New Layered Material Confirmed As Unique Altermagnet, Paving Way for Spintronics

Researchers are increasingly focused on the V2Se2O-family of altermagnets, recently confirmed as d-wave systems like RbV2Te2O and KV2Se2O, representing a unique class of van der Waals layered materials. Xingkai Cheng, Yifan Gao, and Junwei Liu, all from the Department of Physics and IAS Center for Quantum Matter at The Hong Kong University of Science and Technology, have developed a realistic tight-binding model for six representative members of this family, parameterised by first-principles calculations and benchmarked against experimental data. This work is significant because the model accurately describes key altermagnetic properties, including crystal-symmetry-paired spin-momentum locking and noncollinear spin-conserved currents, and crucially incorporates strain-coupling parameters to simulate tunable responses. By providing a framework to systematically explore the interplay of spin, valley, and layer degrees of freedom, this research establishes a foundation for both fundamental understanding and potential applications in spintronics and the integration of these materials with topological insulators and superconductors.

These materials uniquely combine nonrelativistic spin-splitting with zero net-magnetization, offering a promising platform for advanced spintronic devices.

A new realistic tight-binding model, parameterised by first-principles calculations and validated with experimental measurements, accurately captures the essential altermagnetic electronic properties of this family, including crystal-symmetry-paired spin-momentum locking and noncollinear spin-conserved currents. This model incorporates strain-coupling parameters, allowing for the simulation of strain-tunable responses such as the piezo-Hall effect, a phenomenon where mechanical stress induces a measurable voltage.
The development of this model addresses a critical need for a theoretical foundation to understand and utilise these novel materials effectively. By accurately representing the interplay between crystal symmetry, spin momentum, and electronic structure, the research provides a means to systematically explore multiple quantum degrees of freedom, spin, valley, and layer, within a single system.

This capability is crucial for investigating how these materials couple with other quantum materials like topological insulators and superconductors, potentially leading to innovative device designs. The work goes beyond simplified models, offering a versatile computational platform for predicting material behaviour and optimising performance in low-dimensional spintronics.
Specifically, the research team investigated six representative members of the V2Se2O family, constructing a tight-binding model that accurately reproduces key characteristics observed in experiments. The model successfully simulates the emergence of noncollinear spin currents and layer polarization under applied electric fields, demonstrating the potential for manipulating spin transport.

Furthermore, the inclusion of strain-coupling parameters enables the prediction of piezo-Hall effects, opening avenues for developing strain-based spintronic devices. This realistic model not only advances fundamental understanding of altermagnetism but also lays the groundwork for exploring novel device applications in this emerging class of layered materials.

Computational methodology for altermagnetic property determination

First-principles calculations underpinned this work, employing the density functional theory framework as implemented within the Vienna ab initio simulation package. The projector-augmented wave potential was adopted, utilising a plane-wave energy cutoff of 440 eV to ensure convergence criteria of 10−6 eV were met.

Exchange-correlation effects were accounted for using the Perdew, Burke, Ernzerhof functional, alongside a 11×11×9 gamma-centred Monkhorst, Pack mesh for accurate k-point sampling. To construct a realistic tight-binding Hamiltonian for Berry curvature calculations, the WANNIER90 interface was implemented, incorporating both V d and Se/Te p orbitals.

This model was then parametrised by the first-principles calculations and benchmarked against experimental measurements, accurately capturing essential altermagnetic electronic properties such as crystal-symmetry-paired spin-momentum locking and noncollinear spin-conserved currents. The inclusion of strain-coupling parameters within the model facilitated the simulation of strain-tunable responses, notably the piezo-Hall effect.

