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 on this issue can be obtained by frequency-resolved probes that measure the complex optical conductivity .
Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator [2,3]. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation . The strong increase of the resistivity beyond the Ioffe-Regel-Mott limit is accompanied by a `displaced Drude peak' in the optical conductivity. 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 .
 Rev. Mod. Phys. 83, 471 (2011)
 Nat. Mater. 17, 773-777 (2018)
 npj Quantum Mater. 6, 9 (2021); arXiv:1911.06766
 accepted at Nat. Commun. (2021), arXiv:2101.07201