Quadrupolar Charge Dynamics in the Magnetically Ordered Superconductor EuRbFe₄As₄

G. Blumberg,^1,2 M. Thinel,³ C. Garrity,¹ S. Ishida,⁴ Akira Iyo,⁴ H. Eisaki,⁴ and A. N. Pasupathy,³

¹Department of Physics & Astronomy, Rutgers University, New Jersey, USA

²National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

³Department of Physics, Columbia University, New York, NY 10027, USA

⁴National Institute of Advanced Industrial Science and Technology, Tsukuba 205-8568, Japan

We employ polarization-resolved Raman spectroscopy to study the helical ferromagnetic superconductor EuRbFe₄As₄, where translational symmetry breaking unidirectional gap modulations have been observed below T_m = 15K [1]. At this temperature, we detect a change in the screening of the Raman vertex in the fully symmetric A_1g channel and find evidence for phonon softening consistent with a bulk phase transition. Below the critical temperature of 35 K, we observe opening of multiple superconducting gap features in the B_2g symmetry channel of the Raman response. These are accompanied by a sharp in-gap collective mode which we attribute to under-damped quadrupolar nematic fluctuations. The Raman and tunneling spectroscopy measurements indicate coupling of these bosonic fluctuations to continuum quasiparticles. Remarkably, the collective mode is strongly enhanced at the onset of the smectic pair density wave phase. The study points to cohesive interactions between quadrupolar fluctuations, magnetic order, and superconductivity.

[1] H. Zhao et. al., Nature 618, 940 (2023).