Ideal semimetallic Weyl ferromagnet finally realized experimentally
By Department of Physics & Astronomy
A team of researchers at the RIKEN Institute in Japan has realized an ideal Weyl semimetal, theoretically predicted in 2011 by University of Waterloo’s faculty Anton Burkov.
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.
Massless Weyl fermions are the building blocks of the Standard Model of particle physics, but are not observed in the high-energy physics context due to mass generation. Surprisingly, they can arise as quantum excitations (quasiparticles) 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 recently 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 by a University of Waterloo faculty physics professor Anton Burkov, but then abandoned and almost 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 all modern electronics. 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, the only other known example being graphene, which has found uses in 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)2Te . This new material is a ferromagnet and, as the researchers were able to convincingly demonstrate, an ideal Weyl semimetal with a pair of opposite-chirality massless Weyl fermions and no trivial states at the Fermi energy.
Ideal Weyl semimetals, such as the one realized in this work, are anticipated to have multiple practical applications in high-performance quantum sensors, low-power electronics, and novel optoelectronics devices.
Anton Burkov has been a faculty member at the Department of Physics and Astronomy, University of Waterloo, since 2007. He received his PhD from Indiana University in 2002 and has held postdoctoral appointments at University of California in Santa Barbara and Harvard University. He is also an Associate Faculty at Perimeter Institute for Theoretical Physics. He is interested in the physics of quantum materials, especially in the interplay of topology, disorder and electron-electron interactions in gapless quantum materials, such as Weyl semimetals.