Axion Electrodynamics in Topological Antiferromagnets

Axions are hypothetical particles that were originally proposed to solve the so-called strong charge–parity problem in quantum chromodynamics and have also been speculated as candidates for cold dark matter. However, in condensed matter physics, the term “axion electrodynamics” is used to describe effective electromagnetic responses in certain materials with electronic bandstructure topology. Most prominent, experimental observations of the quantum anomalous Hall effect have been interpreted in terms of this effective field theory description [1,2].

This project aims to deepen our understanding of a new class of materials that exhibit axion-like electrodynamic through the study of epitaxial thin films and heterostructures of topological antiferromagnets (AF). Remarkably, these materials preserve Kramers degeneracy despite breaking time-reversal symmetry, presenting rich physics for exploration. In that regard, the van der Waals layered compound MnBi₂Te₄ is of particular interest due to its layered antiferromagnetism, where magnetic order can switch between compensated or uncompensated depending on whether the number of septuple layers is even or odd [3]. The quantum anomalous Hall effect has already been reported for an odd-number of septuple layers [4], while a transition to a topological antiferromagnetic state has been inferred upon stabilizing the even-number septuple layer counterpart [5]. This state is associated with remarkable magnetoelectric responses, which can be modeled effectively using the framework of axion electrodynamics [6,7].

(a) Crystal structure of ABC-stacked MnBi2Te4 with an A-type AF ordering. (b,c) Illustration of intrinsic axion insulators in even layers and QAH insulators in odd layers. The intrinsically gapped surfaces on the top and bottom surfaces have half-quantized Hall conductances, whose signs are opposite (the same) in even (odd) layers, leading to Chern number C = 0 (C = 1). In even layers, an electric field (Ex)could induce magnetization effects identical to the depicted circulating Hall current, showing quantized magnetoelectric effects. Adapted from Ref. 3. — (a) Crystal structure of ABC-stacked MnBi₂Te₄ with an A-type AF ordering. (b,c) Illustration of intrinsic axion insulators in even layers and QAH insulators in odd layers. The intrinsically gapped surfaces on the top and bottom surfaces have half-quantized Hall conductances, whose signs are opposite (the same) in even (odd) layers, leading to Chern number C = 0 (C = 1). In even layers, an electric field (E_x)could induce magnetization effects identical to the depicted circulating Hall current, showing quantized magnetoelectric effects. Adapted from Ref. 3.

(a) Crystal structure of ABC-stacked MnBi₂Te₄ with an A-type AF ordering. (b,c) Illustration of intrinsic axion insulators in even layers and QAH insulators in odd layers. The intrinsically gapped surfaces on the top and bottom surfaces have half-quantized Hall conductances, whose signs are opposite (the same) in even (odd) layers, leading to Chern number C = 0 (C = 1). In even layers, an electric field (E_x)could induce magnetization effects identical to the depicted circulating Hall current, showing quantized magnetoelectric effects. Adapted from Ref. 3.

The Ph.D candidate will benefit from strong interactions with theoreticians in the field of axion electrodynamics as well as use state-of-the-art facilities at the Max Planck Institute for Chemical Physics of Solids in Dresden, to grow high-quality epitaxial thin films by molecular beam epitaxy (MBE), which will allow the control of the topological properties of material candidates (besides the canonical MnBi₂Te₄) via atomically precise thickness tuning, epitaxial strain, or chemical doping. In addition to the available characterization techniques which include X-ray diffraction, transmission electron microscopy, SQUID magnetometry, or atomic force microscopy, the candidate is expected to perform magnetotransport experiments in lithographically patterned samples at cryogenic temperatures in a variety of cryostats (400 K - 50 mK and up to 18 T). The MBE chamber is further connected in vacuo to a cluster including two magnetron sputtering chambers, a glove box, a cryogenic scanning tunneling microscope (STM) and an angle-resolved photoemission spectroscopy (ARPES) apparatus.

The Ph.D candidate should have an excellent understanding of solid-state physics and materials science, a good command of spoken and written English, and be highly motivated to work in a fast-paced, collaborative and international research environment.

[1] A. Sekine & K. Nomura

Axion electrodynamics in topological materials

J. Appl. Phys. 129, 141101 (2021)

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[2] D.M. Nenno, C.A.C. Garcia, J. Gooth, C. Felser & P. Narang

Axion physics in condensed-matter systems

Nature Review Physics 2, 682–696 (2020)

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[3] J. Li, Y. Li, S. Du et al.

Intrinsic magnetic topological insulators in van der Waals layered MnBi₂Te₄-family materials

Science Advances 5, eaaw5685 (2019)

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