The distinction between stacking geometries sounds arcane, but it is the entire story. In the most common bilayer arrangement, known as 2H stacking, the top sulfur plane sits rotated by 180 degrees relative to the bottom layer, which restores inversion symmetry and drives the valence band maximum away from the valleys where the conduction band minimum resides. The 3R, or rhombohedral, arrangement keeps the layers aligned in the same orientation and breaks that symmetry, enabling ferroelectric behaviour through interlayer sliding. The 1H geometry, the one the Taiwanese-led team targeted, places the second layer in perfectly commensurate vertical alignment with the first in a specific relative orientation that keeps the band extrema aligned in momentum space. Achieving this configuration in a scalable, crystal-growth setting rather than by manually transferring and stacking exfoliated flakes is what makes the new work remarkable.

The researchers accomplished it with a modified two-step chemical vapour deposition scheme in which growth temperature is modulated between the nucleation of the first layer and the growth of the second. Their Extended Data analysis traces the mechanism to the diffusion behaviour of molybdenum adatoms landing on the completed monolayer. Depending on temperature, arriving adatoms follow either edge diffusion pathways, hopping along equivalent crystallographic directions to attach at the flake boundary, or surface diffusion pathways, migrating across the terrace of the underlying monolayer. An Arrhenius analysis of the hopping-rate ratio shows how the balance between these two regimes selects the resulting stacking polytype: 1H, 3R or 2H. Under the conditions favouring 1H growth, molybdenum adatoms form ribbons along preferred crystallographic directions that branch and eventually coalesce into seamless bilayer triangles, a process confirmed by bright-field and dark-field transmission electron microscopy showing fully stitched, unidirectional bilayer nanoribbons.

Structural verification was exhaustive. High-angle annular dark-field scanning transmission electron microscopy, cross-sectional STEM prepared by focused ion beam milling, and selected-area diffraction patterns were compared against simulated diffraction patterns for each candidate polytype. The examined regions consistently showed the 1H stacking arrangement with atomic-scale uniformity, and interlayer spacings measured at ten separate locations in the cross-sectional images matched the expected commensurate geometry. Second-harmonic generation microscopy and atomic force microscopy provided additional, wafer-scale confirmation that the resulting flakes were structurally distinct from the 3R and 2H bilayers grown under the alternative temperature conditions of the same process.

With the structure nailed down, the team turned to the electronic structure. Angle-resolved photoemission spectroscopy performed with a photoelectron momentum microscope at the Taiwan Photon Source probed the valence bands directly, and the experimental spectra were compared with density functional theory calculations. In conventional bilayer MoS2, the valence band maximum at the K valley sits measurably higher in energy than at the Gamma point only in monolayers; in bilayers the Gamma point wins and the gap becomes indirect. For the 1H bilayers, the measured energy separation between the valence band at K and at Gamma remained consistent with a direct gap, and the extracted value for a monolayer reference in the same experiment, 146 millielectronvolts, agreed with published literature, providing an internal calibration for the measurement.

The optical consequences followed immediately. Photoluminescence mapping across the 1H bilayer flakes revealed intensified excitonic emission, and, crucially, the spectra lacked the low-energy indirect-gap emission features that usually betray bilayer character. In ordinary 2H bilayers, most electron-hole recombination funnels through the indirect transition and emits weakly at longer wavelengths; the 1H bilayers showed no such signature, indicating that radiative recombination proceeds through the direct, momentum-conserving channel. In other words, the extra layer adds carrier capacity and mobility without exacting the usual optical penalty.

