Multipolar interactions and related ordering phenomena have attracted great interest. Multipoles are related to exotic phases including possible multipole-fluctuation induced superconductivity and hidden-order phase. For a systematic investigation of the collective behaviors of multipole moments, f-electron systems are suitable choices since the interplay of spin and orbital degrees of freedom of f-electrons facilitates multipole formation.

In this work, we report Raman-scattering results of YbRu2Ge2 single crystals. This heavy-fermion metal has been suggested to enter a ferroquadrupolar (FQ) phase below T0=10K. We determined the crystal-field (CF) level scheme of Yb3+ ground multiplet in YbRu2Ge2. Importantly, the lowest-energy CF transition couples to the quadrupolar fluctuations above T0. From symmetry analysis, we further suggest that the FQ order has B1g symmetry.

Our study demonstrates the ability of Raman-scattering technique to probe the symmetry of dynamical fluctuations above unusual electronic phase transitions. Moreover, determination of the CF level scheme, and identification of the symmetry of the FQ order of YbRu2Ge2  will not only serve as basis for future study of this model system, but also inspire more interest in multipole-related phenomena.

The figure above shows a schematic energy diagram of the CF states (red horizontal lines) and the phonon modes (blue horizontal lines). The coupled CF transition and phonon modes are circled in purple. On the left are the angular electron-cloud distributions of the four CF states; on the right are the vibration patterns of the phonon modes.

Authors: W.-L. Zhang, Y. Song, W.-Y. Wang, C.-D. Cao, P.-C. Dai, C.-Q. Jin, and G. Blumberg

Abstract: We use polarization-resolved Raman scattering to study lattice dynamics in  NaFe0.53Cu0.47As single crystals. We identify four A1g phonon modes, at 126, 172, 183, and 197 cm-1, and four B3g phonon modes at 101, 139, 173, and 226
cm-1(D2h point group). The phonon spectra are consistent with the Ibam space group, which confirms that the Cu and Fe atoms form a stripe order. The temperature dependence of the phonon spectra suggests weak electron-phonon and magnetoelastic interactions.


Authors:  M.Ye, H.-S.Kim, J.-W.Kim, C.-J.Won, K.Haule, D.Vanderbilt, S.-W.Cheong and G. Blumberg

Figure 1

We report ab-initio density functional theory calculation and Raman scattering results to explore the electronic structure of Ba5CuIr3O12 single crystals. This insulating iridate, consisting of face-sharing IrO6 octahedra forming quasi-one-dimensional chains, cannot be described by the local jeff = 1/2 moment picture commonly adopted for discussing electronic and magnetic properties of iridate compounds with IrO6 octahedra. The shorter Ir-Ir distance in the face-sharing geometry, compared to corner- or edge-sharing structures, leads to strong covalency between neighboring Ir. Then this strong covalency results in the formation of molecular orbitals (MO) at each Ir trimers as the low-energy electronic degree of freedom. The theoretically predicted three-peak structure in the joint density of states, a distinct indication of deviation from the jeff = 1/2 picture, is verified by observing the three-peak structure in the electronic excitation spectrum by Raman scattering.


We are proud to announce that Dr. Hsiang-Hsi (Sean) Kung has been awarded the 2018 Richard J. Plano Dissertation Prize. This Prize is awarded to the graduate student with the best Ph.D. dissertation this academic year. For more information please see here.

Also congratulations to undergraduate student Viktor Krapivin for receiving the Richard T. Weidner Physics Prize for outstanding academic performance. For more information please see here.
Viktor will be attending Stanford University in the graduate program in applied physics next year.

Authors:  W.-L.Zhang, W.R.Meier, T.Kong, P.C.Canfield and G. Blumberg

The B2g symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant d-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant λΓ=0.0015.


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

Screen Shot 2017-12-06 at 10.26.55 AM


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.