  Heavy fermion materials gain high electronic masses and expand Fermi surfaces
when the high-temperature localized f electrons become itinerant and hybridize
with the conduction band at low temperatures. However, despite the common
application of this model, direct microscopic verification remains lacking.
Here we report high-resolution angle-resolved photoemission spectroscopy
measurements on CeCoIn5, a prototypical heavy fermion compound, and reveal the
long-sought band hybridization and Fermi surface expansion. Unexpectedly, the
localized-to-itinerant transition occurs at surprisingly high temperatures, yet
f electrons are still largely localized at the lowest temperature. Moreover,
crystal field excitations likely play an important role in the anomalous
temperature dependence. Our results paint an comprehensive unanticipated
experimental picture of the heavy fermion formation in a periodic multi-level
Anderson/Kondo lattice, and set the stage for understanding the emergent
properties in related materials.
