Realistic tight-binding model for V2Se2O-family altermagnets

Following earlier theoretical prediction, intercalated V2Se2O-family altermagnets such as RbV2Te2O and KV2Se2O have now been experimentally confirmed as d-wave altermagnets, representing the only known van der Waals layered altermagnetic systems. By combining crystal-symmetry-paired spin-momentum locking (CSML) with the layered structure, the V2Se2O-family offers a suitable platform for studying low-dimensional spintronic responses and exploring the interplay among multiple quantum degrees of freedom. To establish a concrete theoretical foundation for understanding and utilizing these materials, we investigate six representative members of the V2Se2O-family and construct a realistic tight-binding model parameterized by first-principles calculations, which is benchmarked by experimental measurements. This model accurately captures essential altermagnetic electronic properties, including CSML and noncollinear spin-conserved currents. It further incorporates strain-coupling parameters, enabling the simulation of strain-tunable responses such as the piezo-Hall effects. This realistic model allows systematic exploration of multiple degrees of freedom (like spin, valley, and layer) within a single system, and lays the groundwork for understanding their coupling with other quantum materials, such as topological insulators and superconductors, thereby advancing both the fundamental understanding and potential device applications of this novel class of layered altermagnets.

💡 Research Summary

This paper presents a comprehensive, realistic tight‑binding (TB) description of the newly confirmed d‑wave altermagnets in the V₂Se₂O family, such as RbV₂Te₂O and KV₂Se₂O, which are the only known van‑der‑Waals layered altermagnetic systems. By combining crystal‑symmetry‑paired spin‑momentum locking (CSML) with the intrinsic two‑dimensional nature of these compounds, the authors construct a four‑band TB model that faithfully reproduces first‑principles band structures and a wide range of experimentally observed phenomena.

The model is built on the P4/mmm space group, which contains two vanadium sublattices (V_A and V_B) related by a four‑fold rotation (C₄z) and mirror operations (M_xy, M̅_xy). First‑principles orbital projections reveal that the low‑energy sector is dominated by four orbitals: d_{xz}↑ and d_{xy}↑ on V_A, and d_{yz}↓ and d_{xy}↓ on V_B. These form the basis {d_A↑xz, d_A↑xy, d_B↓yz, d_B↓xy}. Hopping integrals up to third‑nearest neighbours are constrained by the crystal symmetries, leading to a Hamiltonian that is block‑diagonal in spin when spin‑orbit coupling (SOC) is absent. The symmetry‑enforced equivalence between x‑direction hopping on sublattice A and y‑direction hopping on sublattice B produces the characteristic C‑paired nodal lines (or points) that define altermagnetism.

SOC is incorporated via the magnetic space group P4′/mm′m, adding off‑diagonal spin‑mixing terms that open small gaps at former spin‑degenerate crossings. Although the antiferromagnetic exchange splitting is much larger than the SOC energy scale, these gaps generate sizable Berry curvature, enabling strain‑tunable Hall responses. The fitted TB parameters (Tables I and II) reproduce both the SOC‑free and SOC‑included DFT bands with excellent agreement, and they capture the experimentally observed spin‑polarization reversal between X and Y valleys, a hallmark of CSML that survives SOC.

Using the model, the authors calculate transport under in‑plane electric fields. Because the conductivity is anisotropic under the C₄ symmetry, the longitudinal and transverse spin‑resolved conductivities (σ↑, σ↓) depend strongly on the field angle θ. For E‖