tracking (Qurack) for kHz and quantum heterodyne (Qdyne) imaging for MHz frequencies—they realized wide-range AC magnetic field imaging. This study was carried out in collaboration with Harvard University and Hitachi, Ltd.

The researchers conducted a proof-of-principle wide-frequency-range magnetic field imaging experiment by applying an AC current to a 50-turn coil and sweeping the frequency from 100 Hz to 200 kHz for Qurack and 237 kHz to 2.34 MHz for Qdyne. As expected, the uniform AC Ampere magnetic field’s amplitude and phase were imaged using NV centers with high spatial resolution (2–5 µm), validating both measurement protocols.

Using this innovative imaging system, the team could simultaneously map the amplitude and phase of stray magnetic fields from the CoFeB–SiO2 thin films, which have been developed for high-frequency inductors. Their findings revealed that these films exhibit near-zero phase delay of up to 2.3 MHz, indicating negligible energy losses along the hard axis. Moreover, they observed that energy loss depends on the material’s magnetic anisotropy—when magnetization is driven along the easy axis, phase delay increases with frequency, signifying higher energy dissipation.

Overall, the results showcase how quantum sensing can be used to analyze soft magnetic materials operating at higher frequencies, which is considered to be a major challenge in developing highly efficient electronic systems. Notably, the capacity to resolve domain wall motion, one of the magnetization mechanisms strongly related to energy losses, is a pivotal step, leading to important practical advances and optimizations in electronics.

Looking forward, the researchers hope to further improve the proposed techniques in various ways. “The Qurack and Qdyne techniques used in this study can be enhanced by engineering improvements,” says Hatano. “Qurack’s performance can be enhanced by adopting high-performance signal generators to extend its amplitude range, whereas optimizing spin coherence time and microwave control speed would broaden Qdyne’s frequency detection range.”

“Simultaneous imaging of the amplitude and phase of AC magnetic fields across a broad frequency range offers numerous potential applications in power electronics, electromagnets, non-volatile memory, and spintronics technologies,” remarks Hatano. “This success contributes to the acceleration of quantum technologies, particularly in sectors related to sustainable development goals and well-being.”

by Institute of Science Tokyo

edited by Stephanie Baum, reviewed by Robert Egan

Hyperspectral imaging (HSI), or imaging spectroscopy, captures detailed information across the electromagnetic spectrum by acquiring a spectrum for each pixel in an image. This enables precise identification of materials through their spectral signatures.

HSI supports Earth remote sensing applications such as automated classification, abundance mapping, and estimation of physical and biological properties like soil moisture, sediment density, water quality, biomass, leaf area, and pigment content.

Although HSI offers detailed insight into a remote sensing scene, HSI data may not be readily available for an intended application. Recent studies have attempted to combine HSI with traditional red-green-blue (RGB) video acquisition to lower costs and improve performance. However, this fusion technology still faces technical challenges.

In a recent study published in the Journal of Applied Remote Sensing, researchers from the Chester F. Carlson Center for Imaging Science at the Rochester Institute of Technology developed a bimodal video platform that combines a 371-band hyperspectral imaging system, operable in a low-rate video mode, with a standard RGB video camera. Led by Chris H. Lee, the team designed this system to bridge the gap between high-cost hyperspectral imaging and widely available RGB video technology.

The team demonstrated their proof-of-concept by capturing video data of the Lake Ontario shoreline at Hamlin Beach State Park in Rochester, New York.

“We developed a workflow that links reflectance data from a line-scanning hyperspectral imaging spectrometer with RGB video frames to predict hyperspectral imagery,” Lee says. “We established a correlation between the two data streams during a specific time segment, then used it to predict hyperspectral frames both before and after that segment using only RGB video.”

They captured visible to near-infrared hyperspectral video using a Headwall Hyperspec micro-High Efficiency imaging spectrometer, operating in its low-rate video mode. RGB data came from a widely available, low-cost GoPro Hero 8 Black. Lee’s group pushed the systems to their operational limits, acquiring video data at rates on the order of milliseconds and correlating the RGB and HSI data in both time and space.

To assess the accuracy of their workflow, the researchers compared the predicted reflectance with measured reflectance, after correcting for sensor and atmospheric effects. The results varied by wavelength range. In the visible spectrum, the platform predicted 95% of the water scene within 2% absolute reflectance, or about 30% of the water signal level.

In contrast, the near-infrared range showed larger errors: for 95% of the scene, the normalized residual error reached up to 90%. The team attributed this increase to the limited spectral data in RGB video in the shallow water scene.

