Large Fermi surface expansion through anisotropic mixing of conduction and f electrons in the semimetallic Kondo lattice CeBi

Using angle-resolved photoemission spectroscopy (ARPES) and resonant ARPES, we report evidence of strong anisotropic conduction-$f$ electron mixing ($c\text{\ensuremath{-}}f$ mixing) in CeBi by observing a largely expanded Ce $5d$ pocket at low temperature, with no change in the Bi $6p$ bands. The anisotropic Fermi surface (FS) expansion is accompanied by a pronounced spectral weight transfer from the local $4{f}⁰$ peak of Ce (corresponding to $\mathrm{C}{\mathrm{e}}³⁺$) 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 $(\ensuremath{\sim}1%)$ 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 highly dependent on the orbital character and momentum and is made possible by the low carrier density.