Researchers have made a significant advancement by demonstrating an ideal Weyl semimetal, solving a problem that has persisted for nearly ten years in the realm of quantum materials.
An international team led by the Strong Correlation Quantum Transport Laboratory at the RIKEN Center for Emergent Matter Science (CEMS) has achieved a world-first by creating an ideal Weyl semimetal, marking a major milestone in a long-standing challenge in quantum materials.
Weyl fermions are collective quantum excitations of electrons found in crystals, theorized to exhibit unique electromagnetic properties, which have sparked considerable global interest. Nevertheless, despite extensive investigations into thousands of crystal structures, most known Weyl materials primarily exhibit electrical conduction dominated by unwanted, trivial electrons, which obscure the presence of Weyl fermions. Finally, researchers have successfully synthesized a material that contains a singular pair of Weyl fermions without any irrelevant electronic states.
This study, which has been detailed in the latest issue of Nature, is the result of a four-year collaboration between CEMS, the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS), the Quantum-Phase Electronics Center (QPEC) at the University of Tokyo, Tohoku University’s Institute for Materials Research, and Nanyang Technological University in Singapore. The team developed a Weyl semimetal from a topological semiconductor by revisiting a strategy originally proposed in 2011 that had been set aside and forgotten.
Semiconductors are characterized by a small ‘energy gap’ that allows them to switch between insulating and conducting states, forming the foundation of modern transistors. In contrast, semimetals can be seen as a special case of semiconductors with a zero ‘energy gap’, sitting right at the boundary between insulators and metals. Such materials are exceptionally rare in the real world, with graphene being one of the most well-known examples, used in moiré physics and flexible electronics.
The topological semiconductor studied here is bismuth telluride, noted as Bi2Te3. The researchers methodically altered the chemical composition by substituting chromium for bismuth, resulting in (Cr,Bi)2Te3. Ryota Watanabe, a Ph.D. student and co-first author of the study, mentioned, “At first, we were fascinated by the significant anomalous Hall effect (AHE) in (Cr,Bi)2Te3, which hinted at new physics beyond traditional topological semiconductors.” Collaborator Ching-Kai Chi from iTHEMS highlighted that, “Differing from previous Weyl materials, the remarkably straightforward electronic structure of (Cr,Bi)2Te3 allowed us to explain our experimental findings using a precise theoretical approach. This enabled us to link the large AHE directly to the emergence of Weyl fermions.”
Ilya Belopolski, the first author of the paper from CEMS, expressed surprise at the discovery, recalling how both he and his colleagues had not anticipated the outcome. “Various research groups had already established critical theoretical and experimental knowledge required for creating this Weyl semimetal. However, a lack of communication led us to overlook this significant finding. Looking back, it seems this discovery should have happened nearly a decade ago.”
Regarding why this breakthrough occurred at RIKEN, Belopolski attributes it to the exceptional combination of brilliant minds, substantial funding, and an intellectually stimulating environment at CEMS. “Numerous talented groups in the United States, China, and across Europe have been focused on similar subjects for years. The reason this discovery happened here likely stems from RIKEN’s highly creative and collaborative setting.”
One promising application for this discovery is in terahertz (THz) devices. Standard semiconductors can only absorb photons with energy above their energy gap, which generally excludes the THz frequency range. Yuki Sato, a postdoctoral researcher and co-author, stated, “Unlike semiconductors, semimetals possess a negligible energy gap, allowing them to absorb low-frequency light, including THz frequencies. We are keen on utilizing our ideal Weyl semimetal for generating and detecting THz light.”
The team is also looking forward to exploring high-performance sensors, low-power electronics, and innovative optoelectronic devices. Postdoctoral researcher Lixuan Tai, who joined the Strong Correlation Quantum Transport Laboratory just as this work was being published, shared her enthusiasm about the new research opportunities presented by this quantum phase of matter. “Joining this research team at such an exciting time, as we finally have access to a true Weyl semimetal after all these years, is sure to lead to many fascinating breakthroughs.”
