Tunneling into the Mott insulator ${\text{Sr}}_2$${\text{IrO}}_4$

We studied the single-layered iridate ${\text{Sr}}_2$${\text{IrO}}_4$ with a scanning tunneling microscope. The finite low temperature conductance enables the electronic structure of this antiferromagnetic Mott insulator to be measured by tunneling spectroscopy. We imaged the topography of freshly cleaved surfaces and measured differential tunneling conductance at cryogenic temperatures. We found the Mott gap in the tunneling density of states to be $2\Delta =615$ meV. Within the Mott gap, additional shoulders are observed which are interpreted as inelastic loss features due to magnons.

Figure 1

Crystal structure of ${\text{Sr}}_2$${\text{IrO}}_4$ with a projection along (a) an arbitrary direction and (b) the $c$ axis, where the Ir, Sr, and O atoms are black, blue, and red, respectively. The ${\text{IrO}}_6$ octahedra are colored dark yellow. (c) Illustration of how the 5$d$ orbital is split due to crystal field splitting and spin-orbit coupling. (d) Illustration of the effect of the Coulomb interaction on the $J_{\mathrm{eff},1/2}$ band and the ground state configuration of the system.

Figure 2

A high-resolution atomically resolved topographic image of $12.2\times 12.2$ nm${}^2$ area of surface of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ obtained with a constant current of $I_{\mathrm{set}}=600$ pA and $V_{\mathrm{bias}}=450$ mV. (Inset) Fast Fourier transform of topographic image.

Figure 3

Scanning tunneling spectroscopy (solid red) of ${\text{Sr}}_2$${\text{IrO}}_4$ at 77 K with modulation voltage 4 mV and frequency 703.4 Hz. The energy gap $\Delta$ and width $\Gamma$ of the density of states from fits to Eq. (2) (dashed line) are displayed. (Inset) Inelastic electron tunneling spectroscopy (blue) and magnon density of states (light gray). The magnon dispersion has exchange energies ${\mathrm{J}}_1=60$ meV, ${\mathrm{J}}_2=-20$ meV, and ${\mathrm{J}}_3=15$ meV (Ref. [4]).

Figure 4

Scanning tunneling spectroscopy (red) of ${\text{Sr}}_2$${\text{IrO}}_4$ at 4.2 K with modulation voltage 10 mV and frequency 704.2 Hz. (Inset) Inelastic electron tunneling spectroscopy (blue).