“Our platform shows that we can predict hyperspectral frames from RGB video with reasonable accuracy in the visible range,” Lee notes. “The drop in performance at longer wavelengths highlights the need for broader spectral coverage of fewer-band data for the prediction algorithm.”

Looking ahead, Lee sees opportunities to enhance the system, “Future improvements will focus on aligning and calibrating the spectrometer and camera fields of view more precisely, and on developing more advanced prediction models.”

By combining affordable RGB cameras with hyperspectral technology, this new platform opens the door to more accessible environmental video monitoring. With further refinement, it could support a broad range of applications, from water quality assessment to vegetation analysis and beyond.

Diamond is one of the most prized materials in advanced technologies due to its unmatched hardness, ability to conduct heat and capacity to host quantum-friendly defects. The same qualities that make diamond useful also make it difficult to process.

Engineers and researchers who work with diamond for quantum sensors, power electronics or thermal management technologies need it in ultrathin, ultrasmooth layers. But traditional techniques, like laser cutting and polishing, often damage the material or create surface defects.

Ion implantation and lift-off is a way to separate a thin layer of diamond from a larger crystal by bombarding a diamond substrate with high-energy carbon ions, which penetrate to a specific depth below the surface. The process creates a buried layer in the diamond substrate where the crystalline lattice has been disrupted. That damaged layer effectively acts like a seam: Through high-temperature annealing, it turns into smooth graphite, allowing for the diamond layer above it to be lifted off in one uniform, ultrathin wafer.

A team of researchers at Rice University has developed a simpler and more effective way to achieve lift-off: instead of high-temperature annealing, they discovered that growing an extra epilayer of diamond atop the substrate after ion implantation is enough to turn the damaged layer graphite-like.

According to a study published in Advanced Functional Materials, the refined technique can bypass the high-temperature annealing and generates higher-purity diamond films than the original substrates. Moreover, the substrate sustains minimal damage in the process and can be reused, making the whole process resource-efficient and scalable.

(a) Theoretical model of the diamond block used in the MD simulations, where vacancies are confined to the central region. (b) Normalized pair radial distribution function, g(r), for pristine diamond (black curve) and diamond with vacancy densities of 1.0 × 10²² vac/cm³ (green curve), 2.8 × 10²² vac/cm³ (red curve), and 9.0 × 10²² vac/cm³ (blue curve). (c), (d), and (e) show MD snapshots of the final simulation frame for vacancy densities of 1.0 × 10²² vac/cm³, 2.8 × 10²² vac/cm³, and 9.0 × 10²² vac/cm³, respectively. Credit: Advanced Functional Materials (2025). DOI: 10.1002/adfm.202423174
“We found that diamond overgrowth converts the buried damage layer into a thin graphitic sheet, removing the need for energy‑heavy annealing,” said Xiang Zhang, assistant research professor of materials science and nanoengineering at Rice and a corresponding author on the study. “The resulting diamond film is purer and higher-quality than the original diamond, matching the electronic-grade quality.”

According to Zhang, these ultrapure diamond films “could revolutionize electronics, enabling faster, more efficient devices, or serve as the foundation for quantum computers that solve problems beyond today’s reach.”

To grow a new layer of diamond on the substrate, the researchers used microwave plasma chemical vapor deposition, a method that deposits new diamond material onto the surface in perfect alignment with the underlying crystal. The researchers hypothesized that the conditions of the growth process itself were enough to drive the conversion of the buried damage layer into graphite, without the need for additional heating.

To confirm this theory, the team examined how the interfaces between the diamond substrate, the buried damage layer and the overgrown film evolved during diamond overgrowth using a combination of transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy and photoluminescence mapping.

“By correlating atomic‑level imaging with spectroscopic signatures, we demonstrate that diamond overgrowth is sufficient to form a clean graphitic release layer, preserve substrate smoothness and yield electronic‑grade diamond films, which is crucial for quantum technologies,” Zhang said.

By simplifying production and boosting sustainability, the new method could enable the development of transformative diamond-based technologies.

by Rice University

edited by Gaby Clark, reviewed by Robert Egan

Blue phosphorescent OLEDs can now last as long as the green phosphorescent OLEDs already in devices, University of Michigan researchers have demonstrated, paving the way for further improving the energy efficiency of OLED screens.

“This moves the blues into the domain of green lifetimes,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Electrical Engineering and corresponding author of the study in Nature Photonics.

“I can’t say the problem is completely solved—of course it’s not solved until it enters your display—but I think we’ve shown the path to a real solution that has been evading the community for two decades.”

