Phys. Rev. B 99, 121114(R) (2019)
Spin-polaron ladder spectrum of the spin-orbit-induced Mott insulator Sr2IrO4 probed by scanning tunneling spectroscopy
Jose M. Guevara1, Zhixiang Sun1, 2, Ekaterina M. Pärschke1,3, Steffen Sykora1, Kaustuv Manna1,*, Johannes Schoop1, Andrey Maljuk1, Sabine Wurmehl1,4, Jeroen van den Brink1, Bernd Büchner1,4,5, and Christian Hess1,5,†
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.
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 Sr2IrO4. 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 Sr2IrO4.