Charge density wave transition in the magnetic topological semimetal EuAl4

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 EuAl4 with tetragonal lattice. Comparing with its counterpart EuGa4, which does not show intricate magnetism, the difference lies in the presence of charge-density wave (CDW) order in EuAl4. 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 EuAl4 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 EuGa4, 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.