A universal trend in Hall coefficient and superconducting transition temperature in FeSe electric double layer transistors

Since the discovery of high-T_c superconductivity in monolayer FeSe on SrTiO₃ [1], tremendous efforts have been addressed onto exploration of electronic origin for the high-T_c superconductivity using in-situ spectroscopic measurements. We have recently demonstrated ex-situ electrical transport measurements of the high-T_c FeSe with detailed thickness dependences using electrochemical etching technique in electric double layer transistor (EDLT) configuration [2]. In this talk, we will present a systematic thickness dependence of superconducting properties of FeSe-EDLT deposited on various oxide substrates. We found the universal relationship between the Hall coefficient and T_c, pointing out the importance of charge balance between electron and hole concentrations for emergence of high-T_c at 40 K [3]. We will also report on thickness dependences of the upper critical magnetic field and critical current density, both of which sharply increase towards thin film limit. The large values of the critical parameters clearly indicate the robustness of the high-T_c state in FeSe ultrathin films [1] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012). [2] J. Shiogai et al., Nature Phys. 12, 42 (2016). [3] J. Shiogai et al., Phys. Rev. B 95, 115101 (2017).