Perhaps the most surprising result concerns valley physics. Monolayer MoS2 owes its valleytronics credentials to broken inversion symmetry, which couples the spin and valley degrees of freedom and allows circularly polarized light to selectively populate one of two inequivalent valleys. Bilayers with 2H stacking restore inversion symmetry and destroy this valley contrast. The 1H bilayers, however, not only preserved valley-selective circular polarization but actually exhibited stronger valley polarization than monolayers under both resonant and nonresonant excitation. The authors attribute this enhancement primarily to suppressed intervalley scattering in the top layer of the stack, meaning that once excitons are injected into a valley they are less likely to relax into the opposite valley before recombining. Robust polarization under off-resonant excitation is particularly valuable for practical devices, since it relaxes the demanding requirement for exactly resonant optical pumping.

The implications reach across several device families. Bilayer transition metal dichalcogenides are already attractive for next-generation transistors because they offer higher carrier mobility than monolayers and, in suitably stacked forms, electrically switchable polarity. They also underpin emerging sliding ferroelectric devices, in which an interlayer displacement toggles a polarization state. The demonstration that a specific, growth-accessible stacking order can restore a direct band gap means engineers may no longer have to choose between the electronic merits of two layers and the optical merits of one. Light-emitting transistors, valleytronic logic and integrated optoelectronic circuits built on MoS2 all become more plausible when the same material platform supports charge transport, light emission and valley polarization simultaneously.

There are also broader lessons for the rapidly growing field of stacking-engineered quantum materials. The explosion of interest in twisted and commensurate bilayers of graphene and transition metal dichalcogenides has shown that interlayer registry, not chemistry alone, dictates electronic behaviour. Most such studies, however, rely on mechanical assembly of exfoliated flakes, which is artisanal, slow and poorly suited to manufacturing. The two-step CVD approach reported here shows that a desired polytype can be selected during synthesis by tuning adatom diffusion kinetics, and that adjacent bilayer ribbons can stitch together seamlessly into larger crystals. If the same kinetic control can be extended to other materials and other stacking targets, it would move stacking engineering closer to wafer-scale production.

Caveats and open questions remain, as with any single study. The photoemission measurements probed multiple crystallographic orientations of the grown flakes within the probing area, requiring careful angular slicing of the momentum images, and the reported direct-gap character rests on the agreement between experiment and density functional theory rather than on a direct measurement of the conduction band. Whether the enhanced valley polarization survives at elevated temperatures, in encapsulated device structures, and across wafer-scale films will need to be established. Still, the core message stands: the indirect band gap of bilayer MoS2 is not an immutable fact of nature but a consequence of stacking geometry, and that geometry can now be grown on demand. As the authors put it in their abstract, the results establish 1H MoS2 as a model system for stacking-engineered quantum materials, and they underscore its potential for valleytronic and optoelectronic applications that the field has been pursuing since monolayer MoS2 first announced its direct gap sixteen years ago.

Subject of Research: Stacking-controlled electronic and optical properties of CVD-grown bilayer molybdenum disulfide

Article Title: Stacking-induced direct band gap in CVD-grown 1H MoS2 bilayers

Article References: Yang, T. H., Chen, I.-T., Zhang, M.-J., Huang, J.-Y., Kuo, T.-H., Chen, S.-Y., Li, H.-Y., Chao, Y.-C., Hennighausen, Z. B., Dien, V. K., Wei, H.-W., Wu, M.-C., Yen, H.-W., Chuang, T.-H., Wei, D.-H., Kong, J., Lu, T.-H., Lin, K.-I., & Lan, Y.-W. (2026). Stacking-induced direct band gap in CVD-grown 1H MoS2 bilayers. Nature. https://doi.org/10.1038/s41586-026-11069-3

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11069-3

Keywords: MoS2, transition metal dichalcogenides, two-dimensional materials, band gap engineering, chemical vapour deposition, stacking polytypes, photoluminescence, angle-resolved photoemission spectroscopy, valleytronics, excitons, optoelectronics, density functional theory

Spearheaded by a team of four distinguished physicists—Pengcheng Dai, Ming Yi, Emilia Morosan, and Qimiao Si—this collaboration unites diverse expertise in experimental and theoretical condensed matter physics. The team’s collective aim is to unravel the mysteries behind unconventional superconductivity, quantum magnetism, and topological states of matter. These emergent phases, which arise from complex many-body interactions, hold the promise to revolutionize material design and propel transformative breakthroughs across computing, storage, and energy sectors.

