Emergence of helimagnon bands in MnSi

Recent theoretical studies predict that the broken inversion symmetry in the helical phase of MnSi will lead to a rich spectrum of helimagnons for wave vectors that are small compared to the helical propagation vector $\vec{k}$. Our extensive inelastic neutron scattering study in the helical phase shows the existence of broad dispersive excitations. Using a parameter free model we quantitatively establish that these excitations represent broad spin wave bands that are purely caused by the tiny magnetic propagation vector. The small magnetic Brillouin zone leads to multiple Umklapp interactions and thus many helimagnon modes. Our study provides a tractable show-case how collective spin excitations may be radically modified even in simple systems by seemingly harmless small magnetic propagation vectors.