Atomic Insights into Hidden Structural Order in Altermagnets and Chirality-Driven Topology

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Symmetry and order parameters in crystalline solids play a fundamental role in shaping their physical properties. The intimate interplay between orbital, spin, and lattice degrees of freedom has enabled a broad range of exotic phases and emergent phenomena [1]. Currently, there is a growing interest in exploring the momentum-dependent spin splitting and orbital texture driven by their potential for next-generation magnetic memory and logic applications. In this context, altermagnets and chiral materials have emerged as particularly promising platforms. Altermagnets exhibit fully compensated collinear magnetic order in real space but support strong momentum-dependent spin-splitting in their electronic bands—even without spin–orbit coupling. This unique feature provides a promising pathway to high-speed, energy-efficient spintronics [2]. Chirality refers to a material system that cannot be superimposed on its mirror reflection due to the absence of mirror planes and inversion centers. Chiral materials often host nontrivial band topology in both electronic and phonon states, opening potential applications such as chiral-induced spin selectivity and circularly polarized beam splitting [3]. Despite growing experimental evidence confirming the unique properties related to altermagnetic order and chiral symmetry, direct atomic-scale characterization of these phenomena remains limited. In this work, we directly resolve non-centrosymmetric distortions in MnTe. To reveal the ferroelectric and magnetic order in MnTe, we carried out atomic-resolution structural and spectroscopic characterization using aberration-corrected scanning transmission electron microscopy (STEM). We also reveal the unique atomic arrangements in chiral ZrOS crystal using differential phase contrast (DPC-STEM) imaging. The orbital texture in ZrOS crystal was investigated by electron magnetic chiral dichroism (EMCD). To examine the phonon dispersion of ZrOS crystal across the opposite momenta, we performed momentum-resolved vibrational electron energy-loss spectroscopy (EELS) measurements. To further gain microscopic insights into the hidden structural order in MnTe and chirality-driven topology in ZrOS, we performed density-functional theory (DFT) calculations as well as the STEM simulation [4]. (a) Schematic of MnTe lattice projected along [100] direction. The magnetic sublattices with antiparallel spin moments (MnA and MnB) are highlighted in different colors and also indicated by opposite arrows. (b) A representative STEM-HAADF image of MnTe showing the local atomic structure along the [100]-axis. (c) Schematic of chiral ZrOS along [111] direction. (d) A representative STEM-HAADF image of ZrOS crystal viewed along the [100]-axis.

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