Engineering a Correlated Narrow-Gap Semiconductor: Effects of Ga Substitution in EuZn$_2$P$_2$

TL;DR

This work demonstrates that Ga substitution in the Zintl semiconductor $EuZn_2P_2$ substantially narrows the electronic gap and increases free-carrier density, while preserving short-range magnetic correlations. Through a combination of electrical transport, ESR, and terahertz time-domain spectroscopy, the authors show a transition from insulating-like to enhanced metallic-like behavior, with a pronounced Dysonian ESR signature and a strong Drude response in the Ga-substituted compound. The results reveal three transport regimes tied to magnetic fluctuations and magnetic-polaron physics, and quantify the boosted Drude weight and carrier scattering times, underscoring Ga substitution as an effective route to engineer correlated narrow-gap magnetic semiconductors. Overall, EuZn$_{2-x}$Ga$_x$P$_2$ emerges as a versatile platform for tuning electronic, optical, and spin-dependent functionalities relevant to spintronics, optoelectronics, and quantum sensing, including dark-metection schemes relying on low-energy excitations.

Abstract

The effect of Ga substitution on the electronic, magnetic, and low-energy responses of the Zintl phase EuZn$_2$P$_2$ is investigated by electrical transport, electron spin resonance (ESR), and terahertz time-domain spectroscopy (THz-TDS). Incorporating Ga into EuZn$_2$P$_2$ (EuZn$_{1.8}$Ga$_{0.2}$P$_2$) reduces the electrical resistivity, indicating enhanced free-carrier density and a narrowed semiconducting gap. ESR confirms the persistence of Eu$^{2+}$ moments while showing a crossover from a Lorentzian to a Dysonian lineshape, consistent with reduced skin depth, increased carrier density, and the emergence of diffusive contributions. Ga-substituted compound display pronounced negative magnetoresistance linked to magnetic-polaron formation. THz-TDS reveals strong low-frequency absorption and a notable enhancement of the Drude conductivity in the substituted material, together with an increased carrier scattering time and enhanced carrier-density--to--effective-mass ratio. These results demonstrate that Ga substitution tunes charge transport, carrier dynamics, and short-range magnetic correlations in EuZn$_2$P$_2$, establishing EuZn$_{1.8}$Ga$_{0.2}$P$_2$ as a promising platform for engineering correlated narrow-gap magnetic semiconductors with enhanced electronic and spin-dependent functionalities.

Figures (9)

• Figure 1: powder X-ray diffraction pattern of crushed single crystals of a) EuZn$_2$P$_2$ and b) EuZn$_{1.8}$Ga$_{0.2}$P$_2$. The experimental data are presented in blue and black for EuZn$_2$P$_2$ and EuZn$_{1.8}$Ga$_{0.2}$P$_2$, respectively. The Rietveld refinement is shown in red, the difference between experiment and model is shown in gray, and the Bragg reflections corresponding to EuZn$_2$P$_2$, EuZn$_{1.8}$Ga$_{0.2}$P$_2$ and Sn flux are shown as vertical lines.
• Figure 2: Comparison of the electrical resistances of EuZn$_2$P$_2$ (blue) and EuZn$_{1.8}$Ga$_{0.2}$P$_2$ (black). The activated regime in EuZn$_2$P$_2$ begins around 150 K, whereas in EuZn$_{1.8}$Ga$_{0.2}$P$_2$ it starts near 50 K.
• Figure 3: a) Electrical resistivity of EuZn$_{1.8}$Ga$_{0.2}$P$_2$ from 45.0 to 300. At least three distinct transport regimes can be identified in this temperature range. b) Arrhenius plot with linear fitting, yielding an estimated energy gap of approximately 63.
• Figure 4: Isothermal magnetoresistance of EuZn$_{1.8}$Ga$_{0.2}$P$_2$ with $i \perp c$ and $H \parallel c$ at various temperatures. a) MR as a function of applied magnetic field from 0.0 to 9. b) MR as a function of $(\mu_0 H)^2$.
• Figure 5: ESR spectra at 300 K under various microwave powers for EuZn$_2$P$_2$ for a) $H \parallel c$ and b) with $H \parallel ab$, both showing a Lorentzian lineshape. EuZn$_{1.8}$Ga$_{0.2}$P$_2$ for c) $H \parallel c$ and d) $H \parallel ab$. These spectra show a Dysonian lineshape with clear signatures of diffusion.