Spectroscopic Studies of the Topological Magnon Band Structure in a Skyrmion Lattice

When magnons propagate across a non-collinear magnetic texture, they collect a Berry phase that can be interpreted as the flux of a synthetic orbital magnetic field. This Berry flux is related to the topological skyrmion density of the magnetic texture. For a topologically non-trivial skyrmion configuration, the magnon thus experiences a synthetic Lorentz force that bends the quasi-classical trajectories of the magnons. For a skyrmion lattice, the average synthetic field is finite such that the magnons are confined to cyclotron orbits of the corresponding magnon Landau levels. This is reflected in a magnon band structure with finite Chern numbers. Here, we report on a series of spectroscopic studies of this band structure using various techniques that allow to probe different regimes in energy-momentum space: magnetic resonance spectroscopy [1], spin-wave spectroscopy [2], inelastic neutron scattering [3] and Brillouin light scattering [4,5]. These techniques have been applied to various cubic chiral magnets like MnSi, FeCoSi and Cu₂OSeO₃, that are well described by a universal effective continuum theory parametrized by a few parameters only. The combined data from all techniques are found to be in quantitative agreement with theory.<br/><br/>[1] R. Takagi, M. Garst, J. Sahliger, C. H. Back, Y. Tokura, S. Seki, Phys. Rev. B <b>104</b>, 144410 (2021).<br/>[2] S. Seki, M. Garst, J. Waizner, R. Takagi, N. D. Khanh, Y. Okamura, K. Kondou, F. Kagawa, Y. Otani, Y. Tokura, Nat. Commun. <b>11</b>, 256 (2020).<br/>[3] T. Weber, D. M. Fobe, J. Waizner, P. Steffens, G. S. Tucker, M. Boehm, L. Beddrich, C. Franz, H. Gabold, R. Bewley, D. Voneshen, M. Skoulatos, R. Georgii, G. Ehlers, A. Bauer, C. Pfleiderer, P. Boeni, M. Janoschek, M. Garst, Science <b>375</b>, 1025 (2022).<br/>[4] N. Ogawa, L. Koehler, M. Garst, S. Toyoda, S. Seki, Y. Tokura, PNAS 118, e2022927118 (2021).<br/>[5] P. Che, R. Ciola, V. Kravchuk, M. Garst, D. Grundler, unpublished.