Gate-Tuned Superconductor-Insulator Transition In (Li,Fe)Ohfese

The antiferromagnetic (AFM) insulator-superconductor transition has always been a center of interest in the underlying physics of unconventional superconductors. However, in the family of iron-based high-T-c superconductors, no intrinsic superconductor-insulator transition has been confirmed so far. Here, we report a first-order transition from superconductor to AFM insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe. The superconducting transition temperature (T-c) is continuously enhanced with electron doping by ionic gating up to a maximum T-c of 43 K, and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest T-c. A phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out, indicating that the superconductor-insulator transition is a common feature for unconventional superconductivity. These results help to uncover the underlying physics of iron-based superconductivity as well as the universal mechanism of high-T-c superconductivity. Our finding also suggests that the gate-controlled strong charge doping makes it possible to explore novel states of matter in a way beyond traditional methods.