Terahertz light, situated between microwave and infrared frequencies on the electromagnetic spectrum, oscillates at an extraordinary rate of over a trillion cycles per second. These oscillation frequencies closely correspond to the natural vibrational frequencies of atoms and electrons within various materials, rendering terahertz radiation a potentially ideal probe for capturing intrinsic quantum motions. However, the relatively long wavelengths of terahertz waves—hundreds of microns in length—have historically precluded their use in high-resolution microscopy. This diffraction limit dictates that the minimum achievable focus size for any electromagnetic wave is constrained by its wavelength, thus hampering the ability to resolve features smaller than tens of microns when employing terahertz illumination.

MIT’s innovative solution hinges on the utilization of spintronic terahertz emitters—composite multilayer metallic structures that produce ultrashort, intense pulses of terahertz radiation upon laser excitation. By positioning a microscopic sample in immediate proximity to the emitter, the researchers effectively confined the terahertz electromagnetic field within subwavelength dimensions, thereby compressing the radiation into a spatially localized hotspot far below the standard diffraction limit. This proximity-induced confinement enabled the team to interact strongly with microscopic quantum states and extract signals that embody the subtle electron dynamics within materials like bismuth strontium calcium copper oxide (BSCCO), a prominent layered high-temperature superconductor.

BSCCO, renowned for its relatively elevated superconducting transition temperature, served as an ideal candidate for demonstrating this terahertz microscope’s capabilities. When cooled to near absolute zero, the researchers transmitted tightly confined terahertz pulses into an atomically thin BSCCO sample and monitored the resultant electromagnetic responses. They discovered a striking dynamic: a frictionless “superfluid” of superconducting electrons collectively oscillating at terahertz frequencies. These oscillations manifested as modulations or distortions in the reflected terahertz signal, indicating that the sample was not merely a passive medium but an active emitter of terahertz waves induced by internal quantum mechanical excitations.

Prior to this work, such collective electron oscillations within superconductors had been predicted theoretically but remained experimentally elusive due to the spatial and temporal scales involved. The terahertz superfluid plasmon, as it is termed, exemplifies a new quantum mode of coherent electron flow that exhibits zero resistance and could hold the key to unraveling the fundamental physics underpinning high-temperature superconductivity. Observing these modes directly opens potential avenues for engineering materials with enhanced superconducting properties, possibly bringing the longstanding dream of room-temperature superconductors closer to reality.

A central challenge the team overcame was the mitigation of background noise and interference from the optical pump laser used to excite the spintronic emitters. To achieve this, the experimental setup incorporated a sophisticated Bragg mirror, a multilayered reflective filter designed to selectively transmit terahertz frequencies while blocking detrimental shorter-wavelength laser light. This intricate design safeguarded the sample and ensured that the emitted terahertz pulses maintained coherence and spectral purity, critical factors for accurate imaging at such finely resolved scales.

Beyond its profound implications for fundamental physics, this terahertz microscopy technique holds transformative potential for applied sciences and emerging technologies. Terahertz frequencies are poised to revolutionize wireless communication by providing dramatically faster data transmission rates and enhanced bandwidth compared to current microwave-based systems. However, the development of devices capable of efficiently emitting and detecting terahertz radiation remains a technological frontier. The ability to image interactions between terahertz waves and microscopic device components promises to accelerate the design and optimization of next-generation terahertz antennas, sensors, and circuits, facilitating future advancements in telecommunications infrastructure.

Moreover, the nonionizing nature of terahertz radiation, combined with its capacity to penetrate a diverse array of nonmetallic materials—including fabrics, plastics, ceramics, and biological tissues—renders it a compelling candidate for safe, noninvasive imaging applications. Potential uses range from security screening systems capable of discerning concealed objects to medical diagnostic tools that visualize soft tissue anomalies without harmful ionizing radiation exposure. The enhanced spatial resolution provided by MIT’s terahertz microscope could refine these imaging techniques, enabling detailed characterization at cellular or molecular levels.

The research team comprises a collaborative ensemble of physicists and materials scientists, including lead author Alexander von Hoegen and Nobel-winning Donner Professor of Physics Nuh Gedik, alongside other MIT experts and international partners from Harvard University, the Max Planck Institutes, and Brookhaven National Laboratory. Their collective expertise spans quantum physics, spintronics, and advanced microscopy, facilitating this interdisciplinary breakthrough that fuses cutting-edge quantum materials science with state-of-the-art photonics engineering.

