Antichiral spin order, its soft modes, and their hybridization with phonons in the topological semimetal Mn3Ge

We report the magnetic structure and spin excitations of Mn3Ge, a breathing kagome antiferromagnet with transport anomalies attributed to Weyl nodes. Using polarized neutron diffraction, we show the magnetic order is a k = 0 coplanar state belonging to a Gamma(9) irreducible representation, which can be described as a perfect 120 degrees antichiral structure with a moment of 2.2(1) mu(B)/Mn, superimposed with weak collinear ferromagnetism. Inelastic neutron scattering shows three collective Q = 0 excitations at Delta(1) = 2.9(6) meV, Delta(2) = 14.6(3) meV, and Delta(3) = 17.5(3) meV. A field theory of Q approximate to 0 spin waves in triangular antiferromagnets with a 120 degrees spin structure was used to classify these modes. The in-plane mode (alpha) is gapless, Delta(1) is the gap to a doublet of out-of-plane spin excitations (beta(x), beta(y)), and Delta(2), Delta(3) result from hybridization of optical phonons with magnetic excitations. While a phenomenological spin Hamiltonian including exchange interactions, Dzyaloshinskii-Moriya interactions, and single-ion crystal field terms can describe aspects of the Mn-based magnetism, spin-wave damping [Gamma = 25(8) meV] and the extended range of magnetic interactions indicate itinerant magnetism consistent with the transport anomalies.