Discovery of topological magnetic Weyl loops by spectroscopy and transport

There is a great zoo of topological electronic states in quantum matter, comprising the quantum Hall state, topological insulators, Weyl semimetals and other exotic states. In this talk, I'll introduce a new animal in this zoo: the magnetic Weyl loop [1,2]. I'll describe the electronic dispersion of this object, discuss its π Berry phase topological invariant and argue that it should arise naturally in ferromagnets with mirror symmetry. Then I'll present our recent experiments using angle-resolved photoemission spectroscopy (ARPES) on two such ferromagnets, Co₂MnGa and Co₃Sn₂S₂. We directly observe the characteristic cone dispersions of Weyl loops on the bulk Brillouin zone mirror planes. We can further measure the energy dispersions of the Weyl loops, as well as map out their trajectories in momentum space and even observe topological drumhead surface states. Weyl loops are typically expected to gap out under finite spin-orbit coupling, leaving behind a concentrated Berry curvature distribution which can drive anomalous transport. I'll provide evidence that Weyl loops drive the giant anomalous Hall and Nernst effects in Co₂MnGa and Co₃Sn₂S₂. I'll also comment on the relationship of the Weyl loop with the Weyl points expected in Co₃Sn₂S₂. Broadly, I'll argue that Weyl loops naturally provide a powerful mechanism for generating large, robust topological response. Time permitting, I'll comment on related exotic phenomena in other topological magnets.

[1] Ilya Belopolski et al. Science 365, 1278 (2019)
[2] Ilya Belopolski et al. arXiv:2005.02400 (2020)