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.

Close

PrevNext

View figure in article

• Charge density waves
• Electronic structure
• Magnetic anisotropy
• Magneto-optical effect
• Topological materials
• Single crystal materials
• Optical spectroscopy
• Reflectivity