Phys. Rev. B 99, 121114(R) (2019)

Spin-polaron ladder spectrum of the spin-orbit-induced Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ probed by scanning tunneling spectroscopy

Jose M. Guevara¹, Zhixiang Sun^{1, 2}, Ekaterina M. Pärschke^1,3, Steffen Sykora¹, Kaustuv Manna^1,*, Johannes Schoop¹, Andrey Maljuk¹, Sabine Wurmehl^1,4, Jeroen van den Brink¹, Bernd Büchner^1,4,5, and Christian Hess^1,5,†

¹Leibniz-Institute for Solid State and Materials Research, IFW-Dresden, 01069 Dresden, Germany
²Center for Joint Quantum Studies and Department of Physics, Tianjin University, 300072 Tianjin, China
³Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA
⁴Institute for Solid State Physics, TU Dresden, 01069 Dresden, Germany
⁵Center for Transport and Devices, TU Dresden, 01069 Dresden, Germany
^*Present address: Max-Planck-Institute for Chemical Physics of Solids, 01187 Dresden, Germany.
^†c.hess@ifw-dresden.de

Abstract

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 $\Delta \approx 620$ 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 ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.