LAUSR

We have developed a field-effect transistor (FET) device using a solid ion conductor (SIC) as the gate dielectric, which can tune the carrier density of FeSe by driving lithium ions in and out of the FeSe thin flakes and consequently control the physical properties and phase transitions. A dome-shaped superconducting phase diagram was mapped out with increasing Li content, with ${T}_{c}\ensuremath{\sim}46.6$ K for optimal doping, and an insulating phase was reached at the extremely overdoped regime. Our study suggests that, by using a solid ion conductor as the gate dielectric, the SIC-FET device is able to induce much higher carrier doping in the bulk, suit many surface-sensitive experimental probes, and stabilize structural phases that are inaccessible in ordinary conditions.