Spin-polaron ladder spectrum of the spin-orbit-induced Mott insulator Sr 2 IrO 4 probed by scanning tunneling spectroscopy

The motion of doped electrons or holes in an antiferromagnetic lattice with strong on-site Coulomb interactions touches one of the most fundamental open problems in contemporary condensed matter physics. The doped charge may strongly couple to elementary spin excitations, resulting in a dressed quasiparticle which is subject to confinement. This “spin polaron” possesses internal degrees of freedom with a characteristic “ladder” excitation spectrum. Despite its fundamental importance for understanding high-temperature superconductivity, clear experimental spectroscopic signatures of these internal degrees of freedom are scarce. Here, we present scanning tunneling spectroscopy results of the spin-orbit-induced Mott insulator Sr 2 IrO 4 . Our spectroscopy data reveal distinct shoulder-like features for occupied and unoccupied states beyond a measured Mott gap of Δ ≈ 620 meV. Using the self-consistent Born approximation we assign the anomalies in the unoccupied states to the spin-polaron ladder spectrum with excellent quantitative agreement and estimate the Coulomb repulsion U = 2.05 ... 2.28 eV in this material. These results confirm the strongly correlated electronic structure of this compound and underpin the previously conjectured paradigm of emergent unconventional superconductivity in doped Sr 2 IrO 4 .