Spectroscopic signatures of magnetization-induced band renormalization and strong spin-charge-lattice coupling in EuZn${}_2$As${}_2$

We report an infrared spectroscopy study of the antiferromagnetic (AFM) insulator EuZn${}_2$As${}_2$ over a broad frequency range, spanning temperatures both above and below the AFM transition $T_{\mathrm{N}}\simeq$ 20 K. The optical response reveals an insulating behavior, featuring two prominent infrared-active phonon modes at around 95 and 190 cm${}^{-1}$, and two subtle absorption peaks at around 130 ($\alpha$ peak) and 2700 cm${}^{-1}$ ($\beta$ peak), along with a strong absorption edge rising around 9000 cm${}^{-1}$ ($\gamma$ peak). Significantly, the temperature-dependent changes in these peaks show noticeable anomalies across the AFM transition, particularly the emergence of the $\alpha$ peak and an unusual redshift of the $\gamma$ peak, suggesting a strong interaction between the charge excitations and the AFM order. Band structure calculations reveal that these anomalies arise from magnetization-induced band renormalizations, including shifts and foldings. Additionally, both phonon modes feature asymmetric Fano line shapes at low temperatures, with the 95 cm${}^{-1}$ phonon mode exhibiting strong coupling to the fluctuations of Eu spins. These findings highlight a complex interplay of spin, charge, and lattice degrees of freedom in EuZn${}_2$As${}_2$.