Discrete superconducting phases in proton-intercalated FeSe thin flakes

We have successfully manipulated the electronic properties of FeSe thin flakes based on a field-effect transistor with a solid proton conductor as the gate dielectric. The phase diagram as a function of gate voltage demonstrates three evident discrete superconducting phases with ${T}_{\mathrm{c}}$ corresponding to $\ensuremath{\sim}8, \ensuremath{\sim}34$, and $\ensuremath{\sim}42$ K, respectively, which is similar to those observed in ${\mathrm{Li}}_x\mathrm{FeSe}$ and ${\mathrm{Na}}_x\mathrm{FeSe}$ by tuning Li or Na content with the solid-ion-conductor gating technique. In the protonation process, the final state with a saturation of the amount of hydrogen ions is reached at a moderately low gate voltage. It suggests that such a device based on a proton conductor can be used to modulate the physical properties by controlling the intercalated proton content and to explore the hydrogen storage capacity of materials.