title: Spin-state transition in the Fe pnictides.
abstract: We report an Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca(1-x)RE(x)Fe(2)As(2) (RE = La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼ 0.9 μ(B) at T = 300 K to ∼ 0.45 μ(B) at T = 70 K. In the collapsed tetragonal phase of Nd- and Pr-doped samples (T<70 K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca(1-x)RE(x)Fe(2)As(2) (RE = Pr and Nd) exhibits a spin-state transition and provide direct evidence for a nonmagnetic Fe(2+) ion in the collapsed tetragonal phase; spin state as predicted by Yildirim. We argue that the gradual change of the spin state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.
authors.affiliation: Department of Physics, University of Toronto, 60 Saint George Street, Toronto, Ontario M5S 1A7, Canada.
authors.forename: H
authors.initials: H
authors.lastname: Gretarsson
authors.affiliation: Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Saha
authors.affiliation: Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: T
authors.initials: T
authors.lastname: Drye
authors.affiliation: Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: J
authors.initials: J
authors.lastname: Paglione
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Jungho
authors.initials: J
authors.lastname: Kim
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: D
authors.initials: D
authors.lastname: Casa
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: T
authors.initials: T
authors.lastname: Gog
authors.affiliation: Department of Physics, University of Toronto, 60 Saint George Street, Toronto, Ontario M5S 1A7, Canada.
authors.forename: W
authors.initials: W
authors.lastname: Wu
authors.affiliation: Department of Physics, University of Toronto, 60 Saint George Street, Toronto, Ontario M5S 1A7, Canada.
authors.forename: S R
authors.initials: SR
authors.lastname: Julian
authors.affiliation: Department of Physics, University of Toronto, 60 Saint George Street, Toronto, Ontario M5S 1A7, Canada.
authors.forename: Young-June
authors.initials: YJ
authors.lastname: Kim