This work not only heralds a new era in terahertz spectroscopy but also exemplifies how overcoming fundamental physical constraints can unlock entirely new vistas in the study of complex quantum systems. By successfully imaging the coordinated terahertz oscillations of superconducting electrons, MIT researchers have illuminated a hidden layer of material behavior that had, until now, remained a theoretical abstraction. The implications ripple outward, promising future discoveries in two-dimensional quantum materials, novel device architectures, and enhanced control over electromagnetic phenomena at terahertz frequencies.

Looking ahead, the team plans to extend their investigations to a wider range of two-dimensional and layered materials, seeking to capture and characterize other collective excitations such as lattice vibrations and spin dynamics that similarly unfold within the terahertz regime. These efforts will deepen understanding of emergent quantum phases and may catalyze the invention of transformative technologies based on quantum coherence and ultrafast electron dynamics. As terahertz microscopy matures, it is poised to become an indispensable tool across physics, materials science, and engineering disciplines, bridging the gap between quantum theory and observable phenomena at microscopic scales.

In sum, this landmark accomplishment showcases how innovation in light-matter interaction techniques can reveal the intricate dance of electrons within superconductors—material systems that hold promise for revolutionizing energy transmission, computing, and communications. By capturing the elusive terahertz superfluid plasmon directly, MIT scientists have illuminated a new dimension of superconducting behavior, laying the groundwork for a future where quantum materials are not only understood but harnessed with precision innovation.

Subject of Research: Imaging and characterization of quantum electron dynamics in layered high-temperature superconductors using terahertz microscopy.

Article Title: “Imaging a terahertz superfluid plasmon in a two-dimensional superconductor”

Web References:
DOI link to article

Image Credits: Sampson Wilcox and Emily Theobald

Keywords

Electrons, Particle physics, Physics, Subatomic particles, Quantum mechanics, Mechanics, Electromagnetism, Superconductivity, Superconduction, Electromagnetic properties, Superconductors, Electrical conductors, Electrical engineering

Historically, the redox chemistry of iron in battery cathodes has been constrained by the metal’s tendency to participate in oxidation-reduction processes with a maximum valence change involving two or three electrons. This limitation restricts the attainable energy storage capacity inherent to iron, which ironically remains one of the most abundant, cost-effective, and environmentally benign transition metals. The potential to push iron into higher oxidation states and reverse these changes in a stable, repeatable fashion has been a coveted goal—one that had remained elusive due to structural instabilities and unwanted side reactions within the materials.

The pivotal breakthrough emerged from the collaborative effort spearheaded by Stanford PhD candidates Hari Ramachandran, Edward Mu, and Eder Lomeli, who meticulously refined the synthesis and characterization of a new lithium-iron-antimony-oxygen (LFSO) cathode material. Their team hypothesized that spatial separation of iron atoms within the host crystal structure would prevent deleterious oxygen bonding and other side reactions, thereby enabling iron to reversibly lose and regain as many as five electrons. The crux lay in engineering nanoscale particles—mere hundreds of nanometers in diameter—far smaller than previous attempts. Such nano-dimensions stabilized the crystal framework during charge-discharge cycles, a feat previously unattainable.

Their approach involved growing nanocrystals from an intricate liquid medium solution, a technically challenging process that required balancing complex chemical interactions to yield uniformly small and stable particles. Electrochemical testing confirmed that the LFSO cathode maintained structural integrity and exhibited reversible redox activity consistent with the unprecedented five-electron transition. However, this apparent expansion of iron’s electronic shuttling raised critical questions about the underlying electronic structure.

To unravel the atomic-level nuances, the team incorporated advanced spectroscopic techniques combined with theoretical modeling. Collaborator Lomeli, leveraging state-of-the-art numerical simulations at SLAC National Accelerator Laboratory, discerned that the additional electrons were not sourced solely from iron atoms but instead involved a cooperative interplay between iron and surrounding oxygen atoms within the crystal lattice. This emergent behavior exemplifies a sophisticated collective electronic structure, where iron and oxygen participate as a unified redox entity rather than independent actors—a conceptual leap reflecting the complexity and subtlety of transition metal oxides.

The implications extend beyond battery technology. The team envisions applications in fields dependent on iron’s magnetic properties, such as magnetic resonance imaging (MRI) and magnetic levitation systems, and even anticipates ramifications for high-temperature superconductors, where electron transfer dynamics are critical. The broader material science community has long sought sustainable alternatives to cobalt and nickel—metals that dominate current lithium-ion battery cathodes but pose supply chain vulnerabilities, geopolitical concerns, and ethical issues linked to mining practices in regions with problematic labor conditions.

