Electron-Doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$: An Analogue of Hole-Doped Cuprate Superconductors Demonstrated by Scanning Tunneling Microscopy

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ was predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure, and magnetic coupling constants. Here, we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ with surface electron doping by depositing potassium (K) atoms. We find that as the electron doping increases, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogaplike phase, and a phase with a sharp, V-shaped low-energy gap with about 95% loss of density of state (DOS) at ${\mathrm{E}}_F$. At certain K coverage (0.5–0.6 monolayer), the magnitude of the low-energy gap is 25–30 meV, and it closes at around 50 K. Our observations show that the electron-doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ remarkably resembles hole-doped cuprate superconductors.

Popular Summary

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, a $5d$ transition-metal oxide, strongly resembles high-temperature-superconducting cuprates in its structure and Mott insulator nature, and it is predicted to be a high-temperature superconductor upon electron doping. However, no experimental evidence of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ superconductivity has been reported up until now. Here, we present low-temperature scanning tunneling microscopy and spectroscopy of electron-doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ subjected to in situ surface potassium (K) dosing. We also provide an indication of possible superconductivity in this system.

We evaporate K atoms onto the surface of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ and hold our experimental setup at a temperature of 4.5 K. We observe a sharp, V-shaped gap with about 95% spectral weight suppression and visible coherence peaks in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ with 0.5–0.7 monolayer (ML) K coverage. We also demonstrate that, given increased surface K coverage, the electronic state of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ evolves from a Mott insulating state to a normal metallic state with more than 1 ML K coverage via a pseudogaplike state and a V-shape-gapped state, sequentially. The remarkable analogy between this system and the hole-doped cuprates, particularly in the tunneling spectral line shapes, characteristic temperatures, gap energy scales, and the evolution of various electronic states with doping, hints at possible superconductivity in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, which is consistent with previous theoretical predictions. Efforts are currently underway to look for further evidence of superconductivity in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ such as magnetic vortex, zero resistance, and the Meissner effect. If confirmed, superconductivity in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ would help to build a universal theme of high-temperature superconductivity and solve a long-standing mystery.

We expect that our findings will pave the way for a more thorough understanding of the electronic properties of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$.

Figure 1

Surface topography and $d\mathrm{I}/d\mathrm{V}$ spectrum of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ with and without K coverage. (a) Sketch of the sample configuration. (b) Typical topographic image on the SrO-terminated surface of pristine ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, measured at 77 K. The top and bottom insets represent the atomically resolved image and the FT of data in panel (b), respectively. Four different kinds of defects are indicated by arrows. (c) Spatially averaged $d\mathrm{I}/d\mathrm{V}$ spectrum on the SrO-terminated surface. An insulating energy gap as large as 700 meV is observed on the defect-free region, while a reduced gap is observed on defects 3. (d) Typical topographic image of the SrO-terminated surface adjacent to Au contacts (measured at 20 K), in which a few Au clusters are observed as the white spots. (e) Spatially averaged $d\mathrm{I}/d\mathrm{V}$ spectra measured on panels (d) and (f), showing a reduced insulating gap and a metallic state, respectively. (f) Typical topographic image after depositing 0.6 ML K atoms on the SrO-terminated surface that is adjacent to the Au contacts (measured at 20 K). (g) Representative $d\mathrm{I}/d\mathrm{V}$ spectra on K-doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ with various K coverages (10 K for 0.5 ML, 20 K for 0.6 ML, 4.5 K for 0.7 ML), showing V-shaped gaps.

Figure 2

Gap inhomogeneity of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ with 0.6 ML K taken at 20 K. (a,b) $d\mathrm{I}/d\mathrm{V}$ maps taken at $V_b=20\mathrm{meV}$ and 70 meV, respectively. Representative areas with different electronic states are marked by dotted lines. Each map has $100\times 100\text{pixels}$. (c) Typical $d\mathrm{I}/d\mathrm{V}$ spectra taken at the positions marked by dots in panels (a) and (b), showing the evolution of electronic states across the regions. The horizontal bars indicate the zero conductance position of each curve. Dashed lines located at 20 meV and 70 meV are added as guides to the eye. The labels I, T, HEG, and LEG on the color bars indicate the insulating, transition, HEG-dominated, and LEG-dominated regions, respectively.

Figure 3

K coverage dependence of $d\mathrm{I}/d\mathrm{V}$ spectra. (a) The representative spectra taken at the HEG-dominated regions for various K coverages. Inhomogeneity of the pseudogap-like feature is illustrated by two spectra shown for each coverage. The arrows indicate the averaged energy locations of ${\mathrm{\Delta }}_H$. (b) The representative spectra taken at the LEG-dominated regions for 0.5–0.7 ML K coverage and arbitrary regions for 1–2 ML K coverage. The curves are offset vertically for clarity, and the horizontal markers indicate the zero conductance position of each curve. The data were taken at 10 K for 0.5 ML, 20 K for 0.6 ML, and 4.5 K for 0.7–2 ML.

Figure 4

Temperature dependence of the V-shaped gap. (a)–(c) Temperature dependence of the spatially averaged $d\mathrm{I}/d\mathrm{V}$ spectra for K coverage of (a) 0.5 ML, (b) 0.6 ML, and (c) 0.7 ML, respectively. The spectra shown here are the average of the $d\mathrm{I}/d\mathrm{V}$ spectra with large gap depth and visible peaks at the gap edges taken in the LEG-dominated regions of a $30\times 30{\mathrm{nm}}^2$ area. The line-shape variations at different temperatures are caused by the lack of precisely tracking the same location on a clustered surface with strong LDOS inhomogeneity. The curves are offset vertically for clarity. (d) Gap depth (as defined in the main text) as a function of temperature, which decreases gradually with increasing temperature. $T_L$ is defined by the temperature at which the gap depth stops decreasing quickly upon warming.

Figure 5

${\mathrm{\Delta }}_H$, ${\mathrm{\Delta }}_L$, and $T_L$ as a function of the surface coverage of potassium. The gap magnitude of the pseudogap observed at the antinode by ARPES is shown by empty triangles for comparison [14].