Emergent Landau Levels of Topological Magnons in a Skyrmion Lattice

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 investigated the propagation of magnons, i.e., bosonic collective spin excitations, in a lattice of skyrmion tubes in MnSi using polarized inelastic neutron scattering. For wavevectors perpendicular to the skyrmion tubes the magnon spectra bear strong witness of the formation of finely spaced emergent Landau levels that are characteristic of the fictitious magnetic field used to account for the non-trivial topological winding of the skyrmion lattice. This provides microscopic evidence of a topological magnon band structure in reciprocal space, which is born out of the non-trivial real-space topology of a magnetic order.