Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy

Abstract

The effective Mott gap measured by scanning tunneling microscopy (STM) in the lightly doped Mott insulator $({\mathrm{Sr}}_{1-\mathrm{x}}{\mathrm{La}}_{\mathrm{x}}{)}_2{\mathrm{IrO}}_4$ differs greatly from values reported by photoemission and optical experiments. Here, we show that this is a consequence of the poor electronic screening of the tip-induced electric field in this material. Such effects are well known from STM experiments on semiconductors, and go under the name of tip-induced band bending (TIBB). We show that this phenomenon also exists in the lightly doped Mott insulator $({\mathrm{Sr}}_{1-\mathrm{x}}{\mathrm{La}}_{\mathrm{x}}{)}_2{\mathrm{IrO}}_4$ and that, at doping concentrations of $x\le 4\%$, it causes the measured energy gap in the sample density of states to be bigger than the one measured with other techniques. We develop a model able to retrieve the intrinsic energy gap leading to a value which is in rough agreement with other experiments, bridging the apparent contradiction. At doping $x\approx 5\%$ we further observe circular features in the conductance layers that point to the emergence of a significant density of free carriers in this doping range, and to the presence of a small concentration of donor atoms. We illustrate the importance of considering the presence of TIBB when doing STM experiments on correlated-electron systems and discuss the similarities and differences between STM measurements on semiconductors and lightly doped Mott insulators.

9 pages, 5 figures

References in corpus (8)

• Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor
• Novel Jeff = 1/2 Mott State Induced by Relativistic Spin-Orbit Coupling in Sr2IrO4
• Appearance of fluctuating stripes at the onset of the pseudogap in the high-Tc Superconductor Bi2Sr2CaCu2O8+x
• Visualizing the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator
• Universality of pseudogap and emergent order in lightly doped Mott insulators
• Magnetic couplings, optical spectra, and spin-orbit exciton in 5d electron Mott insulator Sr2IrO4
• Imaging Transport Resonances in the Quantum Hall Effect
• First-order melting of a weak spin-orbit Mott insulator into a correlated metal

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