Iridates represent a nique system to study the interplay and competition of comparable electronic correlations and SOC. As one of the most intensively studied systems, Sr₂IrO₄ possesses a Jeff=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 andlectronically analogous cuprate HTSC, and Sr₂IrO₄. A signature of superconductivity has been indeed detected in the electron-injected samples. Nevertheless, the bulk superconductivity in Sr₂IrO₄ seems a long way off.

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

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.

Electrical and magnetic measurements taken for Sr_2-xGaxIrO₄

(a)Resistivities vs. temperature for Sr_2-xGaxIrO₄. (b) Susceptibilities vs. temperature. (c)-(e) Magnetic hysteresis loops. (f) Phase diagram of temperature vs. doping. (Image by ZHU Wenka)