Electron-Doped Sr2IrO4: An Analogue of Hole-Doped Cuprate Superconductors Demonstrated by Scanning Tunneling Microscopy

Abstract

Sr2IrO4 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 Sr2IrO4 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 EF . 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 Sr2IrO4 remarkably resembles hole-doped cuprate superconductors.

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Plain Language Summary

Sr2IrO4 , 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 Sr2IrO4 superconductivity has been reported up until now. Here, we present low-temperature scanning tunneling microscopy and spectroscopy of electron-doped Sr2IrO4 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 Sr2IrO4 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 Sr2IrO4 with 0.5–0.7 monolayer (ML) K coverage. We also demonstrate that, given increased surface K coverage, the electronic state of Sr2IrO4 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 Sr2IrO4 , which is consistent with previous theoretical predictions. Efforts are currently underway to look for further evidence of superconductivity in Sr2IrO4 such as magnetic vortex, zero resistance, and the Meissner effect. If confirmed, superconductivity in Sr2IrO4 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 Sr2IrO4 .