Zero-Energy Excitation In The Classical Kagome Antiferromagnet Naba2mn3f11

We performed inelastic neutron scattering measurements on a polycrystalline sample of a classical kagome antiferromagnet NaBa2Mn3F11 to investigate the possibility of a dispersionless zero-energy excitation associated with rotation of spins along the chains. The observed spectra indeed exhibit such an excitation with strong intensity at low energy, as well as dispersive excitations with weak intensity at high energy. Combining the measurements with calculations from linear spin-wave theory reveals that NaBa2Mn3F11 is a good realization of the classical kagome antiferromagnet which exhibits a dispersionless mode lifted by the magnetic dipole-dipole interaction.