Rise and Fall of Landau's Quasiparticles While Approaching the Mott Transition

Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m*. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Crucial information can be obtained by frequency-resolved probes that measure the complex optical conductivity σ₁(ω) + iσ₂(ω). Here we explore the electrodynamic response of correlated metals at half filling upon approaching a Mott insulator. The correlation strength U/W is varied by partial chemical substitution. We reveal persistent Fermi-liquid behavior with T² and ω² dependences of the optical scattering rate γ(ω), along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe-Regel-Mott limit ρ ≫ ρ_IRM is accompanied by a 'displaced Drude peak' in σ₁(ω). Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.