Effects of Surface Electron Doping and Substrate on the Superconductivity of Epitaxial FeSe Films

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• 作者：W. H. Zhang ; X. Liu ; C. H. P. Wen ; R. Peng ; S. Y. Tan ; B. P. Xie ; T. Zhang ; D. L. Feng
• 刊名：Nano Letters
• 出版年：2016
• 出版时间：March 9, 2016
• 年：2016
• 卷：16
• 期：3
• 页码：1969-1973
• 全文大小：366K
• ISSN：1530-6992

文摘

Superconductivity in FeSe is greatly enhanced in films grown on SrTiO₃ substrates, although the mechanism behind remains unclear. Recently, surface potassium (K) doping has also proven able to enhance the superconductivity of FeSe. Here, by using scanning tunneling microscopy, we compare the K doping dependence of the superconductivity in FeSe films grown on two substrates: SrTiO₃ (001) and graphitized SiC (0001). For thick films (20 unit cells (UC)), the optimized superconducting (SC) gaps are of similar size (∼9 meV) regardless of the substrate. However, when the thickness is reduced to a few UC, the optimized SC gap is increased up to ∼15 meV for films on SrTiO₃, whereas it remains unchanged for films on SiC. This clearly indicates that the FeSe/SrTiO₃ interface can further enhance the superconductivity, beyond merely doping electrons. Intriguingly, we found that this interface enhancement decays exponentially as the thickness increases, with a decay length of 2.4 UC, which is much shorter than the length scale for relaxation of the lattice strain, pointing to interfacial electron–phonon coupling as the likely origin.