This methodology enabled systematic exploration of multiple degrees of freedom, spin, valley, and layer, within a single material system. By establishing a concrete theoretical framework, the research lays the groundwork for understanding the coupling of these materials with topological insulators and superconductors, thereby advancing both fundamental understanding and potential device applications of this novel class of layered altermagnets. Lattice constants of 4.020 Å, 3.942 Å, 4.008 Å, 3.952 Å, 3.928 Å, and 3.887 Å were used for materials RbV2Te2O, RbV2Se2O, KV2Te2O, KV2Se2O, V2Te2O, and V2Se2O respectively, during these calculations.

D-wave altermagnetism confirmed in layered V2Se2O compounds via tight-binding modelling

Researchers have experimentally confirmed that intercalated V2Se2O-family altermagnets, including RbV2Te2O and KV2Se2O, function as d-wave altermagnets, representing the only known van der Waals layered altermagnetic systems. A realistic tight-binding model, parameterised by first-principles calculations, accurately captures essential altermagnetic electronic properties, including crystal-symmetry-paired spin-momentum locking and noncollinear spin-conserved currents.

This model was benchmarked against experimental measurements, demonstrating strong correspondence between calculations and data obtained from Rb-intercalated V2Te2O and K-intercalated V2Se2O. The symmetry-restricted four-band tight-binding Hamiltonian, without spin-orbit coupling, exhibits a strictly diagonal structure arising from decoupled spin-up and spin-down blocks and inhibited inter-orbital coupling.

Hopping parameters, detailed in Table I, were fitted to match first-principles calculations, and a direct comparison of band structures from both methods is shown in Fig0.2. Incorporation of spin-orbit coupling into the model, through the symmetry-allowed terms detailed in Appendix V C, introduces finite interaction between spin-up and spin-down blocks, opening gaps at spin-degenerate points.

The resulting spin-split bands exhibit only minor modifications with the inclusion of spin-orbit coupling, however sizable Berry curvature emerges near the crossing points, potentially enabling tunable Hall effects. Fitted parameters for the model including spin-orbit coupling are listed in Table II, with corresponding spin-polarized band structures presented in Fig0.3.

Analysis reveals that even with spin conservation broken by spin-orbit coupling, the magnetic crystal symmetries preserve the essential crystal-symmetry-paired spin-momentum locking pattern, with opposite signs of spin polarization Sz observed in the C-paired X and Y valleys. Noncollinear spin currents were also investigated, revealing spin-polarized current Js parallel to Jc when an electric field E is applied.

Pure spin current Js perpendicular to Jc emerges when E is oriented at specific angles, as detailed in Fig0.4, with longitudinal and transverse conductivities for each spin channel normalized by σ↑,L(θ = 0) + σ↓,L(θ = 0). Furthermore, the study demonstrates electric-field control of layer polarization, inducing changes in the band projections onto both the top and bottom layers, as illustrated in Fig0.5.

D-wave altermagnetism and strain-tunable spintronics in van der Waals layered V2Se2O compounds

Researchers have experimentally confirmed that intercalated V2Se2O-family altermagnets, including RbV2Te2O and KV2Se2O, exhibit d-wave altermagnetism, representing the only known van der Waals layered altermagnetic systems. These materials combine crystal-symmetry-paired spin-momentum locking with a layered structure, creating a platform for investigating low-dimensional spintronic responses and the interplay between various degrees of freedom.

A realistic tight-binding model, informed by first-principles calculations and validated by experimental measurements, accurately captures the essential altermagnetic electronic properties of six representative V2Se2O-family members. This model incorporates strain-coupling parameters, allowing for the simulation of strain-tunable responses such as the piezo-Hall effect, and facilitates systematic exploration of spin, valley, and layer degrees of freedom within a single system.

The developed model establishes a foundation for understanding the coupling of these materials with topological insulators and superconductors, potentially advancing both fundamental knowledge and device applications. The authors acknowledge a focus on intralayer hopping due to experimentally confirmed negligible dispersion along the kz direction as a limitation of the current model. Future research may focus on extending the model to include interlayer interactions and exploring the behaviour of these materials in heterostructures, potentially leading to novel spintronic devices.