Iron chalcogenide superconductor, FeSe, has attracted much attention for high tunability of the superconducting transition temperature (T_c). Although T_c of FeSe is 8 K in bulk form, it increases up to approximately 40 K by applying hydrostatic pressure, intercalation etc. Furthermore, ARPES measurements reported that a monolayer epitaxial film grown on SrTiO₃ (STO) substrate shows superconductivity [1] with T_c as high as 65 K [2]. The high T_c in the monolayer FeSe/STO is attributed to interface effects such as electron doping from STO substrate and electron-phonon interaction via the substrate. One important problem in the high T_c superconductivity in monolayer FeSe/STO is discrepancy in reported values of T_c depending on the measurement techniques. The 65 K T_c was reported by ARPES measurements. However, almost all of resistivity measurements reported the onset T_c (T_c^onset) of about 40 K and much lower zero-resistance T_c (T_c^zero) [1,3]. The only exception is the report of 109 K T_c^zero by Ge et al.[4], for which a skepticism was proposed in terms of reproducibility and reliability [5]. The disagreement between resistance T_c and those obtained by ARPES measurements casts a question whether the real T_c in monolayer FeSe/STO is 65 K or not. We have worked on the growth of epitaxial thin films of FeSe by a pulsed laser deposition (PLD) technique [6]. We investigated electron doping effect by fabricating electric double layer transistor (EDLT) structure on the films, and found that even only electron doping increases T_c^zero up to 46 K [7,8]. This is the highest T_c^zero of FeSe ever reported including monolayer FeSe/STO except for ref. [4]. Thus, it is intriguing how T_c of FeSe films grown by PLD changes as the thickness of FeSe films is reduced. It should be noted that all reports of high T_c in monolayer FeSe/STO is obtained in films grown by molecular-beam epitaxy (MBE). It might be possible that T_c^zero of monolayer FeSe/STO is higher than those ever reported when fabricated by PLD. In this work, we report growth and transport properties of FeSe/STO ultrathin films by the PLD technique.

Prior to the deposition, the STO substrates were thermally treated so that the step and terrace structure on the surface was observed. This is very important to obtain films with good superconducting properties. Fig. 1 shows the temperature dependence of resistivity for three films with different thicknesses, namely 30 nm, 20 nm, and 11 nm. All films show superconducting transition at low temperatures. T_c^onset increases as the thickness is reduced, and the 11-nm-thick film shows the highest T_c^onset = 26 K and T_c^zero = 11 K. These results are remarkable since these T_c^onset values are exceptionally high in terms of the relationship between T_c and in-plane strain reported for bulk superconductivity in FeSe films [9]. All of these films have in-plane lattice constant, a ≧ 3.77 Å, leading to T_c < 8K according to the relationship reported in ref. [9]. In addition, the behavior that T_c^onset increases with decreasing thickness is in good agreement with those of MBE-grown films [10] even quantitatively. These results indicate that the high T_c in the grown films is the result of the interface effects other than the lattice strain. In other words, we succeeded in realizing interface superconductivity in FeSe/STO by the PLD technique for the first time.
In the presentation, we will also show results of thinner films.

Figure 1 : Temperature dependence of resistivity for 30, 20, 11 nm films of FeSe/STO

References
[1] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012)
[2] S. He et al., Nat. Mater. 12, 605 (2013)
[3] A. K. Pedersen et al., Phys. Rev. Lett. 124, 227002 (2020)
[4] J. F. Ge et al., Nat. Mater. 14, 285 (2014)
[5] I. Bozovic and C. Ahn, Nat. Phys. 10, 892 (2014)
[6] Y. Imai, F. Nabeshima, A. Maeda Condens. Matter 2, 25 (2017)
[7] N. Shikama et al., Appl. Phys. Express 13, 083006 (2020)
[8] N. Shikama et al., Phys. Rev. B 104, 094512 (2021)
[9] F. Nabeshima et al., JJAP 57, 120314 (2018)
[10] Q. Wang et al., 2D Mater. 2, 044012 (2015)

Keywords: Iron chalcogenide, Pulsed laser deposition, Supercondutivity, ultrathin films