Superconductivity is critical for a variety of advanced technologies, where efficient energy transfer is essential. Cooper pairs, which consist of pairs of electrons bound together at low temperatures, play a fundamental role in the emergence of superconductivity. The energy required to break apart these Cooper pairs is known as the superconducting gap, and the behavior of this gap is pivotal in determining the operational efficacy of superconductors. Traditionally, the larger the superconducting gap, the more likely it is for superconductivity to persist at higher temperatures, making it indispensable for accessible technological applications. This study emphasizes the importance of controlling the superconducting gap, particularly in light of demands for improving the functionality of quantum devices.

Historically, attempts to manipulate the superconducting gap have been concentrated on controlling the physical properties at the real-space level, focusing on where particles are situated within the material. However, efforts to achieve similar levels of control within momentum space—a framework that represents the energy states of a system—have proven elusive until now. The ability to fine-tune the superconducting gap in momentum space is seen as a necessary step to escalate the development of superconductors and their applications in quantum computing, essentially a prerequisite for the next generation of high-performance superconducting materials.

To unveil this potential, the research team focused on ultrathin layers of niobium diselenide (NbSe2), a well-regarded superconductor, laid upon a graphene substrate. By employing state-of-the-art imaging and fabrication techniques, notably spectroscopic-imaging scanning tunneling microscopy coupled with molecular beam epitaxy, the researchers were able to precisely vary the twist angles of these layers. This delicate adjustment resulted in measurable alterations in the superconducting gap as observed within momentum space. This key observation introduces a previously unexplored method for tuning superconducting properties, paving the way for vast enhancements in material design and function.

Masahiro Naritsuka, the study’s lead author, noted that twisting the ultra-thin layers provides an exquisite control mechanism over superconductivity by selectively adjusting the superconducting gap across targeted regions within momentum space. Among the striking discoveries from this research were the emergence of unique flower-like modulation patterns within the superconducting gap, patterns that do not align with the crystallographic axes of either niobium diselenide or graphene. This unexpected finding highlights the pivotal role that twisting plays in influencing superconducting properties, a nuance that may have significant implications for designing future superconducting materials.

The research team’s findings not only deepen the fundamental understanding of how superconducting systems interact across layers but also mark a critical step toward the engineering of superconductors that exhibit tailored properties. By controlling the superconducting gap through twists, the researchers have laid the groundwork for future innovations that could lead to more energy-efficient technologies and groundbreaking advances in quantum computing. Tetsuo Hanaguri, a senior author of the paper, emphasizes that this research opens the door to further inquiries, particularly concerning the integration of magnetic layers into these structures. Such additions could enable selectivity in both spin and momentum, thereby unveiling entirely new research avenues in the field of superconductivity.

As scientists delve deeper into understanding the complex interplay of factors affecting superconductivity, the implications of this research are vast. The ability to manipulate superconducting properties through twisting may revolutionize not only the materials engineering landscape but also the design and function of devices that rely on superconductivity. By enhancing energy efficiency and lowering operational thresholds, the potential applications of these findings could extend into various domains, including power transmission, electromagnetic enhancements, and next-generation quantum computing hardware.

Moreover, continued exploration into the integration of magnetic elements into this framework may lead to materials that exhibit both superconductive and magnetic properties concurrently, vastly expanding the capabilities of conventional superconductors. This multidisciplinary approach to material science could yield breakthroughs that transcend current limitations, culminating in the practical application of superconductors in areas previously thought impossible.

As contemporary challenges in energy consumption and computation intensify, the relevance of such research becomes ever more critical. Innovations in superconducting materials are not merely theoretical exercises; they represent tangible solutions to the world’s growing energy demands. The journey toward high-temperature superconductors that operate at ambient conditions may still be in its infancy, but findings like those from the RIKEN CEMS team serve as vital stepping stones on this path.

In conclusion, the research conducted at the RIKEN Center for Emergent Matter Science exemplifies a significant leap forward in our capability to control superconductivity through strategic manipulation of material properties. As scientists harness these emergent techniques, the implications on a global scale could translate into benefits that extend beyond mere energy efficiency. With continued research, we are poised to unravel even more about superconductivity and its transformative potential for technology and society at large.

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Subject of Research:
Article Title: Superconductivity controlled by twist angle in monolayer NbSe2 on graphene
News Publication Date: 20-Mar-2025
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References: DOI: 10.1038/s41567-025-02828-6
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Keywords

Superconductivity, Quantum Computing, Materials Engineering, Niobium Diselenide, Graphene, Energy Efficiency

Despite extensive research on a multitude of crystalline materials, most Weyl materials hitherto discovered have been overwhelmed by the influence of trivial electrons that obscure the presence of Weyl fermions. The successful synthesis of a material that supports a single pair of Weyl fermions without the interference of irrelevant electronic states represents not only a significant scientific breakthrough but also a culmination of years of theoretical predictions and experimental endeavors.

