  The motion of a spin excitation across topologically non-trivial magnetic
order exhibits a deflection that is analogous to the effect of the Lorentz
force on an electrically charged particle in an orbital magnetic field. We used
polarized inelastic neutron scattering to investigate the propagation of
magnons (i.e., bosonic collective spin excitations) in a lattice of skyrmion
tubes in manganese silicide. For wave vectors perpendicular to the skyrmion
tubes, the magnon spectra are consistent with the formation of finely spaced
emergent Landau levels that are characteristic of the fictitious magnetic field
used to account for the nontrivial topological winding of the skyrmion lattice.
This provides evidence of a topological magnon band structure in reciprocal
space, which is borne out of the nontrivial real-space topology of a magnetic
order.
