Formation of an incoherent metallic state in Rh-doped Sr$_2$IrO$_4$

TL;DR

This study investigates the insulator-to-metal transition in Rh-doped Sr$_2$IrO$_4$, revealing an incoherent metallic state with a pseudogap and no quasiparticles, driven by Fermi level shifts into the lower Hubbard band.

Contribution

It provides detailed ARPES analysis showing the evolution of electronic structure and the nature of the metallic state in Rh-doped Sr$_2$IrO$_4$, highlighting its incoherent, non-Fermi liquid character.

Findings

Metallic state appears for x > 0.07

Absence of quasiparticles in the metallic phase

Presence of a ~30 meV pseudogap across the Fermi surface

Abstract

Sr$_2$IrO$_4$ is the archetype of the spin-orbit Mott insulator, but the nature of the metallic states that may emerge from this type of insulator is still not very well known. We study with angle-resolved photoemission the insulator-to-metal transition observed in Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ when Ir is substituted by Rh (0.02 < $x$ < 0.35). The originality of the Rh doping is that Ir and Rh, which are formally isovalent, adopt different charge states, a rather unusual and inhomogeneous situation. We show that the evolution to the metallic state can be essentially understood as a shift of the Fermi level into the lower Hubbard band of Sr$_2$IrO$_4$. The Mott gap appears quite insensitive to the introduction of up to $\sim$20\% holes in this band. The metallic phase, which forms for $x$ > 0.07, is not a Fermi liquid. It is characterized by the absence of quasiparticles, unrenormalized band dispersion compared to calculations and an $\sim$30-meV pseudo-gap on the entire Fermi surface.