Colossal Terahertz Magnetoresistance from Magnetic Polarons in EuZn$_2$P$_2$

Abstract

Magnetic polarons can generate colossal magnetoresistance in magnetic semiconductors, yet their terahertz electrodynamics remain largely unexplored. Here we report magneto-terahertz spectroscopy of the Eu-based Zintl antiferromagnet EuZn$_2$P$_2$. The low-frequency conductivity shows pronounced non-Drude behavior consistent with an evolution from isolated to overlapping magnetic polarons upon cooling. The polaron relaxation time reaches a maximum at the Néel temperature and exhibits a strong magnetic-field dependence. This polaron-driven reshaping of the conductivity leads to a strongly frequency-dependent magnetoresistance that becomes colossal in the terahertz range, reaching about 90 % at 1.5 THz, roughly three times larger than the zero-frequency limit value. These results demonstrate that magnetic polarons strongly govern the low-energy electrodynamics and highlight the sensitivity of terahertz spectroscopy to polaronic magnetotransport in correlated magnetic semiconductors.

Figures (7)

• Figure 1: (a) Real part of the optical conductivity at selected temperatures in zero magnetic field. Solid lines are power-law fits to the low-frequency response below the conductivity minimum. Inset: CaAl$_2$Si$_2$-type crystal structure and schematic of the magneto-terahertz time-domain spectroscopy in Faraday geometry. (b) Temperature dependence of $\sigma_1$ at 1.5 THz, a representative frequency near the conductivity minimum. (c) Effective scattering time extracted from the fits in (a). The dashed line marks the Néel temperature. Error bars are smaller than the data point symbols.
• Figure 2: Optical conductivity at dc (circles) and the low-energy scale $\sigma_p$ evaluated at $\nu_0$ (squares), extracted from the fits in Fig. 1. Solid lines are guides to the eye. The dashed line marks $T_\mathrm{N}$. Right panels: schematic spin-plane snapshots (black: down; white: up) illustrating (I) isolated magnetic polarons, (II) their overlap and connection (percolative transport), and (III) the persistence of polaronic correlations in the antiferromagnetic phase. Error bars are smaller than the data point symbols.
• Figure 3: (a) Optical conductivity at representative magnetic fields. Solid lines are power-law fits to the low-frequency response. (b) Magnetic-field dependence of $\sigma_1$ at 0 THz (dc, filled symbols) and at 1.5 THz (open symbols), measured at $T=1.6$ and 50 K. (c) Effective scattering time extracted from the low-frequency fits in (a), shown at the same temperatures as in (b). Error bars are smaller than the data point symbols.
• Figure 4: (a--c) Terahertz magnetoresistance (%) as a function of magnetic field and frequency at $T=1.6$, 50 and 120 K. (d) Field dependence of the terahertz magnetoresistance at dc (filled symbols) and at 1.5 THz (open symbols), for $T=1.6$ K (dark blue) and 50 K (light blue). Solid lines are guides to the eye. Error bars are shown where visible; otherwise, they are smaller than the data point symbols.
• Figure S1: Semilogarithmic plot of the real part of the optical conductivity, $\sigma_1$, at 1.5 THz for $\mathrm{EuZn}_2\mathrm{P}_2$ as a function of inverse temperature. The dashed line is the Arrhenius fit over $60\ \mathrm{K} \leq T \leq 150\ \mathrm{K}$. The dot-dashed and dotted lines mark 60 K and $T_N = 23.5$ K, respectively.