OLED screens are standard in flagship smartphones and high-end televisions, providing high contrast and energy efficiency as variations in brightness are achieved by the light emitters rather than a liquid crystal layer over the top. However, not all OLEDs are equally energy efficient.

In current displays, red and green OLEDs produce light through the highly efficient phosphorescent route, whereas blue OLEDs still use fluorescence. This means while red and green OLEDs have a theoretical maximum of one photon for every electron running through the device, blue OLEDs cap out at a far lower efficiency.

Claire Arneson, PhD student in Forrest’s lab, demonstrates the glowing blue PHOLED. Its structure shows a pathway for efficient blue OLEDs that can last as long as the efficient green and red OLEDs already in high end televisions and flagship smartphone displays. Credit: Jero Lopera, Electrical and Computer Engineering, University of Michigan.
The trouble is that blue light is the highest energy that an RGB device must produce: The molecules in blue phosphorescent OLEDs (PHOLEDs) need to handle higher energies than their red and green counterparts. Most of the energy leaves in the form of blue light, but when it is trapped, it can instead break down the color-producing molecules.

Previously, Forrest’s team discovered that there was a way to get that trapped energy out faster by including a coating on the negative electrode that helps the energy convert into blue light. Haonan Zhao, a recent Ph.D. graduate in physics, said it was like creating a fast lane.

“On a road that doesn’t have enough lanes, impatient drivers can crash into one another, cutting off all traffic—just like two excitons bumping into one another create a lot of hot energy that destroys the molecule,” said Zhao, first author of that study as well as the new one. “The plasmon exciton polariton is our optical design for an exciton fast lane.”

The details are based in quantum mechanics. When an electron comes in through the negative electrode, it creates what’s called an excited state in one of the molecules that produces blue light. That state is a negatively charged electron that jumps into a higher energy level and a positively charged “hole” that the electron leaves behind—together, they make an exciton.

Ideally, the electron would quickly jump back to its original state and fire off a blue photon, but excitons that use the phosphorescent route tend to hang around. Simply relaxing into their original state would violate a law of quantum mechanics. However, excitons very near the electrode produce photons faster because the shiny surface supports another quantum quasiparticle—surface plasmons. These are like ripples in the pond of electrons on the surface of the metal.

The new PHOLED, developed in the lab of Steve Forrest at the University of Michigan, shows a pathway for efficient blue OLEDs that can last as long as the efficient green and red OLEDs already in high end televisions and flagship smartphone displays. Credit: Marcin Szczepanski/Michigan Engineering
If the exciton in the light-emitting material is close enough to the electrode, it gets a little help with the conversion to blue light because it can dump its energy into a surface plasmon—a phenomenon known as the Purcell effect. It does this because the exciton oscillates a little like a broadcast antenna, which creates waves in the electrons in the electrode.

This isn’t automatically helpful, though, as not all surface plasmons produce photons. To get the photon, the exciton must attach itself to the surface plasmon, producing a plasmon exciton polariton.

Forrest’s team encouraged this route by adding a thin layer of a carbon-based semiconductor onto the shiny electrode that encourages the exciton to transfer its energy and resonate in the right way. It also extends the effect deeper into the light-emitting material, so excitons further from the electrode can benefit.

The team reported on this last year, and they have since been putting this effect together with other approaches to finally produce a blue PHOLED that can last as long and burn as bright as a green one. These are the highlights of the design:

Two light-emitting layers (a tandem OLED): This cuts the light-emitting burden of each layer in half, reducing the odds that two excitons merge.
Adding a layer that helps the excitons resonate with surface plasmons near both electrodes, so that both emitting layers have access to the fast lane
The whole structure is an optical cavity, in which blue light resonates between the two mirror-like electrodes. This pushes the color of the photons deeper into the blue range.

by Kate McAlpine, University of Michigan

edited by Gaby Clark, reviewed by Robert Egan

When exposed to periodic driving, which is the time-dependent manipulation of a system’s parameters, quantum systems can exhibit interesting new phases of matter that are not present in time-independent (i.e., static) conditions. Among other things, periodic driving can be useful for the engineering of synthetic gauge fields, artificial constructs that mimic the behavior of electromagnetic fields and can be leveraged to study topological many-body physics using neutral atom quantum simulators.

Researchers at Ludwig-Maximilians-Universität, Max Planck Institute for Quantum Optics and Munich Center for Quantum Science and Technology (MCQST) recently realized a strongly interacting phase of matter in large-scale bosonic flux ladders, known as the Mott-Meissner phase, using a neutral atom quantum simulator. Their paper, published in Nature Physics, could open new exciting possibilities for the in-depth study of topological quantum matter.

