Tunable and Controllable Monoatomic Layer Two-Dimensional Electron Gas Discovered at the Heterointerface of 5d Iridates 

Recently, Professor YANG Xiaoping's research group at the High Magnetic Field Laboratory, the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, discovered a tunable and controllable monoatomic layer two-dimensional electron gas (2DEG) localized at the heterointerface. 

The research results were published in ACS Applied Electronic Materials.

The Mott insulator-metal transition is a key topic in condensed matter physics due to its potential for device applications and superconductivity when doped. In 5d iridates, the spin-orbit coupling (SOC) is much stronger than in 3d transition metal oxides, making it comparable to crystal field splitting and electron-electron interactions. This results in the Ir 5d-t_2g bands splitting into J_eff = 3/2 and J_eff = 1/2 subbands. 

Currently, artificial heterointerfaces techniques are widely employed to manipulate the electronic structure and properties of materials.

In this study, researchers explored the electronic properties of (SrIrO₃)_m/(LaTiO₃)₁ superlattices using density functional theory. They observed that an integer charge transfer occurs between LaTiO₃ and SrIrO₃, driven by the combined effects of interfacial polarity differences and oxygen octahedral distortions. The number of transferred electrons on each Ir atom can be controlled by doping the A-site of LaTiO₃ or varying the SrIrO₃ layer number m, thereby modulating the oxidation states of Ir. This led to a variety of electronic states, including nonmagnetic band insulators, ferromagnetic metals, ferrimagnetic Mott insulators, and ferrimagnetic metals. 

A mixed valence state emerges when there are at least two layers of SrIrO₃. This leads to an insulator-metal transition when the SrIrO₃ layer number m is greater than or equal to 3. 

The most interesting thing is that the charge transfer and the formation of a two-dimensional electron gas (2DEG) only occur at the single atomic layer of IrO₂ where the materials meet,  regardless of the SrIrO₃ layer thickness. This is different from the 3d LaAlO₃/SrTiO₃ system, where the 2DEG extends deeper into the material beyond just the interface.

These findings provide fresh insights into the development of novel nanoscale oxide electronic devices and the exploration of two-dimensional unconventional iridate superconductivity.

Schematic illustration of band alignment in different SrIrO₃-based superlattices. (Image by YANG Xiaoping)