Exploring Axion physics in quantum materials via magnetoelectric coupling

Abstract

Axion, a hypothetical particle in high-energy physics, was originally proposed to solve both the strong CP problem in quantum chromodynamics and the dark matter problem. Due to its extremely weak interaction with ordinary matter, the Axion has remained elusive despite decades of searches. In electromagnetism, Axion physics introduces an additional E · B term to Maxwell’s equations, which couples electric and magnetic fields. In condensed matter physics, this framework finds an analog in the magnetoelectric coupling of topological materials. In vacuum or conventional materials, an electric field induces electrical polarization, while a magnetic field generates magnetization—typically without cross-coupling between the two. However, beyond this paradigm, certain novel materials could exhibit magnetoelectric coupling, where an electric field induces magnetization (M = αE) or a magnetic field induces electric polarization (P = αH). The magnetoelectric effect is commonly observed in a special class of wide-bandgap magnetic insulators, known as magnetoelectric or multiferroic insulators (e.g., Cr₂O₃ and BiFeO₃), where the effect arises from localized magnetic ions. More recently, theoretical advances have predicted fundamentally new types of magnetoelectric coupling in topological materials—systems characterized by nontrivial topological invariants and Berry curvature. Unlike conventional insulators, these materials can support robust dissipationless edge states and exhibit magnetoelectric phenomena rooted in their topological electronic structure. In particular, magnetic topological insulators provide a platform where magnetism and topology intertwine, giving rise to rich physics, including a quantized magnetoelectric coefficient in topological insulators, Axion quasiparticles in antiferromagnetic (AFM) topological insulators, and the chiral anomaly in magnetic Weyl semi-metals. In this thesis, we present several experimental discoveries, uncovering the unique magnetoelectric effect in magnetic topological material. First, we investigate the optical magnetoelectric effect in the prototypical AFM topological insulator MnBi₂Te₄, which enables unique reflection circular dichroism in an antiferromagnet. Furthermore, we achieved the optical control of the antiferromanetic order by circularly polarized light for the first time. Both the optical detection and control could be understood within the framework of optical Axion electrodynamics. Next, we report the direct observation of the Axion quasiparticle in MnBi₂Te₄, which is a condensed matter analog of the dark matter Axion. Using ultrafast optical pump-probe techniques, we observe coherent oscillations of the magnetoelectric coefficient α in MnBi₂Te₄, which is the smoking-gun evidence for the Axion quasiparticles. Microscopically, the Axion quasiparticle is enabled by magnon-induced Berry curvature modulation. The observed Axion quasiparticle not only can serve as a simulator for the elusive dark matter Axion, but it could also serve as a potential Axion detector, providing a novel path for dark matter detection. Lastly, we shift our focus to explore the magnetoelectric coupling in a magnetic Weyl semimetal CeAlSi. Firstly, we uncover a broadband nonlinear optical diode effect (NODE) in CeAlSi, where the magnetization induces a pronounced directional asymmetry in the optical second-harmonic generation (SHG). DFT calculations also show that this broadband NODE effect originates from the Weyl fermions. By applying an electrical current, we further explore the magnetoelectric coupling in this magnetic Weyl semimetal, in which we discover the electric control over the magnetic domains.