“Our work was inspired by a long-standing effort across the field of neutral atom quantum simulation to study strongly interacting phases of matter in the presence of magnetic fields,” Alexander Impertro, first author of the paper, told Phys.org. “The interplay of these two ingredients can create a variety of quantum many-body phases with exotic properties.

“While their microscopic mechanisms are typically well understood, the emergent many-body properties are elusive and hard to probe in conventional solids, with a notable example being the (fractional) quantum Hall effect. Unfortunately, it turned out that the Floquet engineering technique, which is one of the primary methods for obtaining an effective magnetic field for neutral atoms, generally causes strong heating in interacting quantum systems.”

The heating processes prompted by Floquet engineering techniques are known to rapidly destroy fragile quantum states. As a result, most previous experiments probing exotic quantum many-body phases only focused on non-interacting or weakly interacting systems, while strongly-correlated ones remained limited to only two particles.

The first objective of the recent study by Impertro and his colleagues was to leverage the capabilities of a new experimental quantum simulation platform they developed to realize quantum many-body states with artificial magnetic fields and strong interactions producing little heating. In addition, they hoped to simulate larger systems that reached well beyond the two-particle systems realized in previous experiments.

In their experiments, the researchers utilized optical superlattices, a short-spacing vertical lattice and a so-called Feshbach resonance that provides an important tuning knob. In addition, they employed a recently developed technique for the precise measurement of particle currents.

“Using the optical superlattices, we partitioned a two-dimensional optical lattice into an independent array of ladder systems, in which we realize the experimental studies,” explained Impertro. “Additionally, the double-wells that form the rungs of the ladders are also the basis for the Floquet engineering technique that we use to create an artificial magnetic field.

“Intuitively, the technique modifies the motion of particles in the lattice using additional laser beams, which in turn imprint the effect of a magnetic field onto the atoms to mimic a Lorentz force or Hall deflection.”

Finally, Impertro and his colleagues leveraged the Feshbach resonance in cesium. This property of cesium allowed them to tune the interaction strength between atoms over a wide range, which is important both for preparing the desired strongly interacting quantum states with low heating and to probe the response of the quantum states to a changing interaction strength.

“The central challenges we encountered when preparing the states were to find suitable parameter regimes where the heating rate due to the periodic modulation (Floquet engineering) is minimal, which is particularly challenging for large many-body systems, and to find preparation paths that allow us to adiabatically transform an easy-to-prepare initial state into the quantum state of interest, without creating excitations,” said Impertro.

“Lastly, a central quantity that characterizes the ground states of flux ladders, such as the Mott-Meissner phase, are persistent particle currents.”

Notably, quantum gas microscopes like the one employed by Impertro and his colleagues can typically only measure local densities and fail to measure currents. To enable the measurement of currents, the team employed a current detection technique that they developed as part of their earlier studies, adapting it for the purpose of their experiment.

“For the first time, we were able to prepare low-temperature states in Floquet engineered quantum systems with a large number of particles and microscopically study their properties,” said Impertro. “We also demonstrated the measurement of particle currents with full spatial resolution across large systems, which constitutes an entirely new way to probe these physics using quantum gas microscopy.

“This constitutes a key step towards studying fractional quantum Hall phases in synthetic quantum systems, which is a longstanding goal in various communities, ranging from superconducting qubits to photonics, neutral atoms and Rydberg atom arrays.”

Impertro and his colleagues hope that their recent efforts will inform future theoretical and experimental studies focusing on topological many-body physics. In the future, the methods they devised could help to realize other complex quantum phases that have so far proved difficult to engineer experimentally.

“On the one hand, we show that it is now indeed feasible to experimentally realize interacting systems with an artificial magnetic field and reach significant system sizes,” said Impertro. “This offers a new playground for quantum simulation of many-body systems in- and out-of-equilibrium in regimes that are extremely difficult to access with classical numerical techniques. On the other hand, a comparison with numerical simulations allowed us to extract an estimate of the effective temperature of the prepared states.”

The new methods introduced by Impertro and his colleagues could soon enable the validation of theoretical models of strongly interacting quantum many-body systems, while also potentially contributing to the future advancement of quantum technologies. In their next studies, the researchers plan to further explore the rich phase diagram of interacting flux ladders beyond the Mott-Meissner phase, for instance by probing vortex or symmetry-broken states.

