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).