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

High transition temperature (Tc) superconductivity in FeSe/SrTiO3 has been widely discussed on the possible mechanisms in conjunction with the various effects of interface between FeSe and SrTiO3 substrate. By employing an electric-double-layer transistor configuration, which enables both the electrostatic carrier doping and electrochemical thickness tuning, we investigated the interfacial effect on the high-Tc phase at around 40 K in FeSe films deposited on SrTiO3, MgO, and KTaO3 substrates. The systematic study on thickness dependence of transport properties under a certain gate voltage reveals the universal trend of the onset Tc against the Hall coefficient in all the FeSe films, irrespective of the substrate materials in which the different contribution of interfacial effect is expected. The independence of the highest Tc on substrate materials evidences that the high-Tc superconductivity at around 40 K does not primarily originate from a specific interface combination but from a charge carrier filling at specific electronic band structure.