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 (Sr_(1−x)La_x)_2IrO_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 (Sr_(1−x)La_x)_2IrO_4 and that, at doping concentrations of x≤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≈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.