Universality of pseudogap and emergent order in lightly doped Mott insulators

Surprising observations in the evolution of electronic states in electron-doped iridates provide fresh insight into the melting of the Mott state and might lead to a fuller understanding of corresponding processes in copper-oxide superconductors. It is widely believed that high-temperature superconductivity in the cuprates emerges from doped Mott insulators 1 . When extra carriers are inserted into the parent state, the electrons become mobile but the strong correlations from the Mott state are thought to survive—inhomogeneous electronic order, a mysterious pseudogap and, eventually, superconductivity appear. How the insertion of dopant atoms drives this evolution is not known, nor is whether these phenomena are mere distractions specific to hole-doped cuprates or represent genuine physics of doped Mott insulators. Here we visualize the evolution of the electronic states of (Sr 1− x La x ) 2 IrO 4 , which is an effective spin-1/2 Mott insulator like the cuprates, but is chemically radically different 2 , 3 . Using spectroscopic-imaging scanning tunnelling microscopy (SI-STM), we find that for a doping concentration of x ≈ 5%, an inhomogeneous, phase-separated state emerges, with the nucleation of pseudogap puddles around clusters of dopant atoms. Within these puddles, we observe the same iconic electronic order that is seen in underdoped cuprates 1 , 4 , 5 , 6 , 7 , 8 , 9 . We investigate the genesis of this state and find evidence at low doping for deeply trapped carriers, leading to fully gapped spectra, which abruptly collapse at a threshold of x ≈ 4%. Our results clarify the melting of the Mott state, and establish phase separation and electronic order as generic features of doped Mott insulators.