Spin-polaron ladder spectrum of the spin-orbit-induced Mott insulator Sr2IrO4 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 ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. Our spectroscopy data reveal distinct shoulder-like features for occupied and unoccupied states beyond a measured Mott gap of $\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}620$ 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 ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$.