title: Block Magnetic Excitations in the Orbitally Selective Mott Insulator BaFe_{2}Se_{3}.
abstract: Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder BaFe_{2}Se_{3}, we fully characterize the static and dynamic spin correlations associated with the Fe_{4} block state, an exotic magnetic ground state observed in this low-dimensional magnet and in Rb_{0.89}Fe_{1.58}Se_{2}. All the magnetic excitations of the Fe_{4} block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for 16(3)μ_{B}^{2} per Fe^{2+}, approximatively 2/3 of the total spectral weight expected for localized S=2 moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: M
authors.initials: M
authors.lastname: Mourigal
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Shan
authors.initials: S
authors.lastname: Wu
authors.affiliation: Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Stone
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.|Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Neilson
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.|Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Caron
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.|Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.|Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: T M
authors.initials: TM
authors.lastname: McQueen
authors.affiliation: Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.|Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.|Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: C L
authors.initials: CL
authors.lastname: Broholm