Incoherent charge dynamics of La_2-xSr_xCuO₄: Dynamical localization and resistivity saturation

We performed the systematic optical and transport experiment for La_2-xSr_xCuO₄ at high temperatures. The in-plane resistivity ρ_ab(T) saturates not at the classical Ioffe-Regel-Mott limit but at much higher value. The in-plane optical conductivity σ_ab(ω) exhibits at high temperatures a less characteristic, nearly flat spectrum over a wide energy range up to 1 eV without a Drude peak, irrespective of the positive slope of ρ_ab. For small ω, σ_ab(ω) even possesses a broad finite-energy peak at high temperatures, suggesting “dynamical” localization. These characteristics are explained in terms of the breakdown of the quasiparticle picture due to strong scattering.