title: Enhancement of the Superconducting Gap by Nesting in CaKFe_{4}As_{4}: A New High Temperature Superconductor.
abstract: We use high resolution angle resolved photoemission spectroscopy and density functional theory with measured crystal structure parameters to study the electronic properties of CaKFe_{4}As_{4}. In contrast to the related CaFe_{2}As_{2} compounds, CaKFe_{4}As_{4} has a high T_{c} of 35 K at stochiometric composition. This presents a unique opportunity to study the properties of high temperature superconductivity in the iron arsenides in the absence of doping or substitution. The Fermi surface consists of several hole and electron pockets that have a range of diameters. We find that the values of the superconducting gap are nearly isotropic (within the explored portions of the Brillouin zone), but are significantly different for each of the Fermi surface (FS) sheets. Most importantly, we find that the momentum dependence of the gap magnitude plotted across the entire Brillouin zone displays a strong deviation from the simple cos(k_{x})cos(k_{y}) functional form of the gap function, proposed by the scenario of Cooper pairing driven by a short range antiferromagnetic exchange interaction. Instead, the maximum value of the gap is observed on FS sheets that are closest to the ideal nesting condition, in contrast to previous observations in other ferropnictides. These results provide strong support for the multiband character of superconductivity in CaKFe_{4}As_{4}, in which Cooper pairing forms on the electron and the hole bands interacting via a dominant interband repulsive interaction, enhanced by band nesting.
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Daixiang
authors.initials: D
authors.lastname: Mou
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Tai
authors.initials: T
authors.lastname: Kong
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: William R
authors.initials: WR
authors.lastname: Meier
authors.affiliation: Institut fur Theoretische Physik III, Ruhr-Universitat Bochum, 44801 Bochum, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Lochner
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.
authors.forename: Lin-Lin
authors.initials: LL
authors.lastname: Wang
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.
authors.forename: Qisheng
authors.initials: Q
authors.lastname: Lin
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Yun
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: S L
authors.initials: SL
authors.lastname: Bud'ko
authors.affiliation: Institut fur Theoretische Physik III, Ruhr-Universitat Bochum, 44801 Bochum, Germany.
authors.forename: Ilya
authors.initials: I
authors.lastname: Eremin
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.|Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: D D
authors.initials: DD
authors.lastname: Johnson
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: P C
authors.initials: PC
authors.lastname: Canfield
authors.affiliation: Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.|Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Adam
authors.initials: A
authors.lastname: Kaminski