Coherent quasiparticles with a small Fermi Surface in lightly doped Sr$_3$Ir$_2$O$_7$

We characterize the electron doping evolution of (Sr$_{1-x}$La$_x$)Ir$_2$O$_7$ by means of angle-resolved photoemission. Concomitant with the metal insulator transition around $x\approx 0.05$ we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume $3x/2$, where $x$ is the independently measured La concentration. The quasiparticle weight $Z$ remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue $Z\approx0.5$ in lightly doped Sr$_3$Ir$_2$O$_7$, in stark contrast with underdoped cuprates.