Central to the research strategy is the fusion of multiple investigative methodologies, spanning guided materials synthesis, thermodynamic and transport property characterization, neutron scattering experiments, and angle-resolved photoemission spectroscopy (ARPES), alongside robust theoretical modeling. The integration of these techniques permits a holistic exploration of how magnetism interweaves with lattice dynamics, electronic band structures, and orbital degrees of freedom—facets critical for decoding the behavior of quantum materials. This comprehensive approach surpasses the limitations of single-technique studies, enabling new insights into quantum phenomena previously obscured in isolated analyses.

Neutron scattering, a cornerstone tool in this endeavor, facilitates the direct measurement of magnetic order and spin fluctuations within crystalline materials. By quantifying momentum transfer during neutron-material interactions, researchers can map spin arrangements and dynamic excitations at an atomic scale. Nevertheless, neutron scattering alone is insufficient to capture the complete electronic topology linked with magnetic phenomena. For this reason, the team pairs neutron scattering with ARPES, which probes the momentum-resolved electronic structure by ejecting electrons using photon excitation, thereby revealing how electronic states couple to magnetic ordering across momentum space in unprecedented detail.

The diverse expertise of the team exemplifies the synergy necessary for pioneering discoveries. Ming Yi emphasizes the importance of aligning experimental probes to uncover hidden aspects of quantum materials that evade detection through conventional methods. This approach promises a more nuanced understanding of how subtle interactions lead to macroscopic emergent properties directly relevant for future quantum devices and energy-efficient materials. By harnessing a variety of advanced techniques, RLEMM aims to demonstrate how collaborative, multidisciplinary research can accelerate materials discovery.

Within the research program, three primary thrusts stand out. First, the study of fractionalized quasiparticles within quantum magnetism tackles exotic excitations resulting from strong electron correlations and entanglement. These quasiparticles challenge classical intuition about particle behavior, offering clues to fundamentally new states of matter. Second, investigations into unconventional superconductivity focus on the role of flat electronic bands—energy dispersions conducive to enhanced electron pairing and robust superconducting states beyond traditional phonon-mediated mechanisms. Finally, altermagnetism, a newly identified form of magnetic order that combines properties of both ferromagnets and antiferromagnets, represents an exciting frontier with potential for novel spintronic applications.

Emilia Morosan brings critical expertise in materials science, particularly in synthesizing novel compounds designed to exhibit targeted quantum phenomena. The ability to tailor crystal compositions and growth conditions is indispensable for creating new material platforms with emergent magnetic and electronic properties. This tailored materials design lays the experimental foundation for probing scientifically rich, previously unexplored regions of the condensed matter phase space. Morosan’s leadership ensures that discovery-driven synthesis and rigorous experimental characterization remain central pillars of the project.

The impacts of this work are envisioned to extend far beyond academic exploration. By decoding the fundamental physics underlying emergent magnetism, the RLEMM team aspires to provide blueprints for materials engineered to host tailored quantum states, optimized for applications in quantum information storage, advanced sensors, and sustainable energy technologies. The knowledge generated here may help overcome long-standing barriers in coherence times, energy dissipation, and scalability, which currently limit the performance of practical quantum and spintronic devices.

Training the next wave of scientific innovators is a critical component of RLEMM’s mission. The laboratory will serve as a vibrant intellectual ecosystem for graduate students and postdoctoral researchers, immersing them in cutting-edge interdisciplinary research. Complementing hands-on experimentation and theoretical work, RLEMM will also propagate its findings and foster dialogue through online seminar series and public lectures designed to engage the global scientific community and general audiences alike. Open dissemination accelerates knowledge transfer and strengthens collaborative networks.