The research, published in the esteemed journal Nature, is the result of a four-year collaborative effort involving CEMS, the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS), the Quantum-Phase Electronics Center (QPEC) at the University of Tokyo, the Institute for Materials Research at Tohoku University, and Nanyang Technological University in Singapore. The team ingeniously transformed a topological semiconductor into a Weyl semimetal, revisiting a strategy that had been theorized in 2011 but subsequently fell into relative obscurity within the scientific community.

Topological semiconductors, characterized by a small energy gap, can transition between insulating and conducting states. On the other hand, semimetals exist at the very brink of this transition, possessing a unique zero energy gap. This characteristic is exceedingly rare in natural materials, with graphene often cited as a prime example of a material featuring similar properties, particularly regarding its applications in flexible electronics and moiré physics.

The core material used in this groundbreaking study is bismuth telluride (Bi₂Te₃), a well-known topological semiconductor. Researchers carefully manipulated the chemical composition of the material by substituting chromium for bismuth, creating a compound denoted as (Cr,Bi)₂Te₃. This meticulous adjustment of the material’s properties allowed the team to unravel new physics beyond previously established topological semiconductor behavior, as evidenced by the observation of a large anomalous Hall effect (AHE).

The AHE observed in (Cr,Bi)₂Te₃ is particularly noteworthy, as it enables researchers to delve deeper into the material’s electronic structure. This uniquely simple electronic configuration has empowered the research team to quantitatively correlate their experimental results with theoretical predictions, thereby establishing a clear link between the large AHE and the emergent Weyl fermions. This connection signifies a pivotal moment in understanding quantum materials and their potential applications.

Leading author Ilya Belopolski expressed surprise at the discovery, noting that different research communities had already developed the necessary theoretical and experimental knowledge to synthesize this Weyl semimetal but had not effectively communicated. The success of this research illustrates the importance of collaboration across disciplines and highlights how missed opportunities can arise in the absence of dialogue between different scientific fields.

Belopolski attributed the emergence of this critical insight to the unique atmosphere fostered at RIKEN, where brilliant researchers come together in a creatively stimulating environment. The collaboration between talented research groups from various countries exemplifies the global pursuit of scientific knowledge and underlines how a collaborative approach can lead to significant breakthroughs that might otherwise remain unrealized.

One of the most exciting potential applications of this newly discovered Weyl semimetal lies in terahertz (THz) technology. Classical semiconductors are generally unable to absorb photons below certain energy thresholds dictated by their energy gaps. However, semimetals, with their zero energy gap, can effectively absorb light across the THz frequency range. This unique property positions Weyl semimetals as promising candidates for creating and detecting THz light, opening doors to potential advancements in communication technologies and sensor applications.

The implications of this discovery extend beyond just terahertz applications, as the research team anticipates exploration into high-performance sensors, low-power electronics, and innovative optoelectronic devices. The enthusiasm surrounding the prospects of this new quantum phase of matter embodies the dynamic research atmosphere at CEMS, where emerging technologies continuously push the boundaries of material science.

Lixuan Tai, a postdoctoral researcher who joined the Strong Correlation Quantum Transport Laboratory close to the publication of the findings, expressed exhilaration regarding the opportunities that this new Weyl semimetal presents for ongoing and future research. The team is poised to leverage the characteristics of this material to further explore its unique phases and properties, potentially sparking a wave of discoveries in quantum materials.

As researchers continue to delve into the properties of the ideal Weyl semimetal, they anticipate a rich landscape of inquiry that will lead to new methodologies and technological innovations. The intersection of theory and experimentation in this context illustrates the remarkable progress being made in the understanding of quantum materials, a field that will undoubtedly yield significant advancements in science and technology for years to come.

The synthesis of the ideal Weyl semimetal thus represents a transformative achievement in the realm of quantum transport and materials science. It paves the way for further exploration and understanding of Weyl fermions and their associated electromagnetic properties, signifying a potential turning point in how researchers approach the study of quantum materials and highlights the value of collaboration in unlocking the mysteries of the universe.

As the research community continues to build upon this foundation, the exciting prospects for the development of new devices, sensors, and methodologies driven by the unique properties of this Weyl semimetal will likely be a central theme in future scientific discourse. This breakthrough not only illustrates the potential of quantum materials but also serves as an exemplar of what can be achieved through sustained collaboration and innovative thinking in scientific research.

Subject of Research: Quantum Materials
Article Title: Synthesis of a semimetallic Weyl ferromagnet with point Fermi surface
News Publication Date: 22-Jan-2025
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