Iron-based cathodes, particularly those combining lithium, iron, phosphorus, and oxygen, already comprise about 40% of global lithium-ion battery cathodes due to their lower cost and more sustainable sourcing. Yet, these iron-phosphate cathodes are inherently limited by relatively low operational voltages. A high-voltage iron cathode that leverages reversible FeIII/V redox activity could revolutionize battery design, overcoming the tradeoffs that have forced manufacturers to rely on costly and ethically challenging metals to achieve higher voltages.

Structurally, the LFSO nanoparticles distinguish themselves by their ability to accommodate lithium extraction without catastrophic lattice collapse. Conventional bulk iron-based cathodes tend to exhibit irreversible twisting and fracturing upon lithium migration during battery charging. By contrast, the nanoscale LFSO material exhibits elastic bending, effectively absorbing mechanical stresses and preserving its structural coherence through multiple cycles. This resilience is critical for practical commercial deployment, where longevity and reliability are paramount.

The team’s integrated methodology combined rigorous experimental electrochemistry, spectroscopy using X-rays and neutrons at prominent national laboratories across the United States, and sophisticated computational modeling. This holistic approach enabled them to move beyond mere empirical observation to a deep understanding of the microscopic processes enabling the five-electron redox cycle. The research underscores the power of interdisciplinary collaboration spanning physics, chemistry, materials science, and engineering.

Despite the monumental progress, a key challenge remains: antimony, a component of the LFSO cathode, shares some of the supply chain and cost concerns familiar to cobalt and nickel. The Stanford-led team is actively exploring alternative dopants and compositional tweaks to substitute antimony without sacrificing the essential electrochemical properties. Such efforts are critical to transitioning this discovery from laboratory curiosity to industrially viable technology.

This research heralds a new era of sustainable energy technologies leveraging the earth-abundant and environmentally favorable element iron. By shattering previously accepted electrochemical limits, the findings open the door to higher performance lithium-ion batteries that could accelerate the adoption of electric vehicles, grid-scale energy storage, and innovative magnetic and superconducting devices. As the scientific community continues to refine and scale these materials, the dream of affordable, durable, and powerful iron-based energy storage moves closer to reality.

Subject of Research: Not applicable

Article Title: A formal FeIII/V redox couple in an intercalation electrode

News Publication Date: 15-Oct-2025

Web References: http://dx.doi.org/10.1038/s41563-025-02356-x

Image Credits: Bill Rivard

Keywords

Lithium ion batteries, Chemical engineering, Chemical physics, Electrochemical energy, Electrochemical reactions, Sustainable energy, Materials engineering, Materials science, Sustainability

The significance of moiré superlattices is underscored by their rarity in organic crystal formations, in stark contrast to their presence in inorganic structures. Achieving such formations requires the materials to be ultrathin and highly crystalline—characteristics that are notoriously challenging to realize in organic substances. The research team’s focus on bilayer COFs is particularly noteworthy as it addresses the intrinsic difficulties associated with imaging organic materials using traditional microscopy techniques, which often fall short when applied to such delicate structures.

Covalent organic frameworks encapsulate a vibrant landscape of possibilities, specifically in applications like catalysis, energy storage, and gas storage. These structures are comprised of covalently bonded layers aggregated through electrostatic interactions and van der Waals forces. However, despite their utility, the transition from a monolayer to a bilayer configuration exemplifies a poorly understood aspect of their synthesis, primarily due to intermolecular bonding complexities. Information regarding the precise alignment and stacking of layers is paramount in determining the resultant material’s crystallinity and overall performance characteristics.

The research undertaken by Professor Loh Kian Ping and his team illuminates the intricate interplay of bonding forces involved in COF assembly, including van der Waals, electrostatic, and hydrogen bonds. Despite previous advancements in producing monolayers, challenges persist in synthesizing single COF crystals exceeding millimeter dimensions due to potential bonding error accumulations in both horizontal and vertical stacking processes. This misalignment can lead to significant complications regarding the crystallinity of layered materials and real-time observation of the stacking process presents an additional hurdle, particularly when dealing with the fluid dynamics involved in solution-based growth.

The research highlights that random stacking tendencies and bond formations during hydrothermal synthesis frequently hinder crystallinity, resulting in crystal domains significantly smaller than expected sizes. Gaining an in-depth understanding of the stacking mechanisms could dramatically enhance the synthesis protocols, possibly enabling the development of larger COF crystals with improved properties. The present advancements particularly in 2D polymers are exciting; however, many opportunities lie within the yet-untapped area of bilayer 2D polymer (2DP) stacks—a field promising exceptional advances through careful control of stacking and twisting of 2D materials.