A fundamental strength of the initiative lies in its seamless bridging of theoretical and experimental efforts. Qimiao Si highlights how the close integration between modeling and laboratory investigations enables prompt feedback loops, whereby emergent experimental anomalies inspire novel theoretical frameworks, and in turn, predictive models guide targeted experiments. This iterative feedback mechanism epitomizes modern condensed matter research, allowing the team to swiftly adapt and refine approaches addressing the most pressing scientific challenges related to magnetism.

The formation of the Rice Laboratory for Emergent Magnetic Materials stands as a testament to the profound value of fostering collaborative environments within academic institutions. By centralizing expertise across synthesis, characterization, and theory, RLEMM is poised to become an epicenter for discovery in emergent quantum phenomena. The awarded funding from the Department of Energy underscores the strategic importance of investing in fundamental research that may define the foundation of future technological landscapes.

In sum, the RLEMM initiative promises to illuminate the enigmatic mechanisms of magnetism in quantum materials, pushing the boundaries of physics and materials science. As investigations progress, novel materials with engineered magnetic and electronic states are expected to emerge, setting the stage for disruptive innovations in computing, data storage, and energy efficiency. This visionary endeavor marks a significant stride towards translating deep scientific inquiry into impactful technologies aimed at addressing some of the most challenging problems of the 21st century.

Subject of Research: Emergent magnetic materials, quantum magnetism, unconventional superconductivity, altermagnetism, topological phases
Article Title: Rice University Launches Pioneering Laboratory to Decipher Quantum Magnetism with $4.4 Million DOE Grant
News Publication Date: Not specified
Web References:

• Pengcheng Dai profile: https://profiles.rice.edu/faculty/pengcheng-dai
• Ming Yi profile: https://profiles.rice.edu/faculty/ming-yi
• Emilia Morosan profile: https://profiles.rice.edu/faculty/emilia-morosan
• Qimiao Si profile: https://profiles.rice.edu/faculty/qimiao-si
• Rice Center for Quantum Materials: https://rcqm.rice.edu/
• Extreme Quantum Materials Alliance: https://eqma.rice.edu/
  Image Credits: Photo by Jorge Vidal/Rice University
  Keywords: Magnetism, Quantum computing, Data storage, Quantum magnetism, Topology, Technology

A recent study helmed by Professor Fahad Mahmood of the University of Illinois has unveiled significant findings regarding magnetically intrinsic topological insulators, particularly focusing on manganese bismuth telluride (MnBi₂Te₄). This research not only sheds light on the band structure and electronic properties of MnBi₂Te₄ but also contests previous assertions regarding its electronic band gap, a contentious issue in the scientific community. The team’s findings mark the first demonstration of how external factors, specifically circularly polarized light, can manipulate the material’s properties in meaningful ways.

Diving deeper into the quantum characteristics of materials, this study elucidates the concept of a hidden gap in the electronic band structure of MnBi₂Te₄ under specific light conditions. While previous studies laid the groundwork, experimental evidence remained elusive, until now. The research clearly illustrates that MnBi₂Te₄ exhibits a gapless condition at equilibrium—an observation consistent with some prior studies—yet intriguingly develops a gap when subjected to different orientations of circularly polarized light.

Through rigorous experimentation, the research team employed angle-resolved photoemission spectroscopy (ARPES) to meticulously examine the band structure of MnBi₂Te₄. This technique detects the electron energies emitted when light shines upon a material’s surface and reveals how these energy levels shift under various external conditions. The intricacies of examining the electronic structure facilitate a comprehensive understanding of a material’s behavior, which is pivotal in describing its physical properties.

A defining characteristic of non-magnetic topological insulators is the adherence to time-reversal symmetry (TRS), a principle asserting that the fundamental laws of physics remain unchanged when time is reversed. For non-magnetic TIs, the electron currents exhibit this symmetry, which grants them their remarkable surface conduction properties. However, in breaking TRS, magnetic topological insulators introduce new quantum phases—one that could potentially yield transformative results for modern technology.

