  The three-dimensional electronic structure and the nature of Ce 4f electrons
of the Kondo insulator CeRu4Sn6 are investigated by angle-resolved
photoemission spectroscopy, utilizing tunable photon energies. Our results
reveal (i) the three-dimensional k-space nature of the Fermi surface, (ii) the
localized-to-itinerant transition of f electrons occurs at a much high
temperature than the hybridization gap opening temperature, and (iii) the
"relocalization" of itinerant f electrons below 25 K, which could be the
precursor to the establishment of magnetic order.
