Magnetic excitons in a topological Kondo insulator YbB12

We report a time-of-flight inelastic neutron scattering experiment on a topological Kondo insulator (TKI) candidate. The spectra reveal dispersion and the Q-dependent oscillator strength of two sharp modes and a continuum throughout the Brillouin zone. The absence of intensity at the Γ point shows the excitations are not simply transitions between crystal electric field levels. Instead, they appear to be collective in-gap states of the d-f hybridized bands. The Q-dependence of the modes at \hbar ω~ 40 meV and \hbar ω~ 15 meV correspond to AFM correlations between nearest and next-nearest Yb neighbors, respectively. Starting from a phenomenological tight-binding band structure, we use the Anderson lattice model to simulate the magnetic exciton modes and determine the topological class of the TKI candidate.