Frontiers of topological quantum matter: linked Weyl rings and ideal Weyl ferromagnets

Quantum science is driven by the synthesis of state-of-the-art quantum materials and the characterization of their exotic topological states [1-7]. In the first part of this talk, I introduce our discovery of linked Weyl rings in the room temperature Heusler ferromagnet Co2MnGa using high-resolution soft X-ray ARPES [1,2]. By combining ideas in condensed matter physics and knot theory, I explicitly draw the Weyl link diagram for the quantum state and show a linking number of (2,2,2), providing a direct experimental measurement of a new kind of topological invariant in physics. In the second part of this talk, I introduce our observation of a semimetallic Weyl ferromagnet in thin films of (Cr,Bi)2Te3 [3]. In transport, we find a record bulk anomalous Hall angle > 0.5 along with non-metallic conductivity, a regime sharply distinct from established Weyl materials and conventional ferromagnets. Together with density functional theory (DFT), our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation > 75% of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using non-equilibrium molecular beam epitaxy (MBE), we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a Murakami-type topological phase diagram with broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our discovery of a topological quantum link and semimetallic Weyl ferromagnet suggests new approaches to non-Abelian quantum states, as well as materials synthesis methods relevant to quantum technology.

1. I.B. et al. Nature 604, 647 (2022)
2. I.B. et al. Science 365, 6459 (2019)
3. I.B. et al. Nature, under review
4. Max T. Birch, I.B. et al. Nature, in press (2024)
5. M. Z. Hasan, G. Chang, I.B. et al. Nat. Rev. Mat. 6, 784 (2021)
6. D. Sanchez*, I.B.* et al. Nature 567, 500 (2019)
7. S. Xu*, I.B.* et al. Science 349, 613 (2015)

If you would like to meet with Ilya during his visit, please contact Susan LaMoreaux.