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Chiral spin interactions are of great interest as a route to multiferroic behavior, unconventional superconducting order parameters and magnetic structures with non-trivial particle-like properties. This has raised the basic question of the Goldstone modes in the simplest kinds of magnetic materials: cubic metals in which a hierarchy of energy scales ranging from ferromagnetic exchange, over chiral interactions to very weak pinning stabilizes a helimagnetic spin spiral. For these systems a new class of magnetic Goldstone modes has been predicted, so-called helimagnons [1,2]. We report inelastic neutron scattering of the low-lying collective spin excitations in the itinerant-electron helimagnet MnSi. In the vicinity of the helical propagation vector we observe an anisotropic dispersion, which demonstrates the predicted sliding and meandering of the helix with respect to its propagation direction. This provides first evidence for the formation of a new class of Goldstone modes in the presence of chiral spin interactions. [1] D. Belitz, T. R. Kirkpatrick, and A. Rosch Phys. Rev. B 73, 054431 (2006), ibid Phys. Rev. B 74, 024409 (2006). [2] S. V. Maleyev, Phys. Rev. B 73, 174402 (2006). work in collaboration with: A. Rosch, F. Bernlochner, S. Dunsiger, C. Pfleiderer, P. Böni, B. Roessli, P. Link |
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