  In Kondo lattice systems with mixed valence, such as YbAl3, interactions
between localized electrons in a partially filled f shell and delocalized
conduction electrons can lead to fluctuations between two different valence
configurations with changing temperature or pressure. The impact of this change
on the momentum-space electronic structure and Fermi surface topology is
essential for understanding their emergent properties, but has remained
enigmatic due to a lack of appropriate experimental probes. Here by employing a
combination of molecular beam epitaxy (MBE) and in situ angle-resolved
photoemission spectroscopy (ARPES) we show that valence fluctuations can lead
to dramatic changes in the Fermi surface topology, even resulting in a Lifshitz
transition. As the temperature is lowered, a small electron pocket in YbAl3
becomes completely unoccupied while the low-energy ytterbium (Yb) 4f states
become increasingly itinerant, acquiring additional spectral weight, longer
lifetimes, and well-defined dispersions. Our work presents the first unified
picture of how local valence fluctuations connect to momentum space concepts
including band filling and Fermi surface topology in the longstanding problem
of mixed-valence systems.
