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TT: Fachverband Tiefe Temperaturen

TT 2: Superconductivity: Fe-based Superconductors - 1111, LiFeAs & As-free Pnictides

TT 2.6: Vortrag

Montag, 26. März 2012, 10:45–11:00, H 2053

Spin fluctuations in the FeAs-based superconductors — •N. Qureshi¹, Y. Sidis², S. Wurmehl³, I. Morozov³, S. Aswartham³, L. Harnagea³, C. Nacke³, B. Büchner³, P. Steffens⁴, and M. Braden¹ — ¹University of Cologne — ²LLB, Saclay (France) — ³IFW Dresden — ⁴ILL, Grenoble (France)

We have performed inelastic neutron scattering (INS) studies on two undoped compounds: superconducting paramagnetic LiFeAs and non-superconducting antiferromagnetic (AF) BaFe₂As₂. INS using LiFeAs single crystals revealed clear evidence for AF fluctuations at an incommensurate Q-vector, which vanish at low energies. The excitations clearly responds to the opening of the superconducting gap by a redistribution of spectral weight. Applying more sophisticated methods like polarized neutron scattering with longitudinal polarization analysis to BaFe₂As₂ single crystals revealed anisotropic fluctuations, where the out-of-plane mode lies lower than the in-plane mode. This finding is astonishing as the static moment aligns along the FeAs layers. The fit of a J₁-J₂ model [1] using the coupling constants of SrFe₂As₂ [2] and taking into account the instrumental resolution yielded the single-ion anisotropies responsible for both gaps. In comparison to theoretical values [3] we find that the order of the gaps is reversed and that the spin-orbit coupling is underestimated in the calculations.
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