Anisotropic and quasi-propagating spin excitations in optimally-doped superconducting Ba(Fe0.926Co0.074)2As2

Haifeng Li (Ames Lab, Iowa)

The suppression of an antiferromagnetic (AFM) phase and the discovery of a spin resonance in the superconducting (SC) state indicate a close connection between magnetism and superconductivity in the recently discovered pnictides. An understanding of the significance of spin fluctuations and their connection with superconductivity requires a detailed exploration of the magnetic excitations. In this talk I will present inelastic neutron scattering from SC Ba(Fe0.926Co0.074)2As2 over a wide range of energies. The in-plane wavevector dependence of the spin resonance at about 8.3 meV in the SC state displays a distinct anisotropy around QAFM = (0.5 0.5). This correlation length anisotropy persists up to the high energy magnetic excitations. Above about 80 meV, the anisotropic spin fluctuations evolve into two maxima only along the direction transverse (TR) to QAFM, indicating quasi-propagating excitations there. We employed the traditional diffusive model to describe the unsplitted magnetic fluctuations, while to model the TR splitting above 80 meV, we proposed a new quasi-propagating model. I will also theoretically discuss the application ranges of both models.

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