In a groundbreaking development that marks a significant milestone in the study of quantum materials, an international research team led by the Strong Correlation Quantum Transport Laboratory at the RIKEN Center for Emergent Matter Science (CEMS) has successfully synthesized an ideal Weyl semimetal. This achievement resolves a decade-old challenge in quantum material science, positioning the research team at the forefront of a highly anticipated breakthrough.
What are Weyl Fermions?
Weyl fermions, a type of quasiparticle, emerge as collective quantum excitations of electrons within certain types of crystals, and they are predicted to exhibit exotic electromagnetic properties. These properties have attracted intense research attention globally, particularly for their potential to revolutionize technologies in areas such as electronics, photonics, and quantum computing.
However, despite substantial experimental efforts to study and synthesize Weyl materials, the vast majority of known Weyl semimetals suffer from an unfortunate problem: their behavior is dominated by trivial electron states, which effectively obscure the Weyl fermions themselves. This issue has been a persistent barrier in realizing the true potential of Weyl materials.
Now, for the first time, researchers have succeeded in synthesizing a Weyl semimetal that hosts a single pair of Weyl fermions, free from irrelevant electronic states. This development represents not only a scientific breakthrough but also a new pathway to explore the unique properties of Weyl fermions in real-world materials.
The Long Road to Discovery
The paper, published in the prestigious journal Nature, stems from a four-year collaboration among several research institutions: CEMS at RIKEN, the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS), the Quantum-Phase Electronics Center (QPEC) of the University of Tokyo, the Institute for Materials Research at Tohoku University, and Nanyang Technological University in Singapore. This collaboration brought together the expertise of scientists from a wide range of disciplines to tackle a fundamental problem in quantum material research.
The breakthrough was made possible by revisiting a theoretical approach that had been proposed back in 2011 but had since been abandoned and largely forgotten by the scientific community. The team synthesized a Weyl semimetal from a topological semiconductor, a material class that has attracted considerable attention due to its special electronic properties. The researchers built upon the theoretical foundations established over a decade ago, applying innovative material engineering techniques to realize their vision.
Engineering the Weyl Semimetal
To create this ideal Weyl semimetal, the research team began with a topological semiconductor known as bismuth telluride (Bi2Te3). Semiconductors, like bismuth telluride, possess a small energy gap, which allows them to be toggled between insulating and conducting states. This duality forms the basis of traditional semiconductor technologies, such as transistors used in computers and electronic devices.
In contrast, semimetals represent a unique and extreme case in which the energy gap is effectively zero, lying at the boundary between insulators and metals. Semimetals are rare in real-world materials, with graphene being one of the most well-known examples. Graphene, composed of a single layer of carbon atoms, has many remarkable properties that have driven its use in a variety of applications, including flexible electronics and moiré physics.
The team focused on (Cr,Bi)2Te3, a compound in which the researchers substituted chromium for bismuth within the bismuth telluride crystal structure. This modification significantly altered the material’s properties, paving the way for the discovery of an ideal Weyl semimetal.
Unexpected Discoveries and New Physics
For Ryota Watanabe, a Ph.D. student and co-first author of the study, the initial intrigue began when the team noticed a large anomalous Hall effect (AHE) in the material, which suggested the presence of new physics beyond that of conventional topological semiconductors. The Hall effect is a phenomenon where a voltage develops perpendicular to an electric current in a conductor subjected to a magnetic field, and it can reveal important information about the properties of materials.
In a breakthrough realization, Ching-Kai Chi, a co-author from iTHEMS, pointed out that (Cr,Bi)2Te3 exhibited an exceptionally simple electronic structure, which allowed the researchers to explain their experimental results using precise theoretical models. The team traced the large anomalous Hall effect to the emergence of Weyl fermions in this material.
The discovery was met with surprise, as Ilya Belopolski, the study’s first author from CEMS, recalled, “Different communities had already established the key theoretical and experimental insights needed to synthesize this Weyl semimetal. But it looks like we were not communicating with one another, so we missed out on this discovery. In retrospect, it should have come about nearly a decade earlier.”
The Role of RIKEN in the Discovery
So why did this breakthrough emerge at RIKEN? According to Belopolski, the unique combination of talented researchers, generous funding, and a dynamic intellectual environment at RIKEN’s CEMS was crucial. “There were many talented research groups in the United States, China, and Europe working on related topics for many years. The reason this discovery took place here is likely because of the highly creative and collaborative atmosphere at RIKEN,” he said.
This illustrates the importance of fostering interdisciplinary collaboration in scientific research, as the diverse insights and expertise brought together by this team were key to achieving the groundbreaking result.
Applications and Future Directions
The practical implications of this discovery are vast, and the team is particularly excited about its potential use in terahertz (THz) devices. Terahertz radiation lies in the frequency range between microwave and infrared light and holds promise for a wide range of applications, including communication technologies and imaging.
Unlike conventional semiconductors, which can only absorb photons with energy greater than their energy gap, semimetals like the Weyl semimetal synthesized by this team can absorb low-frequency light down to THz frequencies. This capability opens the door to novel THz applications and lays the foundation for the development of THz generation and detection devices.
Yuki Sato, a postdoctoral researcher and co-author, highlighted the importance of this discovery, stating, “Unlike semiconductors, semimetals have a vanishing energy gap, so they can absorb low-frequency light, down to THz frequencies. We are currently interested in applying our ideal Weyl semimetal to the generation and detection of THz light.”
In addition to THz applications, the team is also exploring potential uses for the ideal Weyl semimetal in high-performance sensors, low-power electronics, and novel optoelectronic devices. As Lixuan Tai, another postdoctoral researcher, put it: “It makes this a particularly exciting time to join this research team, since having an actual Weyl semimetal available to us after all these years will surely enable many exciting breakthroughs.”
Conclusion
The successful synthesis of an ideal Weyl semimetal by the team led by RIKEN’s Strong Correlation Quantum Transport Laboratory represents a groundbreaking achievement in the field of quantum materials. By overcoming a decade-old problem, this discovery not only unveils the true potential of Weyl fermions but also opens new avenues for research and technological innovation. With the ability to absorb terahertz light and offer novel applications in sensors, low-power electronics, and optoelectronics, the ideal Weyl semimetal promises to have a significant impact on the development of advanced technologies. This breakthrough underscores the importance of interdisciplinary collaboration and highlights the potential of quantum materials to drive progress in various scientific and engineering fields. As researchers continue to explore the unique properties of Weyl semimetals, the future of quantum materials looks increasingly promising, paving the way for next-generation technologies that could revolutionize electronics and beyond.
Reference: Ilya Belopolski et al, Synthesis of a semimetallic Weyl ferromagnet with point Fermi surface, Nature (2025). DOI: 10.1038/s41586-024-08330-y