title: Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure.
abstract: Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.
authors.affiliation: Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Torino, Italy.
authors.forename: R S
authors.initials: RS
authors.lastname: Gonnelli
authors.affiliation: Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Torino, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Daghero
authors.affiliation: Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Torino, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Tortello
authors.affiliation: Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Torino, Italy.
authors.forename: G A
authors.initials: GA
authors.lastname: Ummarino
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław, Poland.
authors.forename: Z
authors.initials: Z
authors.lastname: Bukowski
authors.affiliation: Laboratory for Solid State Physics, Swiss Federal Institute of Technology (ETH), CH-8093 Zürich, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Karpinski
authors.affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.
authors.forename: P G
authors.initials: PG
authors.lastname: Reuvekamp
authors.affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.
authors.forename: R K
authors.initials: RK
authors.lastname: Kremer
authors.affiliation: Dipartimento di Scienze Fisiche e Chimiche, Università dell'Aquila, 67100 Coppito (AQ), Italy.
authors.forename: G
authors.initials: G
authors.lastname: Profeta
authors.affiliation: Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Suzuki
authors.affiliation: Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Kuroki