Spin-Selective Second-Order Topological Insulators Enabling Cornertronics in 2D Altermagnets

Abstract

Recent progress in spintronics within the paradigm of altermagnets (AMs) opens new avenues for next-generation electronic device design. Here, we establish a spin-corner locking mechanism that generates second-order topological states in two-dimensional (2D) altermagnetic systems, through effective model analysis. Remarkably, the breaking of Mxy symmetry under uniaxial strain creates spin-resolved corner modes, driving the system into a corner-polarized second-order topological insulator (CPSOTI). Beyond critical strain, a topological phase transition to quantum anomalous Hall insulator occurs with quantized conductance. Through first-principles calculations, we identify two experimentally viable candidates for 2D intrinsic AM CrO and Cr$_2$Se$_2$O -- which host robust CPSOTI. Moreover, we construct the topological phase diagram of CrO and predict the existence of an altermagnetic Weyl semimetal phase. Our findings open technological avenues in altermagnetism and higher-order topology, while providing opportunities for coupling topological spintronics with cornertronics.

Figures (4)

• Figure 1: Effective model of second-order topological altermagnetic systems. (a) Illustration of the 2D AM SOTI model. (b) AM band structure exhibiting spin splitting. Here, $t_1^x=-1$ eV, $t_1^y=-1.5$ eV, $t_{2}^{x,y} = -0.8 t_1^{x,y}$, $t_r=1$. (c) Spin-polarized QAHI phase under broken $M_{xy}$ symmetry. The dashed lines represent the results without considering SOC, while the solid lines correspond to the results with SOC included. Corresponds to $\delta = -0.1$. (d) Topological phase transition process under broken $M_{xy}$ symmetry, wherein the AMs evolves from a SOTI phase to CPSOTI and QAHI phases. (e) Energy spectrum on a quadrilateral quantum dot in the second-order topological phase, where a set of four corner states (colored dots) can be observed. The inset displays the real-space distribution of these corner states.
• Figure 2: (a) Energy spectrum of a quantum dot with broken $M_{xy}$ symmetry in AMs, showing two sets of corner states (colored dots). (b) Corresponding energy level diagram of the quantum dot, where black horizontal lines denote bulk and edge states. Red and blue dots represent spin-resolved corner states, with their real-space projections shown on the right. (c) 3D spin-resolved band structure of the AM Weyl semimetal. Degenerate points at the Fermi level for different spin bands are marked with colored circles. (d) Fermi surfaces of the AM Weyl semimetal at different chemical potentials.
• Figure 3: (a) Top and side views of CrO. Purple and blue spheres denote Cr and O atoms, respectively. (b) Spin-resolved band structure of CrO in the altermagnetic configuration. Red and blue represent spin-up and spin-down. (c) Band structure of CrO under anisotropic strain along the $x$- and $y$-axes. (d) Topological phase diagram under biaxial continuous strain, characterized by $\eta_{gap}$. (c) Based on the Wannier-based TB model, energy spectrum of the square quantum dot with preserved $M_{xy}$ symmetry, and (d) energy spectrum with broken $M_{xy}$ symmetry. The spin-associated corner states are marked in red and blue. Insets show the total charge distribution of the spin-resolved corner states.
• Figure 4: (a) Top and side views of Cr$_2$Se$_2$O. The purple, green, and blue spheres represent Cr, Se, and O atoms, respectively. (b) Band structure of the altermagnetic configuration of Cr$_2$Se$_2$O. Based on the Wannier-based TB model, energy spectra of the corner quantum dot with preserved (c) and broken (d) $M_{xy}$ symmetry. Insets show the total charge distribution of the spin-resolved corner states.