Signatures of coupling between spin waves and Dirac fermions in YbMnBi2

We present inelastic neutron scattering measurements of magnetic excitations in YbMnBi2, which reveal features consistent with a direct coupling of magnetic excitations to Dirac fermions. In contrast with the large broadening of magnetic spectra observed in antiferromagnetic metals such as the iron pnictides, here the spin waves exhibit a small but resolvable intrinsic width, consistent with our theoretical analysis. The subtle manifestation of spin-fermion coupling is a consequence of the Dirac nature of the conduction electrons, including the vanishing density of states near the Dirac points. Accounting for the Dirac fermion dispersion specific to YbMnBi2 leads to particular signatures, such as the nearly wave-vector-independent damping observed in the experiment.