Magneto-elastic coupling in Fe-based superconductors

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Authors:  S.-F. Wu, W.-L. Zhang, V. K. Thorsmølle, G. F. Chen, G. T. Tan, P. C. Dai, Y. G. Shi, C. Q. Jin, T. Shibauchi, S. Kasahara, Y. Matsuda, A. S. Sefat, H. Ding, P. Richard, and G. Blumberg


We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showing magnetic ordering. We conclude that the small lattice anisotropy is insufficient to induce the in-plane electronic polarizability anisotropy necessary for the observed phonon intensity enhancement, and interpret this enhancement below the Neel temperature in terms of the anisotropy of the magnetic moment and magneto-elastic coupling. We evidence a Fano line-shape in the XY scattering geometry resulting from a strong coupling between the Ag (As) phonon mode and the B2g symmetry-like electronic continuum. Strong electron-phonon coupling may be relevant to superconductivity.