Infrared study of the interplay of charge, spin, and lattice excitations in the magnetic topological insulator EuIn${}_2$As${}_2$

Abstract

We report an infrared spectroscopy study of the axion topological insulator candidate EuIn${}_2$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, a pronounced infrared-active phonon mode at 185 cm${}^{-1}$ and a free carrier plasma edge around 600 cm${}^{-1}$. The interband transitions start above 800 cm${}^{-1}$ and give rise to a series of weak absorption bands at 5\,000 and 12\,000 cm${}^{-1}$ and strong ones at 20\,000, 27\,500 and 32\,000 cm${}^{-1}$. The AFM transition gives rise 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 5\,000 and 12\,000 cm${}^{-1}$. The phonon mode at 185 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 amongst the charge, spin and lattice degrees of freedom.

7 pages, 4 figures