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Real Chern insulators in two-dimensional altermagnetic Fe$_2$S$_2$O and Fe$_2$Se$_2$O

Yong-Kun Wang, Shifeng Qian, An-Dong Fan, Si Li

Abstract

Altermagnets, recently identified as a third class of collinear magnetic materials, have attracted significant attention in condensed matter physics. Despite this growing interest, the realization of real Chern insulators in intrinsic altermagnetic systems has rarely been reported. In this work, based on first-principles calculations and theoretical analysis, we identify monolayer Fe$_2$S$_2$O and Fe$_2$Se$_2$O as a novel class of two-dimensional altermagnetic real Chern insulators. We demonstrate that these materials possess altermagnetic ground states and host a nontrivial mirror real Chern number, leading to the emergence of symmetry-protected zero-dimensional corner states. Notably, these corner modes are spin-polarized, giving rise to a unique spin-corner coupling effect. We further show that the real Chern insulating phases and their associated corner states remain robust against spin-orbit coupling, as well as under both uniaxial and biaxial strain. Additionally, these materials exhibit pronounced linear dichroism and strong optical absorption. Our findings uncover the novel topological character of Fe$_2$S$_2$O and Fe$_2$Se$_2$O, establishing them as promising platforms for exploring real Chern insulators in altermagnetic systems.

Real Chern insulators in two-dimensional altermagnetic Fe$_2$S$_2$O and Fe$_2$Se$_2$O

Abstract

Altermagnets, recently identified as a third class of collinear magnetic materials, have attracted significant attention in condensed matter physics. Despite this growing interest, the realization of real Chern insulators in intrinsic altermagnetic systems has rarely been reported. In this work, based on first-principles calculations and theoretical analysis, we identify monolayer FeSO and FeSeO as a novel class of two-dimensional altermagnetic real Chern insulators. We demonstrate that these materials possess altermagnetic ground states and host a nontrivial mirror real Chern number, leading to the emergence of symmetry-protected zero-dimensional corner states. Notably, these corner modes are spin-polarized, giving rise to a unique spin-corner coupling effect. We further show that the real Chern insulating phases and their associated corner states remain robust against spin-orbit coupling, as well as under both uniaxial and biaxial strain. Additionally, these materials exhibit pronounced linear dichroism and strong optical absorption. Our findings uncover the novel topological character of FeSO and FeSeO, establishing them as promising platforms for exploring real Chern insulators in altermagnetic systems.

Paper Structure

This paper contains 10 sections, 4 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Computation Methods
  3. Results and Discussion
  4. Crystal structure and magnetism
  5. Bulk band structure
  6. Corner states
  7. Effects of SOC
  8. Strain effect
  9. Optical properties
  10. Conclusion

Figures (7)

  • Figure 1: (a) Side view and (b) top view of the crystal structure of monolayer Fe$_2$S$_2$O and Fe$_2$Se$_2$O. The altermagnetic configuration is illustrated in (a). (c) Brillouin zone of the monolayer structure, with high-symmetry points indicated. (d) and (e) Calculated phonon spectra of Fe$_2$S$_2$O and Fe$_2$Se$_2$O, respectively.
  • Figure 2: (a) Band structure and PDOS; (b) two-dimensional band structures of the two lowest conduction bands and the two highest valence bands; and (c) spin splitting of the two highest valence bands for monolayer Fe$_2$S$_2$O. (d)–(f) Corresponding results for monolayer Fe$_2$Se$_2$O. In (a), (b), (d) and (e), red and blue represent spin-up and spin-down bands, respectively. The SOC is not included.
  • Figure 3: (a) Energy spectrum of the square-shaped Fe$_2$S$_2$O nanodisk shown in (b), with energy levels arranged in ascending order. Four zero-energy states are highlighted in color, where red (blue) dots represent spin-up (spin-down) components. (b) Charge distribution of the four zero-energy states, showing clear localization at the corners. (c) and (d) Corresponding results for the monolayer Fe$_2$Se$_2$O nanodisk.
  • Figure 4: (a) Band structure of monolayer Fe$_2$S$_2$O including SOC, with magnetization along the $y$-axis. (c) Band structure of monolayer Fe$_2$Se$_2$O including SOC, with magnetization along the $z$-axis. (b) and (d) Energy spectra of square-shaped nanodisks of monolayer Fe$_2$S$_2$O and Fe$_2$Se$_2$O, respectively, with SOC included. Insets show the charge distributions of the zero-energy states, clearly demonstrating strong corner localization.
  • Figure 5: Band structures of monolayer Fe$_2$S$_2$O under $-2\%$ uniaxial strain along the (a) $x$- and (c) $y$-directions without SOC. (b) and (d) show the corresponding energy spectra of Fe$_2$S$_2$O nanodisks, with insets depicting charge density distributions of two sets of corner states. (e) Band structure under $-2\%$ biaxial strain and (f) the corresponding nanodisk energy spectrum. Red (blue) dots denote spin-up (spin-down) components.
  • ...and 2 more figures