Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal

A superconducting transition temperature ( T c ) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO 3 . The discovery ignited efforts to identify the mechanism for the markedly enhanced T c from its bulk value of 8 K. There are two main views about the origin of the T c enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20 K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO 3 . Without interfacial effects, the surface layer state has a moderate T c of 20 K with a smaller gap opening of 4.2 meV. Our results show that excess electrons with strong correlation cannot induce the maximum T c , which in turn reveals the need for interfacial effects to achieve the highest T c in one monolayer FeSe on SrTiO 3 . Thin FeSe film on SrTiO 3 substrate becomes a superconductor with a transition temperature over 100 K, yet the origin remains controversial. Here, Seo et al . show superconductivity below 20 K on the electron-doped surface of an FeSe crystal, suggesting a decisive role of interfacial effects in the enhancement of superconductivity.