Charge density wave transition in the magnetic topological semimetal ${\mathrm{EuAl}}_4$

In rare-earth intermetallic topological materials, carriers from topological bands mediate the magnetic interactions between local moments, giving rise to a plethora of exotic quantum phenomena. Recently, anomalous magnetic instability, helical spin orders, and skyrmions were found in topological semimetal ${\mathrm{EuAl}}_4$ with tetragonal lattice. Comparing with its counterpart ${\mathrm{EuGa}}_4$, which does not show intricate magnetism, the difference lies in the presence of charge-density wave (CDW) order in ${\mathrm{EuAl}}_4$. Thus, studying the effect of CDW transition on electronic structure is decisive for the final understanding of the intricate magnetism in topological materials. Here, we studied the charge excitations in ${\mathrm{EuAl}}_4$ across the CDW transition through optical spectroscopy and the first-principles calculations. After the CDW transition, a partial gap (60 meV) on the Fermi surface and an enhanced mid-infrared absorption at around 0.4 eV were observed in the optical conductivity. With the magneto-optical spectroscopy, we further observed the evolution of charge excitations alongside the magnetization. Through the first-principles calculations, we have identified that the CDW transition not only partially erodes the Fermi surface contributed by the topological bands but also modulates the high-energy excitations between the bands dominated by Eu $5d$ and Al $3p$ orbitals. In the counterpart ${\mathrm{EuGa}}_4$, the band reconstruction is absent. Since the itinerant carriers and $pd$ hybridizations are usually assigned to mediate the magnetic interactions, our findings offer unprecedented insights to understanding the complex magnetism observed in highly symmetric topological semimetals.