title: Measurement of an enhanced superconducting phase and a pronounced anisotropy of the energy gap of a strained FeSe single layer in FeSe/Nb:SrTiO3/KTaO3 heterostructures using photoemission spectroscopy.
abstract: Single-layer FeSe films with an extremely expanded in-plane lattice constant of 3.99±0.02  Å are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3 heterostructures and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillouin zone corner are resolved with negligible hybridization between them, indicating that the symmetry of the low-energy electronic structure remains intact as a freestanding single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70 K for the iron-based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results place strong constraints on current theories.
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: R
authors.initials: R
authors.lastname: Peng
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: X P
authors.initials: XP
authors.lastname: Shen
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: X
authors.initials: X
authors.lastname: Xie
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: H C
authors.initials: HC
authors.lastname: Xu
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: S Y
authors.initials: SY
authors.lastname: Tan
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: M
authors.initials: M
authors.lastname: Xia
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: T
authors.initials: T
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Key Laboratory for Computational Physical Sciences (MOE), Fudan University, Shanghai 200433, China.
authors.forename: H Y
authors.initials: HY
authors.lastname: Cao
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Key Laboratory for Computational Physical Sciences (MOE), Fudan University, Shanghai 200433, China.
authors.forename: X G
authors.initials: XG
authors.lastname: Gong
authors.affiliation: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China and Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: J P
authors.initials: JP
authors.lastname: Hu
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: B P
authors.initials: BP
authors.lastname: Xie
authors.affiliation: State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People's Republic of China.
authors.forename: D L
authors.initials: DL
authors.lastname: Feng