Superconductivity at 38 K at an electrochemical interface between an ionic liquid and FeSe0.8Te0.2 on various substrates

Abstract

Superconducting FeSe0.8Te0.2 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 Tc values of 8 K, 12 K and 19 K observed before etching on SrTiO3, LaAlO3 and CaF2 substrates, respectively. Tc returned to its original value with the removal of the gate bias. The observation of Tc enhancement for these thick films indicates that the Tc 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 FeSe0.8Te0.2 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 FeSe0.8Te0.2 thin film and the ionic liquid electrolyte.

Enhanced superconductivity induced by several-unit-cells diffusion in an FeTe/FeSe bilayer heterostructure

“…In addition, O and Fe are detected in the top few nanometers of both samples without Se or Te elements, which hints at the probable existence of an Fe-oxide (FeO x ) layer above FeTe (not shown here). Similar oxidized Fe layers were also reported by Kouno et al [60] in their Fe(Se 0.8 Te 0.2 )/CaF 2 thin films.…”

“…Despite the various viewpoints, the intrinsic reason for the suppression of superconductivity in undoped ultrathin Fe-Ch films still remains elusive, which is preventing the discovery of HTS in this system. A very recent work published by Kouno et al [60] reported a T c at 38 K in an Fe(Se 0.8 Te 0.2 ) thin film on CaF 2 substrate. Similar to the top-down approach used by Shiogai [45], they focused on the interface between the Fe(Se 0.8 Te 0.2 ) layer and the ionic liquid electrolyte.…”

“…Even though the T c is currently not comparable with the T c record achieved in ultrathin FeSe thin films, a deep investigation into the Fe-Ch thick film is still of great importance to reveal the universal mechanism behind the superconductivity in Fe-Ch and similar systems. Using the pulsed laser deposition (PLD) method, Fe-Ch films with T onset c over 15 K [55] in FeSe, or 21 K [56] in FeSe 0.5 Te 0.5 , can be easily prepared without external electron doping, although there is a minimum thickness for this system to exhibit superconductivity, empirically over 20 nm for FeSe [53,57,58] and over 30 nm for Fe-Se-Te [43,51,[56][57][58][59][60]. Films with less thickness are generally nonsuperconducting or even insulating.…”

Particulate Generation on Surface of Iron Selenide Films by Air Exposure

“…These results indicate that an in situ process without exposure to air [40] is necessary for electric-field-induced superconductivity in FeSe EDLTs if an electrochemically etching process [34,38,39] is not employed. as-grown film and film exposed to air for 60 min are shown for comparison.…”

“…Another type of T c enhancement of FeSe has been reported by employing electric double-layer transistor (EDLT) configurations with ionic liquid as a gate dielectric material [34][35][36][37][38][39]. In these recent reports, the EDLT configuration serves two roles: it allows electrostatic doping of high-density carriers to the EDLT channels [35][36][37], and electrochemical etching of the EDLT channels under relatively high gate bias (e.g., ≥ 5 V) and at high temperature (close to room temperature) is applied to control the channel thickness and electrical properties [34,38,39]. Both of these factors contribute to achieving a high T c of approximately 40 K, which is close to that for the above cases of high pressure and chemical doping/intercalation.…”