title: Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal.
abstract: A superconducting transition temperature (Tc) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3. The discovery ignited efforts to identify the mechanism for the markedly enhanced Tc from its bulk value of 8 K. There are two main views about the origin of the Tc 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 SrTiO3. Without interfacial effects, the surface layer state has a moderate Tc 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 Tc, which in turn reveals the need for interfacial effects to achieve the highest Tc in one monolayer FeSe on SrTiO3.
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea.|Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.
authors.forename: J J
authors.initials: JJ
authors.lastname: Seo
authors.affiliation: Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.|Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: B Y
authors.initials: BY
authors.lastname: Kim
authors.affiliation: Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.|Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.
authors.forename: B S
authors.initials: BS
authors.lastname: Kim
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea.
authors.forename: J K
authors.initials: JK
authors.lastname: Jeong
authors.affiliation: Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.
authors.forename: J M
authors.initials: JM
authors.lastname: Ok
authors.affiliation: Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.
authors.forename: Jun Sung
authors.initials: JS
authors.lastname: Kim
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Denlinger
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: S-K
authors.initials: SK
authors.lastname: Mo
authors.affiliation: Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.|Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.
authors.forename: C
authors.initials: C
authors.lastname: Kim
authors.affiliation: Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.|Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.
authors.forename: Y K
authors.initials: YK
authors.lastname: Kim