Local density of states study of a spin-orbit-coupling induced Mott insulator ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$

We present scanning tunneling microscopy and spectroscopy experiments on the novel $J_{\mathrm{eff}}=1/2$ Mott insulator ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$. Local density of states (LDOS) measurements show an intrinsic insulating gap of 620 meV that is asymmetric about the Fermi level and is larger than previously reported values. The size of this gap suggests that ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is likely a Mott rather than Slater insulator. In addition, we found a small number of native defects which create in-gap spectral weight. Atomically resolved LDOS measurements on and off the defects show that this energy gap is quite fragile. Together the extended nature of the 5$d$ electrons and poor screening of defects help explain the elusive nature of this gap.

Figure 1

A 20$\times$20 nm${}^2$ topographic image taken with $-$300 mV and 5 pA at 80 K (a) and its Fourier transform (c). The black arrows indicate the $\mathbf{a}$ and $\mathbf{b}$ axes. Green (gray) and blue (dark) arrows indicate two Sr-related defects. (b) Schematic diagrams of the first SrO and second IrO${}_2$ layers. The square lattice in (a) corresponds to Sr.

Figure 2

The 620 meV energy gap. (a) LDOS measured by averaging 36$\times$36 spectra over the 2$\times$2 nm${}^2$ area indicated by the square in the image (inset). The dashed lines are drawn to indicate the band edges at $-$150 mV and +470 mV. Data were taken with $-300$ mV, 10 pA, and ac modulation of 8 mV${}_{\mathrm{rms}}$. (b) and (c) LDOS taken at locations away from the defects, (b) without and (c) with the slow rise. (d) Diagram showing energy bands with two important features: the 620 meV insulating gap and the overlap between the LHB and the $J_{\mathrm{eff}}=3/2$ (quartet) band.

Figure 3

(a)–(d) Images with a wide range of junction resistances: 3–100 G$\Omega$. The + signs indicate defects looking differently with tunneling condition. (e) LDOS measured on the red (dark) encircled plus sign in (a). (f) LDOS measured on the green (gray) encircled plus sign in (a) with 1/10 $\mathrm{G}\Omega$.