  The recent discovery of topological Kondo insulating behaviour in strongly
correlated electron systems has generated considerable interest in Kondo
insulators both experimentally and theoretically. The Kondo semiconductors
CeT2Al10 (T=Fe, Ru and Os) possessing a c-f hybridization gap have received
considerable attention recently because of the unexpected high magnetic
ordering temperature of CeRu2Al10 (TN=27 K) and CeOs2Al10 (TN=28.5 K) and the
Kondo insulating behaviour observed in the valence fluctuating compound
CeFe2Al10 with a paramagnetic ground state down to 50 mK. We are investigating
this family of compounds, both in polycrystalline and single crystal form,
using inelastic neutron scattering to understand the role of anisotropic c-f
hybridization on the spin gap formation as well as on their magnetic
properties. We have observed a clear sign of a spin gap in all three compounds
from our polycrystalline study as well as the existence of a spin gap above the
magnetic ordering temperature in T=Ru and Os. Our inelastic neutron scattering
studies on single crystals of CeRu2Al10 and CeOs2Al10 revealed dispersive
gapped spin wave excitations below TN. Analysis of the spin wave spectrum
reveals the presence of strong anisotropic exchange, along the c-axis (or
z-axis) stronger than in the ab-plane. These anisotropic exchange interactions
force the magnetic moment to align along the c-axis, competing with the single
ion crystal field anisotropy, which prefers moments along the a-axis. In the
paramagnetic state (below 50 K) of the Kondo insulator CeFe2Al10, we have also
observed dispersive gapped magnetic excitations which transform into
quasi-elastic scattering on heating to 100 K. We will discuss the origin of the
anisotropic hybridization gap in CeFe2Al10 based on theoretical models of
heavy-fermion semiconductors.
