Spin fluctuations in the anisotropic Kondo insulator CeRu4Sn6

We report and model anisotropic quasi-elastic magnetic neutron scattering from single crystalline CeRu$_4$Sn$_6$. For $T\approx 2$K the magnetic neutron scattering is broad in momentum ($\bf Q$) with a persistent $1/ \hbar \omega$ spectrum throughout the Brillouin zone. This indicates a lack of spatial coherence and no characteristic energy scale beyond the 0.2 meV resolution of the measurement. We find the Q-dependence of the scattering can be modeled by a Kondo-Heisenberg Hamiltonian that describes residual carriers and incompletely compensated localized electrons. These findings support the interpretation of tetragonal CeRu4Sn6 as an anisotropic or nodal Kondo insulator, markedly different from typical cubic Kondo insulators. We further discuss potential topological implications.