Charge Density Wave Transition in EuAl₄ Revealed by Nonlinear Optics

Charge density wave (CDW) is a topic of long history in condensed matter physics and remains exciting until today as many new aspects of CDW have been revealed lately and constantly. EuM₄ (M = Al, Ga) family exhibits intricate interplays between the CDW phase and their topological and magnetic properties. More interestingly, the CDW in EuM₄ is likely mediated through a transverse phonon, and hence, offers the opportunity for to realize chiral and/or ferro-axial CDWs, which is distinct from the classic picture of CDWs coupled with longitudinal acoustic phonons. Yet, the proof of chiral or ferro-axial CDWs is nontrivial. Here, we report our effort in using second and third harmonic generations to study the CDW phase in EuAl₄. We will show the experimental evidence of breaking both the vertical and diagonal mirrors when entering into the CDW phase, and we will discuss our results found by nonlinear optics in connection with the x-ray diffraction measurements.