Direct observation of the Mott gap in strontium iridate with a scanning tunneling microscope

TL;DR

This study used scanning tunneling microscopy to directly observe the Mott gap in Sr2IrO4, revealing detailed electronic and magnetic properties at cryogenic temperatures, and comparing results with theoretical models and other experimental techniques.

Contribution

First direct measurement of the Mott gap in Sr2IrO4 using STM, providing high-resolution surface images and local density of states data at cryogenic temperatures.

Findings

Measured the insulating gap consistent with optical and spectroscopic methods.

Observed broader gap features than Slater approximation predicts.

Detected low-energy magnetic fluctuation features related to magnetic excitations.

Abstract

The single-layer Mott insulator strontium iridate Sr$_2$IrO$_4$ was studied using a scanning tunneling microscope. This measurement technique is unique due to the transport properties of this Mott insulator allowing tunneling measurements to be performed, even at cryogenic temperatures. We obtained high-resolution images of the sample surface and the differential tunneling conductance at different cryogenic temperatures. The differential conductance is a direct measurement of the local electronic density of states which provided an insulating gap consistent with optical conductivity, angle resolved photoemission spectroscopy and resonant inelastic x-ray scattering (RIXS) experiments. The observed widths of these features is broader than predicted by the Slater approximation and narrower than predicted by dynamical mean field theory. Additionally, the observed density of states due to magnetic fluctuations is found in the derivative of the differential conductance and is consistent with results from Raman scattering and RIXS. At low temperatures, additional low-energy features were observed, suggesting a change in the dispersion of the collective magnetic excitations, which is consistent with the magnetic susceptibility.