Abstract
New materials and quantum phases drive next-generation optoelectronic and quantum technologies. The breakthrough synthesis of the semiconductor GaN in the early 1990s gave us the blue LED, which we use every day for lighting and electronic displays [1]. What would it take for semimetals and topological quantum materials to have the same impact? In the case of Weyl semimetals, one problem is that our Weyl materials are not actually semimetals. An intense worldwide effort starting ~2014 to search all known crystalline materials for Weyl physics gave us TaAs [2], CoSi [3], Co2MnGa [4,5] and Co3Sn2S2 [6,7]. These materials all exhibit conventional metallic resistivity, hosting intrinsic, trivial conduction electrons which obscure the unique Weyl properties. I have now instead used molecular beam epitaxy to dope the topological semiconductor Bi2Te3 with ferromagnetic Cr [8]. I find that (Cr,Bi)2Te3 exhibits a record bulk anomalous Hall angle > 0.5 (the key figure of merit) along with non-metallic conductivity, sharply distinct from earlier Weyl materials and conventional ferromagnets. Our experiments suggest that (Cr,Bi)2Te3 has a semimetallic Fermi surface composed of only two Weyl points, without irrelevant electronic states. This design principle for ‘true’ Weyl semimetals should serve as a bridge to commercial semiconductor technologies. Improving the crystalline quality should increase the figure of merit, which should enable a richer exploration of Weyl light-matter interactions, non-linear response and applications to photovoltaics and THz generation/detection. More broadly, I suggest that integrating spectroscopy, epitaxy and van der Waals/exfoliation methods can address challenges in reproducibility [9] and trigger a revolution in wafer-scale ultra-high-quality materials for quantum science & technology.

[1] I. Akasaki, H. Amano & S. Nakamura. Nobel Prize in Physics (2014)
[2] Su-Yang Xu, I.B. et al. Science 349, 613 (2015)
[3] D. S. Sanchez, I.B. et al. Nature 567, 500 (2019)
[4] I.B. et al. Nature 604, 647 (2022)

[5] I.B. et al. Science 365, 1278 (2019)
[6] Enke Liu et al. Nat. Phys. 14, 1125 (2018)
[7] I.B. et al. Phys. Rev. Lett. 127, 256403 (2021)
[8] I.B. et al. Nature 637, 1078 (2025)
[9] Chun Ning Lau et al. Nature 602, 41 (2022)