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 $\left(T_c\right)$ 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.

Figure 1

Gate-voltage dependence of the resistance for a (Li,Fe)OHFeSe crystal flake (with a thickness of $\simeq 200$ nm) during a continuous upsweep at 330 K. The $V_g$ sweep rate is 1 mV ${\mathrm{s}}^{-1}$. The inset shows an optical image of the iFET device used in our measurements.

Figure 2

Temperature-dependent resistance of a (Li,Fe)OHFeSe iFET at various gate voltages. The longitudinal resistance was measured from 2 to 330 K at different gate biases ranging from 0 to 4.5 V.

Figure 3

(a) Temperature dependence of Hall coefficient $R_H$ at different gate voltages. The thickness $\left(t\right)$ of the measured (Li,Fe)OHFeSe flake is 210 nm. $R_H=(R_{xy}/B)t$ was calculated by linear fitting of $R_{xy}$ versus $B$ plot from $-5$ to 5 T in the superconducting phase, and linear fitting of $R_{xy}$ from 2.5 to 9 T after subtracting the zero-field value in the insulating phase (see the Supplemental Material [16]). Error bars represent the uncertainty of linear fitting. (b) The lower and upper panels show the Hall coefficient $R_H$ and Hall number $n_H=1/R_{H}e$ as a function of $V_g$ at 70 K, respectively. Hall number per Fe site in the FeSe layer was calculated based on the unit-cell volume reported in Ref. [9]. The filled and open circles correspond to the electron-type and hole-type carriers, respectively.

Figure 4

(a) Phase diagram of electronic phases as a function of gate voltage ${\mathrm{V}}_g$ for (Li,Fe)OHFeSe. The boundary of SC phase and metal phase is determined by the midpoint critical temperature $T_c^{\mathrm{mid}}$. Error bars are defined as the width of SC transition. (b) Transverse magnetoresistance (TMR) at $B=9$ T of a superconducting (Li,Fe)OHFeSe iFET (red circle) and an insulating (Li,Fe)OHFeSe iFET (blue triangle), as a function of temperature. The superconducting iFET is tuned to $T_c=42$ K. For both samples, the TMR was measured with the applied magnetic field parallel to the $c$ direction. The solid lines are guides to the eye. (c) Two-magnon Raman spectra for superconducting and insulating (Li,Fe)OHFeSe. Red and blue solid lines stand for the Raman spectra for insulating (Li,Fe)OHFeSe at 10 and 300 K, respectively. The olive solid line stands for Raman spectrum at 300 K for superconducting (Li,Fe)OHFeSe with $T_c^{\mathrm{mid}}=24.4$ K. The gray area is the region for two-magnon mode.