Make it worth Weyl: engineering the first semimetallic Weyl quantum crystal

An international team of researchers led by the Strong Correlation Quantum Transport Laboratory of the RIKEN Center for Emergent Matter Science (CEMS) has demonstrated, in a world’s first, an ideal Weyl semimetal, marking a breakthrough in a decade-old problem of quantum materials.

Weyl fermions arise as collective quantum excitations of electrons in crystals. They are predicted to show exotic electromagnetic properties, attracting intense worldwide interest. However, despite the careful study of thousands of crystals, most Weyl materials to date exhibit electrical conduction governed overwhelmingly by undesired, trivial electrons, obscuring the Weyl fermions. At last, researchers have synthesized a material hosting a single pair of Weyl fermions, and no irrelevant electronic states.

The work, published this week in Nature, arose from a collaboration over four years between CEMS, 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 of Tohoku University, and Nanyang Technological University in Singapore. The researchers engineered a Weyl semimetal from a topological semiconductor, revisiting a strategy which was first theoretically proposed in 2011, but then abandoned and largely forgotten by the community.

Semiconductors have a small ‘energy gap’ which allows them to be switched between insulating and conducting states, forming the basis for the commercial transistor. Semimetals can be viewed as a kind of extreme limit of a semiconductor with zero ‘energy gap’, right at the threshold between insulator and metal. This extreme case remains exceedingly rare in real materials. Perhaps the best-known example is graphene, which has found uses in moiré physics and flexible electronics.

The topological semiconductor used in the current study is bismuth telluride, Bi₂Te₃. The researchers adjusted the chemical composition of the material in a highly controlled way, substituting chromium for bismuth, creating (Cr,Bi)₂Te₃. According to Ryota Watanabe, Ph.D. student and co-first author of the study, “We were intrigued at first by the large anomalous Hall effect (AHE) in (Cr,Bi)₂Te₃, which signaled new physics beyond that of topological semiconductors.” Ching-Kai Chi of iTHEMS and co-author of the work, noted that, “unlike previous Weyl materials, the uniquely simple electronic structure of (Cr,Bi)₂Te₃ enabled us to quantitatively explain our experiments using a precise theory. We could then trace the large AHE back to emergent Weyl fermions.”

First author Ilya Belopolski of CEMS, recalls that the finding came as a shock to both himself and colleagues around the world. “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.”

As for why this insight ultimately emerged at RIKEN, Belopolski credits the unique combination of brilliant researchers, generous research funding and dynamic intellectual atmosphere of CEMS. “There were many talented research groups in the United States, China and across Europe working on related topics for many years. The reason this discovery took place here is likely because of the highly creative and collaborative environment at RIKEN.”

One potential application is in terahertz (THz) devices. Semiconductors can only absorb photons with energy greater than their energy gap, which typically rules out the THz frequency range. According to Yuki Sato, postdoctoral researcher and co-author of the work, “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.”

The team further anticipates research into high-performance sensors, low-power electronics, and novel optoelectronics devices. Postdoctoral researcher Lixuan Tai, who joined the Strong Correlation Quantum Transport Laboratory as this work was nearing publication, expressed excitement about the near-term research enabled by this new quantum phase of matter. “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.”