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.