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 $m^{\star}$. 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 $\sigma_1(\omega)$ + i$\sigma_2(\omega)$ [1].
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 (Fig. 1) [2-4]. We reveal persistent Fermi-liquid behavior with $T^2$ and $\omega^2$ dependences of the optical scattering rate $\gamma(\omega)$ (Fig. 2a-c), along with a puzzling elastic contribution to relaxation [5]. The strong increase of the resistivity beyond the Ioffe-Regel-Mott limit $\rho\gg\rho_{\rm IRM}$ (Fig. 1a) is accompanied by a 'displaced Drude peak' in $\sigma_1(\omega)$ (magenta arrow in Fig. 2d). 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 [5].

[1] D. Basov et al., Rev. Mod. Phys. 83, 471 (2011)
[2] A. Pustogow et al., Nat. Mater. 17, 773-777 (2018)
[3] A. Pustogow et al., npj Quantum Mater. 6, 9 (2021)
[4] Y. Saito et al., J. Mater. Chem. C (2021), DOI: 10.1039/D1TC00785H
[5] A. Pustogow et al., Nat. Commun. 12, 1571 (2021)