Dispersive magnetic-resonance mode in the Kondo semiconductor CeFe 2 Al 10

The Ce${T}_{2}$Al${}_{10}$ family of orthorhombic compounds exhibits a very peculiar evolution from a Kondo insulator ($T$: Fe) to an unconventional long-range magnetic order ($T$: Ru, Os). Inelastic neutron-scattering experiments performed on single-crystal CeFe${}_{2}$Al${}_{10}$ reveal that this material develops a spin gap in its magnetic spectral response below $\ensuremath{\sim}$50 K, with a magnetic excitation dispersing from $E=10.2\ifmmode\pm\else\textpm\fi{}0.5$ meV at the $Y$ zone-boundary point [$\mathbit{q}=(0,1,0)$] to $\ensuremath{\approx}$12 meV at the top of the branch. The excitation shows a pronounced polarization of the magnetic fluctuations along $a$, the easy anisotropy axis. Its behavior is contrasted with that of the (magnonlike) modes previously reported for CeRu${}_{2}$Al${}_{10}$, which have transverse character and exist only in the antiferromagnetic state. The present observation is ascribed to a ``magnetic exciton'' mechanism invoked to explain a similar magnetic response previously discovered in YbB${}_{12}$.