Overdamped antiferromagnetic strange metal state in Sr${}_3$IrRuO${}_7$

The unconventional electronic ground state of Sr${}_3$IrRuO${}_7$ is explored via resonant x-ray scattering techniques and angle-resolved photoemission measurements. As the Ru content approaches $x=0.5$ in Sr${}_3$(Ir${}_{1-x}$Ru${}_x$)${}_2$O${}_7$, intermediate to the $J_{eff}=1\mathrm{/}2$ Mott state in Sr${}_3$Ir${}_2$O${}_7$ and the quantum critical metal in Sr${}_3$Ru${}_2$O${}_7$, a thermodynamically distinct metallic state emerges. The electronic structure of this intermediate phase lacks coherent quasiparticles, and charge transport exhibits a linear temperature dependence over a wide range of temperatures. Spin dynamics associated with the long-range antiferromagnetism of this phase show nearly local, overdamped magnetic excitations and an anomalously large energy scale of 200 meV—an energy far in excess of exchange energies present within either the Sr${}_3$Ir${}_2$O${}_7$ or Sr${}_3$Ru${}_2$O${}_7$ solid-solution endpoints. Overdamped quasiparticle dynamics driven by strong spin-charge coupling are proposed to explain the incoherent spectral features of the strange metal state in Sr${}_3$IrRuO${}_7$.