Authors:  S.-F. Wu, W.-L. Zhang, L. Li, H. B. Cao, H.-H. Kung, A. S. Sefat, H. Ding, P. Richard, and G. Blumberg

Fig 1712.06066
We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe1-xAux)2As2 superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature TS(x) and the Neel transition at TN(x). The static Raman susceptibility in XY symmetry channel increases upon cooling from room temperature following the Curie Weiss law, with Weiss temperature Tθ(x) several tens of degrees lower than TS(x). Data reveals a hidden nematic quantum critical point at x = 0.031 when the system becomes superconducting, indicating a direct connection between quantum critical nematic fluctuations and unconventional superconductivity. We attribute the origin of the nematicity to charge quadrupole fluctuations due to electron transfer between the nearly degenerate dxz/dyz orbitals.


Authors:  S.-F. Wu, W.-L. Zhang, L. Li, H.-B. Cao, H.-H. Kung, A. S. Sefat, H. Ding, P. Richard, and G. Blumberg

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We used polarization-resolved Raman scattering to study magneto-elastic coupling in Ba(Fe1-xAux)2As2 crystals as a function of light Au-doping, materials for which temperatures of the structural transition (TS) and of the magnetic ordering transition (TN) split. We study the appearance of the Ag (As)phonon intensity in the XY scattering geometry that is very weak just below TS, but for which the intensity is significantly enhanced below TN. In addition, the Ag (As) phonon shows an asymmetric line shape below TN and an anomalous linewidth broadening upon Au-doping in the magnetic phase. We demonstrate that the anomalous behavior of the Ag (As) phonon mode in the XY scattering geometry can be consistently described by a Fano model involving the Ag (As) phonon mode interacting with the B2g symmetry-like magnetic continuum in which the magneto-elastic coupling constant is proportional to the magnetic order parameter.


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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.



Authors:  W.-L. Zhang, S.-F. Wu, S. Kasahara, T. Shibauchi, Y. Matsuda, and G. Blumberg


We use polarization-resolved electronic Raman spectroscopy to study charge dynamics in non-magnetic FeSe1-xSx superconductor. We observe two features of the XY quadrupole symmetry: a low-energy quasi-elastic peak (QEP) and an electronic continuum extending to high energy. The QEP exhibits critical enhancement upon cooling towards the structural transition at TS(x). Below TS(x), the QEP diminishes gradually, and a gap with temperature evolution reminiscent to a mean- field order parameter opens up in the continuum. The intensity of the QEP develops with increasing x, while the gap magnitude decreases. We interpret development of the gap in the quadrupole scattering channel as formation of a stripe quadrupole order: a wave of quadrupole moment without charge or spin modulation.


Our new paper "Chiral Spin Mode on the Surface of a Topological Insulator" has been Highlighted by Rutgers Today, and also covered by several News Outlets:

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Authors: H.-H. Kung, S. Maiti, X. Wang, S.-W. Cheong, D.L. Maslov, G. Blumberg


Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi2Se3. We observe a sharp peak at 150 meV in the pseudovector A2 symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.