Inelastic Neutron Scattering Studies of the Spin and Lattice Dynamics inIron Arsenide Compounds

Journal Article · Thu Jan 01 04:00:00 EST 2009 · Physica C

OSTI ID:979596

Christianson, Andrew D ^[1]; Osborn, R. ^[2]; Rosenkranz, Stephen ^[2]; Goremychkin, E. A. ^[3]

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1. ORNL
2. Argonne National Laboratory (ANL)
3. ISIS Facility, Rutherford Appleton Laboratory

Although neutrons do not couple directly to the superconducting order parameter, they have nevertheless played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Measurements of the spin and lattice dynamics have been performed on non-superconducting 'parent' compounds based on the LaFeAsO ('1111') and BaFe{sub 2}As{sub 2} ('122') crystal structures, and on electron and hole-doped superconducting compounds, using both polycrystalline and single crystal samples. Neutron measurements of the phonon density-of-state, subsequently supported by single crystal inelastic X-ray scattering, are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, when combined with estimates of the electron-phonon coupling, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order and a large anisotropy gap. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below T{sub c}, there is evidence in three '122' compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with T{sub c}. Such resonances have been observed in the high-T{sub c} copper oxides and a number of heavy fermion superconductors, where they are considered to be evidence of d-wave symmetry. In the iron arsenides, they also provide evidence of unconventional superconductivity, but a comparison with ARPES and other measurements, which indicate that the gaps are isotropic, suggests that the symmetry is more likely to be extended-s{sub {+-}} wave in character.

• Research Organization: Oak Ridge National Laboratory (ORNL); High Flux Isotope Reactor; Spallation Neutron Source
• Sponsoring Organization: SC USDOE - Office of Science (SC)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 979596
• Journal Information: Physica C, Journal Name: Physica C Journal Issue: 9-12 Vol. 469; ISSN 0921-4534; ISSN PHYCE6
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ANISOTROPY
COPPER OXIDES
CRYSTAL STRUCTURE
ELECTRON-PHONON COUPLING
ENERGY GAP
EXCHANGE INTERACTIONS
FERMIONS
FLUCTUATIONS
IRON
IRON ARSENIDES
MONOCRYSTALS
NEUTRONS
ORDER PARAMETERS
PHONONS
SCATTERING
SPIN
SPIN WAVES
SUPERCONDUCTIVITY
SUPERCONDUCTORS
SYMMETRY
TRANSITION TEMPERATURE