Magnetic topological insulators challenge the conventional understanding of TIs. Unlike their non-magnetic counterparts, the introduction of intrinsic magnetism allows for novel phenomena, such as the quantum anomalous Hall effect (QAHE), which appears when TRS is disrupted. The QAHE facilitates specific energy states that permit currents to flow with minimal resistance—an invaluable property for creating energy-efficient electronic devices. Yet, the inherent challenge is that these magnetic states are typically achieved through external magnetic fields, complicating their practicality for widespread adoption.

Professor Mahmood and his team grappled with the longstanding debate surrounding the existence of a band gap in MnBi₂Te₄. While some experimental research indicated observable gaps, conflicting studies cast doubt on these findings. In their endeavor to clarify this scientific ambiguity, the team utilized Floquet-Bloch manipulation—a state of the art technique that harnesses light to alter material properties and induce new quantum behaviors. By meticulously applying circularly polarized light to MnBi₂Te₄, the researchers successfully induced a band gap, delivering compelling evidence that aligns with theoretical predictions.

The results indicated a striking asymmetry between the responses of the material under right-circularly polarized (RCP) and left-circularly polarized (LCP) light. In the antiferromagnetic low-temperature phase, RCP light opened a gap that was nearly double the size induced by LCP light. This discrepancy in gap sizes robustly signifies the breaking of TRS. The research effectively establishes that altering the direction of light not only influences electron behavior but also has practical implications for the manipulation of quantum states.

Key to these findings is the ability to explore the electronic structure of materials through manipulation techniques such as Floquet-Bloch engineering. By applying these advanced methodologies, scientists now have a tangible way to influence the electronic properties of TIs without relying on cumbersome external fields, leading to more manageable experimental conditions. This breakthrough opens doors to further studies on varied materials and promises an expanded understanding of the mechanisms underlying quantum matter.

As the research progresses, there remains a wealth of uncharted territory awaiting exploration, particularly regarding the broader implications of manipulating magnetic TIs using advanced light techniques. The variations in band gaps identified by the research team not only highlight the interplay between magnetism and electronic states but also raise questions about the underlying mechanisms driving these behaviors.

In the pursuit of deeper insights into MnBi₂Te₄ and similar materials, the potential for real-world applications in electronic devices and quantum computing remains tantalizingly close. By deciphering the complex interactions within these systems, researchers hope to design and develop innovative technologies that could meet the growing demands of modern electronic systems.

The implications of this work extend far beyond the immediate study, as magnetic TIs like MnBi₂Te₄ promise to revolutionize the landscape of condensed matter physics and materials science. Understanding the roles of intrinsic properties like magnetism in determining material behavior sets the stage for potential breakthroughs that could lead to the next generation of electronics, emphasizing the significance of continued exploration in this exciting field.

Lastly, the findings are supported by significant federal grants and institutional support, highlighting the importance of collaborative efforts in driving forward scientific inquiry. As researchers continue to delve into the mysteries of topological insulators, the promise of uncovering further revolutionary discoveries in the physics of condensed matter remains vibrant.

Subject of Research: The hidden gap in the electronic band structure of manganese bismuth telluride (MnBi₂Te₄)
Article Title: Floquet–Bloch manipulation of the Dirac gap in a topological antiferromagnet
News Publication Date: 21-Jan-2025
Web References: https://doi.org/10.1038/s41567-024-02769-6
References: Nature Physics journal
Image Credits: Photo by Heather Coit, Illinois Grainger Engineering

Keywords

1. Topological insulators
2. Quantum anomalous Hall effect
3. Circularly polarized light
4. Manganese bismuth telluride
5. Floquet-Bloch manipulation
6. Electron band structure
7. Time-reversal symmetry