Superconductivity at 38 K at an electrochemical interface between an ionic liquid and FeSe_0.8Te_0.2 on various...

Superconducting FeSe_0.8Te_0.2 thin films on SrTiO₃, LaAlO₃ and CaF₂ substrates were electrochemically etched in an ionic liquid, DEME-TFSI, electrolyte with a gate bias of 5 V. Superconductivity at 38 K was observed on all substrates after the etching of films with a thickness greater than 30 nm, despite the different T_c values of 8 K, 12 K and 19 K observed before etching on SrTiO₃, LaAlO₃ and CaF₂ substrates, respectively. T_c returned to its original value with the removal of the gate bias. The observation of T_c enhancement for these thick films indicates that the T_c enhancement is unrelated to any interfacial effects between the film and the substrate. The sheet resistance and Hall coefficient of the surface conducting layer were estimated from the gate bias dependence of the transport properties. The sheet resistances of the surface conducting layers of the films on LaAlO₃ and CaF₂ showed identical temperature dependence, and the Hall coefficient was found to be almost independent of temperature and to take values of -0.05 to -0.2 m²/C, corresponding to 4-17 electrons per FeSe_0.8Te_0.2 unit cell area in two dimensions. These common transport properties on various substrates suggest that the superconductivity at 38 K appears in the surface conducting layer as a result of an electrochemical reaction between the surface of the FeSe_0.8Te_0.2 thin film and the ionic liquid electrolyte.