A research group led by ZHANG Changjin from the High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) has discovered enhanced electrical conductivity and diluted Ir⁴⁺ spin orders in the research of Sr₂IrO₄: a system with strong spin-orbit couplings (SOC).

Iridates represent a unique system to study the interplay and competition of comparable electronic correlations and SOC, which gives rise to many nontrivial phenomena, such as the SOC-driven Mott insulators, topological crystalline insulator, Weyl semimetal, and quantum spin liquid.

As one of the most intensively studied systems, Sr₂IrO₄ possesses a J_eff=1/2 electronic structure that violates the conventional band theory, as well as a magnetic ground state of canted antiferromagnetism (AFM). Furthermore, Sr₂IrO₄ has been predicted to be a new kind of high-temperature superconductor (HTSC), inspired by the structurally and electronically analogous cuprate HTSC, and Sr₂RuO₄.

Previous studies have indeed discovered a signature of superconductivity in the electron-injected samples, using the angle-resolved photoemission spectroscopy (ARPES) and the scanning tunneling microscopy/spectroscopy (STM/STS), respectively. Nevertheless, the bulk superconductivity in Sr₂IrO₄ seems a long way off.

ZHANG’s group has performed comprehensive studies, including annealing and chemical doping. The chemical doping not only introduces chemical pressure that may distort the crystal structure and electronic structure, but also changes the concentration of carriers, and thus induces dramatic changes in the electrical and magnetic properties.

For the gallium-doped Sr₂IrO₄, the electrical conductivity is enhanced by doping, and a metallic state is achieved when doping x is over 0.1. With the increase of doping, the ferromagnetic phase is suppressed.

In addition to the high-temperature magnetic transition, a low-temperature one is also observed for the x=0.05–0.10 samples. Both of the magnetic states are found to be canted antiferromagnetism. The low-temperature phase is strongly depressed by doping and vanishes when doping is further increased, which is probably associated with the long-way exchange interactions of diluted Ir⁴⁺ spins.

These studies provide an insight into the electrical and magnetic states tuned by chemical doping in Sr₂IrO₄, thereby facilitating the seeking of superconductivity in this system. This work has been published in Applied Physics Letters, entitled “Enhanced electrical conductivity and diluted Ir⁴⁺ spin orders in electron doped iridates Sr_2–xGa_xIrO₄”.

This work was supported by the National Key Research and Development Program of China, National Natural Science Foundation of China, and the CASHIPS Director’s Fund.