Superconductivity above 28 K in single unit cell FeSe films interfaced with GaO2− layer on NdGaO3(1 1 0)

The single unit cell FeSe films grown on GaO2 d terminated perovskite NdGaO3(1 1 0) substrates are found to host enhanced superconductivity with an onset temperature of 28 K. 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved. The discovery of high temperature superconductivity in single unit cell (UC) FeSe on TiO2 d terminated perovskite SrTiO3(0 0 1) substrates [1] has attracted intensive attention on searching for new superconducting systems with engineered interfaces as well as understanding the mechanism of interface high temperature superconductivity. In stark contrast to bulk FeSe—a superconductor with transition temperature Tc 8 K at ambient pressure [2], the single UC FeSe on SrTiO3(0 0 1) becomes superconducting at Tc 65 K [3–5] or higher [6,7], holding the record Tc among all known Fe-based superconductors. The superconducting gap is enhanced by one order of magnitude compared to the value of bulk FeSe, i.e. 20 meV [1] vs. 2.2 meV [8]. What is more interesting is that monolayer FeSe on several TiO2 d terminated substrates, such as BaTiO3 [9], SrTiO3(1 1 0) [10–12] and TiO2 [13,14], exhibit similarly high transition temperatures, despite the variation of lattice constant [15,16]. The common features shared by all these 1-UC FeSe/TiO2 d systems are the enlarged electron pockets at the Brillouin zone (BZ) corners [3,17,18] and emergent replica bands 90–100 meV from the main bands [11,14,18]. They indicate that FeSe is electron-doped and the electrons therein couple with Ti-O stretching phonons [19,20], respectively. On the other hand, electron-doped FeSe systems, such as K-coated multilayer FeSe, exhibit similar Fermi surfaces but universally lower Tc (<45 K) and smaller superconducting gaps (<14 meV) [21–24]. This difference indicates that interface electron-phonon interaction plays an important role in further enhancing superconductivity by cooperatively mediating the cooper pairing [15,16,18,22,25]. The FeSe/TiO2 d interface bears sharp resemblance to the building blocks of cuprate and Fe-based high temperature superconductors. There, superconductivity emerges in the CuO2 and FeAs layers, which are interfaced with oxide layers of BaO/SrO and LaO/SmO, respectively [26–28]. The fact that a variety of oxide layers can act as the charge reservoir layer motivates us to look for an oxide layer that is different from TiO2 d. One recent endeavour by interfacing FeSe with MgO(0 0 1) proved that the onset superconducting temperature can be raised to 18 K [29]. Here, we report on the observation of superconductivity with an onset temperature of 28 K in single UC FeSe films epitaxially grown on perovskite NdGaO3. Scanning transmission electron microscopy (STEM) reveals that FeSe sits on double GaO2 d in a striking similar manner as that of FeSe on TiO2 d. Electron energy loss spectroscopy (EELS) further identifies the charge transfer across the FeSe/GaO2 d heterojunction. NdGaO3 has been commonly used as a substrate for thin-film deposition of high temperature cuprate superconductors due to the matching lattice constants and thermal expansion coefficients [30,31]. NdGaO3 has a distorted orthorhombic structure with lattice parameters: a = 0.543 nm, b = 0.550 nm and c = 0.772 nm [32]. In order to achieve lattice matching between FeSe and NdGaO3, we chose NdGaO3(1 1 0). This crystal orientation corresponds to in-plane lattice constants: a(1 1 0) = 0.384 nm and b(1 1 0) = 0.386 nm (Fig. 1a), which are close to the in-plane lattice constant of FeSe: aFeSe = 0.379 nm [2]. To obtain single GaO2 dterminated surface, we pretreated NdGaO3(1 1 0) substrates by https://doi.org/10.1016/j.scib.2019.03.017 2095-9273/ 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved. ⇑ Corresponding authors. E-mail addresses: liliwang@mail.tsinghua.edu.cn (L. Wang), qkxue@tsinghua. edu.cn (Q.-K. Xue), dingzhang@tsinghua.edu.cn (D. Zhang). 1 These authors contributed equally to this work. Science Bulletin 64 (2019) 490–494