“In these future studies, it will be central to reduce the experimentally accessible temperature scales further, as many of these phases are even more fragile,” added Impertro. “Additionally, a long-term goal is to extend the ladder geometry to full-2D systems, where exotic physics such as anyons in fractional quantum Hall states can be studied.”

by Brookhaven National Laboratory

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and DOE’s Pacific Northwest National Laboratory (PNNL) have used a combination of scanning transmission electron microscopy (STEM) and computational modeling to get a closer look and deeper understanding of tantalum oxide. When this amorphous oxide layer forms on the surface of tantalum—a superconductor that shows great promise for making the “qubit” building blocks of a quantum computer—it can impede the material’s ability to retain quantum information.

Learning how the oxide forms may offer clues as to why this happens—and potentially point to ways to prevent quantum coherence loss. The research was recently published in the journal ACS Nano.

The paper builds on earlier research by a team at Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University that was conducted as part of the Co-design Center for Quantum Advantage (C²QA), a Brookhaven-led national quantum information science research center in which Princeton is a key partner.

“In that work, we used X-ray photoemission spectroscopy at NSLS-II to infer details about the type of oxide that forms on the surface of tantalum when it is exposed to oxygen in the air,” said Mingzhao Liu, a CFN scientist and one of the lead authors on the study. “But we wanted to understand more about the chemistry of this very thin layer of oxide by making direct measurements,” he explained.

So, in the new study, the team partnered with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department to use advanced STEM techniques that enabled them to study the ultrathin oxide layer directly. They also worked with theorists at PNNL who performed computational modeling that revealed the most likely arrangements and interactions of atoms in the material as they underwent oxidation.

Together, these methods helped the team build an atomic-level understanding of the ordered crystalline lattice of tantalum metal, the amorphous oxide that forms on its surface, and intriguing new details about the interface between these layers.

“The key is to understand the interface between the surface oxide layer and the tantalum film because this interface can profoundly impact qubit performance,” said study co-author Yimei Zhu, a physicist from CMPMS, echoing the wisdom of Nobel laureate Herbert Kroemer, who famously asserted, “The interface is the device.”

Emphasizing that “quantitatively probing a mere one-to-two-atomic-layer-thick interface poses a formidable challenge,” Zhu noted, “we were able to directly measure the atomic structures and bonding states of the oxide layer and tantalum film as well as identify those of the interface using the advanced electron microscopy techniques developed at Brookhaven.”

“The measurements reveal that the interface consists of a ‘suboxide’ layer nestled between the periodically ordered tantalum atoms and the fully disordered amorphous tantalum oxide. Within this suboxide layer, only a few oxygen atoms are integrated into the tantalum crystal lattice,” Zhu said.

The combined structural and chemical measurements offer a crucially detailed perspective on the material. Density functional theory calculations then helped the scientists validate and gain deeper insight into these observations.

“We simulated the effect of gradual surface oxidation by gradually increasing the number of oxygen species at the surface and in the subsurface region,” said Peter Sushko, one of the PNNL theorists.

By assessing the thermodynamic stability, structure, and electronic property changes of the tantalum films during oxidation, the scientists concluded that while the fully oxidized amorphous layer acts as an insulator, the suboxide layer retains features of a metal.

“We always thought if the tantalum is oxidized, it becomes completely amorphous, with no crystalline order at all,” said Liu. “But in the suboxide layer, the tantalum sites are still quite ordered.”

With the presence of both fully oxidized tantalum and a suboxide layer, the scientists wanted to understand which part is most responsible the loss of coherence in qubits made of this superconducting material.

“It’s likely the oxide has multiple roles,” Liu said.

First, he noted, the fully oxidized amorphous layer contains many lattice defects. That is, the locations of the atoms are not well defined. Some atoms can shift around to different configurations, each with a different energy level. Though these shifts are small, each one consumes a tiny bit of electrical energy, which contributes to loss of energy from the qubit.

“This so-called two-level system loss in an amorphous material brings parasitic and irreversible loss to the quantum coherence—the ability of the material to hold onto quantum information,” Liu said.

But because the suboxide layer is still crystalline, “it may not be as bad as people were thinking,” Liu said. Maybe the more fixed atomic arrangements in this layer will minimize two-level system loss.

Then again, he noted, because the suboxide layer has some metallic characteristics, it could cause other problems.

“When you put a normal metal next to a superconductor, that could contribute to breaking up the pairs of electrons that move through the material with no resistance,” he noted. “If the pair breaks into two electrons again, then you will have loss of superconductivity and coherence. And that is not what you want.”

Future studies may reveal more details and strategies for preventing loss of superconductivity and quantum coherence in tantalum.

More information: Junsik Mun et al, Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit, ACS Nano (2023). DOI: 10.1021/acsnano.3c10740

Journal information: ACS Nano 

Provided by Brookhaven National Laboratory