Battisti, I; Fedoseev, V; Bastiaans, KM; de la Torre, A; Perry, RS; Baumberger, F; Allan, MP; (2017) Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy. Physical Review B , 95 (23) , Article 235141. 10.1103/PhysRevB.95.235141.

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Abstract

The effective Mott gap measured by scanning tunneling microscopy (STM) in the lightly doped Mott insulator ( Sr 1 − x La x ) 2 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 ( Sr 1 − x La x ) 2 IrO 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.

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