  Using angle-resolved photoemission spectroscopy (ARPES) and resonant ARPES,
we report evidence of strong anisotropic conduction-f electron mixing (c-f
mixing) in CeBi by observing a largely expanded Ce-5d pocket at low
temperature, with no change in the Bi-6p bands. The Fermi surface (FS)
expansion is accompanied by a pronounced spectral weight transfer from the
local 4f 0 peak of Ce (corresponding to Ce3+) to the itinerant conduction bands
near the Fermi level. Careful analysis suggests that the observed large FS
change (with a volume expansion of the electron pocket up to 40%) can most
naturally be explained by a small valence change (~ 1%) of Ce, which coexists
with a very weak Kondo screening. Our work therefore provides evidence for a FS
change driven by real charge fluctuations deep in the Kondo limit, which is
made possible by the low carrier density.