Loh’s team employed a significant methodological leap that allowed them to synthesize large-area bilayer 2D COFs directly at the liquid-substrate interface. By utilizing a direct condensation technique during synthesis, they adhered to the layered structure’s integrity. Their implementation of scanning tunneling microscopy (STM) in solution was revolutionary, as it permitted real-time observation of the molecular assembly during bilayer formation. This method was crucial in revealing how solvent composition and molecular structure influenced bilayer stacking modes, leading to the spectacular emergence of large-area moiré superlattices.

The technical challenges posed by COFs, given their organic and highly porous nature, complicate imaging under traditional conditions. The scenarios necessitating ultra-high vacuum (UHV) or air-exposed conditions often contribute to the degradation of quality essential for atomic-scale imaging. However, by adapting their imaging methods to directly observe COFs while they remain in solution, the research team was able to circumvent many of these obstacles. Prof. Loh expressed the advantage of conducting STM in a liquid medium, remarking that it creates cleaner surfaces than those typically seen when materials are subjected to air.

In pursuit of characterizing the fundamental aspects of twisted bilayers, the research team dedicated significant attention to comparing different isomers, namely pyrene-2,7-diboronic acid (27-PDBA) and pyrene-1,6-diboronic acid (16-PDBA). They discovered that the second layer’s stacking behavior was influenced considerably by the variations in the precursor molecular architecture. Specifically, with 27-PDBA, the stacking could result in either an AA-stacked configuration or a twisted formation, showcasing the potential scalability of tunable properties. Conversely, 16-PDBA yielded a consistent moiré structure without the emergence of dwellings eliciting twist differences, demonstrating the complexity arising from the distinct electrostatic properties of the constituent molecules.

The implications of this research are far-reaching and suggest profound potential applications across various fields. With a foundation built upon controlled synthesis and the ability to manipulate twist angles, the opportunities for tailored materials are vast. The enhancement of ultra-thin porous structures paves the way for innovations in nanofiltration technologies—they could serve as functional barriers and frameworks with tuned channel geometries. Moreover, opportunities for developments that enable optimized light propagation, including manipulation of phase and polarization, are emerging as a critical avenue for further exploration.

Looking towards the future, the research group aims to leverage their foundational knowledge to elaborate upon a broader array of molecular precursors characterized by diverse linkage chemistries. Achieving deterministic control over the twist angles in subsequent bilayer COF systems could unlock previously unimagined applications, further contributing to the rapidly evolving field of organic electronics and nanomaterials. This ambitious initiative signals a promising horizon for researchers and industries alike, as they pursue novel applications driven by understanding and manipulating the molecular architecture of layered organic frameworks.

The intersection of advanced materials science and innovative imaging technologies heralds exciting prospects in the development of next-generation materials. With substantial evidence showcasing the practical applications and a clarified framework for future endeavors, the research conducted at the National University of Singapore establishes itself as a cornerstone in the ongoing quest toward functionalized, smart materials that blur the lines between traditional chemistry and advanced engineering.

Participants in this collaborative research included notable figures from various institutions, extending the impact of their findings across the global scientific community. The collective effort underscores the importance of cross-institutional collaboration in tackling complex challenges and pushing the frontiers of what is achievable in the field of materials science.

The research findings were disseminated through a formal publication in the esteemed journal, "Nature Chemistry," currently heralding significant interest in the scientific community. The implications of these discoveries are poised to inspire an extensive array of future studies exploring the intricate properties and applications of twisted bilayers in diverse scientific domains.

Given the evolving landscape of materials science and the potential for novel innovations to emerge, this research not only contributes to the current body of knowledge but also ignites curiosity for unexplored avenues in bilayer COFs and moiré superlattices. As researchers continue to unravel the complexities within these organic frameworks, we anticipate further revelations and advancements that could redefine technological applications and foster sustainable solutions within our increasingly material-driven world.

Subject of Research: Covalent Organic Frameworks and Moiré Superlattices
Article Title: Moiré two-dimensional covalent organic framework superlattices
News Publication Date: 20-Feb-2025
Web References: Link to Nature Chemistry
References: DOI 10.1038/s41557-025-01748-5
Image Credits: National University of Singapore

Keywords

Superlattices, Discovery Research, Two Dimensional Materials, Covalent Organic Frameworks, Scanning Tunneling Microscopy, Molecular Structure.