Yann Alexanian : An ARPES perspective on layered ruthenates beyond the Fermi liquid paradi

The standard description of metals relies on Fermi-liquid theory, which describes a class of interacting electron systems whose elementary excitations – quasiparticles – are adiabatically connected to the electron and hole excitations of an ideal Fermi gas. In strongly correlated electron systems, however, strong Coulomb interactions give rise to physics beyond such Fermi-liquid paradigm. While the most striking manifestation is the Mott insulating state, a variety of unconventional metallic phases have also been observed, mainly through deviations from the Fermi-liquid resistivity law. The microscopic nature of these non-Fermi-liquid states – and whether they support well-defined quasiparticle excitations – remains a longstanding challenge in condensed-matter physics.

I will start this presentation with a pedagogical introduction to Fermi liquids and strongly correlated electron systems, emphasizing how angle-resolved photoemission spectroscopy (ARPES) provides direct access to the electronic excitations and quasiparticles that underpin these phenomena. I will then present a journey through three different studies of the layered perovskite ruthenates Sr2RuO4 and Sr3Ru2O7, exploring regimes that deviate from their common ultra-clean Fermi-liquid ground state.

We first used ARPES to study the fate of quasiparticles in Sr2RuO4 during the crossover from the Fermi liquid to a bad metallic state, characterized by a resistivity that increases with temperature exceeding the Mott–Ioffe–Regel limit expected for conventional metals. In contrast to previous ARPES experiments, we show that quasiparticles do not disappear with increasing temperature but eventually lose coherence from excessive broadening.

We further investigate the evolution of the quasiparticles of Sr2RuO4 under uniaxial strain, including the regime in which recent transport measurements observed non-Fermi liquid behavior. To this end, we introduce a new method for ARPES experiments under continuously varying strain. Our data monitors the tuning of a van Hove singularity across the chemical potential and shows that quasiparticles remain well-defined in the non-Fermi liquid state at the critical strain. This implies that non-Fermi liquid behavior emerges in Sr2RuO4 from subtle changes in the scattering rate rather than from a breakdown of the quasiparticle picture.

In the last ongoing study, we focus on Sr3Ru2O7, whose rich phase diagram hosts metamagnetism, criticality, and departures from conventional Fermi-liquid behavior. Using precision-ARPES, we first investigate the microscopic origins of its strongly renormalized Fermi-liquid state. We then track the evolution of the electronic structure with temperature across the Fermi-liquid to strange-metal crossover, aiming to identify the mechanisms underlying the emergence of this anomalous metallic behavior