Doping-tunable Fermi surface with persistent topological Hall effect in axion candidate EuIn$_2$As$_2$

TL;DR

This study demonstrates that Ca doping in EuIn$_2$As$_2$ effectively tunes its Fermi surface and preserves its topological Hall effect and antiferromagnetic order, bringing it closer to realizing an axion insulator state.

Contribution

The paper reports the synthesis and characterization of Ca-doped EuIn$_2$As$_2$ crystals, showing Fermi level tuning and persistent topological phenomena, advancing the pursuit of axion insulator realization.

Findings

Ca doping decreases hole carrier density.

Topological Hall effect persists with doping.

Antiferromagnetic order remains despite doping.

Abstract

Rare-earth Zintl compound EuIn$_2$As$_2$ has been theoretically recognized as a candidate for realizing an intrinsic antiferromagnetic (AFM) bulk axion insulator and a higher-order topological state, which provides a fertile platform to explore novel topological transport phenomena. However, the axion state has yet to be realized because EuIn$_2$As$_2$ is highly hole-doped. Here, we synthesized a series of high-quality Ca-doped EuIn2As2 (Ca$_x$Eu$_{1-x}$In$_2$As$_2$, x = 0 ~ 0.25) single crystals to tune the Fermi energy above the hole pocket. Our Hall measurements reveal that the isovalent Ca substitution decreases the hole carrier density by shrinking the lattice spacing, which is also confirmed by our first-principles calculations. We further find that both the temperature dependence of the magnetic susceptibility with a local maximum at the N\'eel temperature and the topological Hall effect originating from the finite real-space spin chirality persist in the Ca-doped samples as observed in the pristine EuIn$_2$As$_2$, despite that the nonmagnetic Ca substitution decreases the effective moment and the N\'eel temperature. These results show that the Ca substitution tunes the Fermi energy while keeping the AFM magnetic structure, suggesting that the axion insulating state may be realized by further Ca substitution.