Lifshitz transition from valence fluctuations in YbAl3

TL;DR

This study demonstrates that valence fluctuations in YbAl3 induce a Lifshitz transition, significantly altering the Fermi surface topology and electronic structure, advancing understanding of mixed-valence Kondo lattice systems.

Contribution

It provides the first direct experimental evidence linking valence fluctuations to Fermi surface topology changes via ARPES in YbAl3.

Findings

Valence fluctuations cause a Lifshitz transition in YbAl3.

A small electron pocket disappears as temperature decreases.

Yb 4f states become more itinerant with temperature change.

Abstract

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.