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.
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.
We are proud to announce that graduate student Mai Ye has been awarded the David C. Langreth Graduate Development Award. This awarded is awarded to "promising early-stage graduate student, typically in the second year of graduate study, providing travel and living expenses for use in attending a scientific summer school or tutorial workshop in condensed matter or related fields." For more information please see here.
Also congradulations to undergraduate students Viktor Krapivin for winning the Mary Wheeler Wigner Memorial Scholarship for "demonstrated outstanding academic excellence" (more here) and William Cheng for winning the Robert L. Sells Scholarship for "demonstrated outstanding academic excellence" (more here).