Electric-field-induced superconductivity in electrochemically etched ultrathin FeSe films on SrTiO3 and MgO

Abstract

(1 − / (0)) with ϕ 0 and ξ(0) being the flux quantum and the GL coherent length at zero temperature, respectively, we obtained ξ(0) = 2.0 nm, which is around two times shorter than the bulk value 3 .8 Getting back to the thickness dependence of T c on in Figs. 2c and 2d, we note that the first single step superconducting transition appears at the thickness of 9.6 nm and 5.9 nm for sample A and N, respectively. The appearance of high-T C superconductivity in the rather thicker conditions offers us to consider that how the electric-field effect contributes to the high-T C superconductivity. Here, we examined the effect of electrostatic doping on the superconductivity in both ultrathin and thicker conditions by different experimental schemes in the same device set-up as sample A. At first, samples B, C on SrTiO 3 and M on MgO were etched at T = 245 K to induce the superconductivity as shown in Fig. 3a, the top panel of Fig. 3c and Fig. 3d (red line data), respectively. The thicknesses of samples B, C and M are tuned to the one/two-unit-cell (the red square in Fig. 2c), 9.4 nm (the blue triangle in Fig. 2c) and 3.7 nm (the green circle in Fig. 2d), respectively, by monitoring the leakage current under the assumption that the same etching rate with sample A holds. After the detection of the high-T C superconducting behavior, we examined the electrostatic effect with removing and then applying V G at T = 220 K (along the blue arrow in Fig. 1b; Note that no electrochemical etching occurs at this temperature as mentioned above).For sample B expectedly in the one/two-unit-cell condition, the initial insulating behavior was not recovered by removing V G from 5 V to 3 and 0 V as shown in Fig. 3b. 9 This result is a good evidence for the thickness reached close to one/two-unit-cell, because the high-T C superconductivity under V G = 0 V is consistent with the result of previous reports on FeSe/SrTiO 3 with charge transfer solely from the SrTiO 3 substrate 8 .Interestingly, the increase of T c on from 43.5 K to 46.3 K as a result of reduced V G implicates the over-doping effect by electrostatic doping. In contrast, in sample C under the thicker 9.4 nm condition, the superconductivity vanishes and the R s -T curve is back to the insulating behavior when V G is removed (blue line in middle panel of Fig. 3c).The reappearance of insulating behavior indicates the uniform etching without local ultrathin region providing a high-T C superconducting current pass. Then, application of V G = 5 V recovers the superconductivity again (blue line in bottom panel of Fig. 3c), demonstrating a reversible control of insulator-superconductor transition by electrostatic mean. These series of results in sample C suggests that the observable thickness condition for high-T c on superconductivity is widely expanded from one/two-unit-cell to above 10-unit-cells by the application of electric field. Additionally, T c on in sample M decreases with reducing the accumulated charge as shown in Fig. 3d. Therefore, we suppose that the high-T C supercondu...

Towards properties on demand in quantum materials

“…The origin of this phenomenon is under debate with both interfacial doping and substrate-modified electron-phonon coupling implicated in the enhancement 80 . The transition temperature of few-layer FeSe can also be modified through electrostatic tuning 81,82 (Fig. 3b).…”

Ionic liquid gating of ultra-thin YBa2Cu3O7−x films

“…Moreover, even if, here, the proposed process is not electrochemical, it was shown recently that etching with an IL can proceed in a controlled way. 33 Further experiments should be performed to elucidate these points, but from atomic force microscopy (AFM) performed after EDL gating, we can already say that no topographical damage could be seen down to AFM in-plane resolution (%10 nm).…”

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…Shiogai et al [153], Lei et al [154] and Hanzawa et al [155] independently demonstrated enhancement of T onset c to above 40 K by electron doping using an electric double-layer transistor (EDLT) technique. As the gate voltage is changed, a rapid increase of T c and sign reversal of Hall coefficient are simultaneously observed, suggesting that enhancement of T c is related to the Lifshitz transition (disappearance of hole Fermi surface) [154].…”

“…As the gate voltage is changed, a rapid increase of T c and sign reversal of Hall coefficient are simultaneously observed, suggesting that enhancement of T c is related to the Lifshitz transition (disappearance of hole Fermi surface) [154]. Shiogai et al successfully controlled thickness of the grown FeSe films by electrochemical etching using EDLT, and investigated the dependence of T c on thickness and substrate [153,156]. They found that electron-doped FeSe films on both STO and MgO substrates show almost the same T c of approximately 40 K. These results seem to be inconsistent with the results of the in situ measurements, which may be due to differences in condition of pretreatment for substrates and post-annealing.…”