Low-Temperature Charging Dynamics of the Ionic Liquid and Its Gating Effect on FeSe0.5Te0.5 Superconducting Films.

Ionic liquids (ILs) have been investigated extensively because of their unique ability to form the electric double layer (EDL), which induces high electrical field. For certain materials, low-temperature IL charging is needed to limit the electrochemical etching. Here, we report our investigation of the low-temperature charging dynamics in two widely used ILs-DEME-TF2N and C4mim-TF2N. Results show that the formation of the EDL at ∼220 K requires several hours relative to milliseconds at room temperature, and an equivalent voltage Ve is introduced as a measure of the EDL formation during the biasing process. The experimental observation is supported by molecular dynamics simulation, which shows that the dynamics are logically a function of gate voltage, time, and temperature. To demonstrate the importance of understanding the charging dynamics, a 140 nm thick FeSe0.5Te0.5 film was biased using the DEME IL, showing a tunable Tc between 18 and 35 K. Notably, this is the first observation of the tunability of the Tc in thick film FeSe0.5Te0.5 superconductors.