Day / Time
Date(s) - 01/03/2016
13 h 00 min - 14 h 00 min
Mardi 1er mars 2016 à 13.00
Ecole de physique
Anisotropy of magnetic correlations in FeAs based superconductors: an orbital finger print
Neutron scattering gives direct insight to the spin-space anisotropy of magnetic correlations driven through spin-orbit coupling. There is clear evidence that magnetic excitations in FeAs-based materials – in antiferromagnetic and in superconducting compounds – exhibit strong anisotropy resulting even in split resonance modes. In the antiferromagnetic BaFe2As2 parent compound  it costs more energy to rotate the spins within the FeAs planes than perpendicular to them in contrast to a simple easy-plane model of magnetic anisotropy. Qualitatively the same anisotropy persists in the superconducting materials [2-4]. All measurements on doped superconducting 122 samples yield significant anisotropies reflecting those in the SDW phase of the parent compound . Spin-orbit coupling remains thus a relevant parameter in the superconducting part of the phase diagrams. For 6% Co doping, there is evidence for a well-defined resonance excitation  sitting in energy below the broader isotropic mode. Superconducting materials that are close to the SDW order and near optimum doping, exhibit thus two characteristics reminiscent of the AFM ordering: a finite l-dispersion  and strong spin-space anisotropy .
In slightly Na-underdoped BaFe2As2 polarized and unpolarized neutron diffraction experiments reveal a spin reorientation from the usual alignment along the in-plane component of the propagation vector towards vertical orientation . This spin reorientation reflects the general spin-space anisotropy of FeAs-based materials which is characterized by an in-plane hard axis. An orbital explanation of these effects will be discussed.
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