Interfacial Superconductivity in FeSe Ultrathin Films on SrTiO3 Probed by In Situ Independently Driven Four-Point-Probe Measurements

Abstract

We investigated the superconducting transport properties of the one-unit-cell FeSe ultrathin films epitaxially grown on undoped SrTiO3(001) (STO) with a well-defined surface structure by in situ independently-driven four-point-probe measurements. Our results unambiguously revealed the detection of the two-dimensional electrical conduction of the films without parallel conduction through the underlying substrate, both in the normal and superconducting states. The monolayer film exhibited a superconducting transition at an onset temperature of 40 K. Surprisingly, the onset of superconductivity was constantly observed at 40 K even for three- and five-unit-cell-thick FeSe films, even though the normal resistivity decreased with increasing thickness. These results agree with the picture of the interfacial superconductivity, where only the FeSe/STO interface and/or the adjacent first layer of FeSe becomes superconducting while the upper layers stay in the normal metallic state. The observed Tc is much lower than that reported by a previous in situ transport measurement for FeSe/Nb:STO but consistent with the results obtained by ex situ measurements for FeSe–undoped STO with a capping layer. This suggests that the capping layer is not an essential factor to limit Tc. We rather propose that the charge transfer from the doped substrate has a key role to achieve the higher temperature superconductivity in the one-unit-cell FeSe.

Source-reported materials — not catalogue approval

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