We report an infrared spectroscopy study of the axion topological insulator candidate $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ for which the Eu moments exhibit an A-type antiferromagnetic (AFM) order below $T_N\simeq 18\mathrm{K}$. The low-energy response is composed of a weak Drude peak at the origin with a plasma frequency of about $2000{\mathrm{cm}}^{-1}$ and a pronounced infrared-active phonon mode at $185{\mathrm{cm}}^{-1}$. The interband transitions start above $800{\mathrm{cm}}^{-1}$ and give rise to a series of weak absorption bands at 5000 and $12000{\mathrm{cm}}^{-1}$ and strong ones at 20 000, 27 500, and $32000{\mathrm{cm}}^{-1}$. The AFM transition leads to pronounced anomalies of the charge response in terms of a cusp-like maximum of the free carrier scattering rate around $T_N$ and large magnetic splittings of the interband transitions at 5000 and $12000{\mathrm{cm}}^{-1}$. The phonon mode at $185{\mathrm{cm}}^{-1}$ has also an anomalous temperature dependence around $T_N$ which suggests that it couples to the fluctuations of the Eu spins. The combined data provide evidence for a strong interaction among the charge, spin, and lattice degrees of freedom.

• Received 6 April 2021
• Accepted 14 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.245101