Unified trend of superconducting transition temperature versus Hall coefficient for ultrathin FeSe films prepared on different oxide substrates

Direct evidence of superconductivity and determination of the superfluid density in buried ultrathin FeSe grown on SrTiO3

“…Even allowing for band bending effects increasing the thickness of the charge transfer layer in the specific electric dipole layer configuration of Ref. [26], it appears reasonable to consider that proximity effects cannot be ignored in > 1 ML thick FeSe layer. In this respect it is interesting to note that the value n 2D s 1.4 × 10 14 cm −2 obtained from the excess electron determination by ARPES is about a factor of four lower than the present determination of the superconducting carrier density n 2D s 6 ± 2 × 10 14 cm −2 of our heterostructure containing 1+4 FeSe layers.…”

“…ARPES measurements indicated charge transfer from the substrate and superconductivity to be restricted to the FeSe interface layer with the top layer displaying charge neutrality. However, in our discussion below we will also address the question of the possible contribution of these additional layers to the observed supercarrier density, in view of our and recent results of charge distributions in ultrathin films 26 . A ∼ 25 nm thick layer of amorphous Se was added for protection.…”

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.…”

“…These findings suggest that the particular structures generated at the surface act as an obstacle to field-induced phase transition and superconductivity in the FeSe EDLTs. For this reason, electrochemical etching [34,38,39], which should be able to remove the particular structures, is effective for achieving high-T c EDLTs if the FeSe channels must be exposed to air during the device fabrication process.…”

“…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.…”