Novel 2D Altermagnetic Vanadium Oxide with a Buckled Lieb Structure

Abstract: Altermagnetism has recently emerged as a highly promising phase for spintronics, offering the combined advantages of both antiferromagnets and ferromagnets. Here, using a first-principles analysis based on density functional theory (DFT), we identify a monolayer V$2$O crystal in a buckled Lieb lattice as a promising two-dimensional altermagnetic material. The structural and thermal stability of V$_2$O is verified through calculations of the crystal's formation energy, phonon structure, room-temperature ab initio molecular dynamics, and stiffness matrix. The system is found to exhibit auxetic behavior with a negative Poisson's ratio. Our calculations indicate an antiferromagnetic ground state with a local magnetic moment of $2.79\,μ{\mathrm{B}}$ per V atom and a magnetocrystalline anisotropy that favors an out-of-plane easy axis. The electronic structure exhibits a momentum-dependent spin splitting of 1.2 eV, which is a characteristic of altermagnets. Inclusion of spin-orbit coupling breaks the symmetry of the quadratic band crossing near the Fermi level, resulting in a large Berry curvature and significant intrinsic spin Hall conductivity around $40\,(\hbar/e)\,\mathrm{S\,cm^{-1}}$. The results demonstrate that monolayer V$_2$O serves as a robust room-temperature altermagnetic platform, exhibiting magnetic anisotropy and spin-dependent transport responses.

• The paper reports that monolayer V₂O exhibits intrinsic altermagnetism in a buckled Lieb lattice with robust room-temperature antiferromagnetic order.
• It employs comprehensive DFT calculations to demonstrate auxetic mechanical behavior and strong spin Hall conductivity, underscoring its spintronic potential.
• The study reveals significant momentum-dependent spin splitting and anisotropic elastic properties, indicating promising avenues for strain-engineered device applications.

First-Principles Investigation of Altermagnetism in 2D Buckled Lieb-Structured V₂O

Introduction

This work presents a comprehensive density functional theory (DFT) investigation of an unexplored two-dimensional vanadium oxide, V₂O, in a buckled inverse Lieb lattice geometry. The study aims to identify V₂O as an intrinsic monolayer altermagnet with robust structural, mechanical, magnetic, and transport properties, revealing its potential as a high-performance spintronic material. The authors specifically address the scarcity of stable 2D altermagnets accessible to device architectures, focusing on symmetry-driven phenomena unattainable in conventional FM or AFM systems.

Structural and Mechanical Properties

The DFT calculations demonstrate that V₂O adopts a buckled inverse Lieb lattice, with pronounced upward and downward displacements of vanadium atoms, resulting in a non-centrosymmetric tetragonal (space group P4m2, No. 115) configuration. Thermodynamic (formation enthalpy $\Delta E_f = -3.64$ eV), dynamical (phonon spectra devoid of imaginary modes), thermal (AIMD stability at 300 K), and elastic (positive-definite stiffness matrix) stability are all verified, suggesting synthesis feasibility.

A key result is the observation of an auxetic response, with a negative Poisson’s ratio $v \approx -0.10$ along principal axes, signifying lateral expansion upon stretching—a rare attribute among 2D systems with potential implications in strain engineering. The angular dependence of Young's modulus reflects significant anisotropy, peaking at $52.0$ N/m and reaching a minimum of $34.9$ N/m along diagonals. These mechanical characteristics underscore V₂O's resilience and tunability for strain-engineered applications.

Predicted IR and Raman spectra, supported by mode symmetry analysis, furnish experimental signatures for future identification of monolayer V₂O.

Magnetic Ground State and Exchange Interactions

The authors systematically compare magnetic states spanning six antiferromagnetic (AFM) configurations and ferromagnetism. The striped (Néel-type) AFM ordering emerges as the ground state, with a substantial local moment of $2.79$ μB per V atom, in agreement with hybrid functional benchmarks. Oxygen maintains negligible magnetic polarization, supporting localized d-electron magnetism predominating on vanadium.

Magnetocrystalline anisotropy calculations indicate an out-of-plane easy axis (MAE $= 0.15$ meV), with in-plane magnetic isotropy, enabling controlled manipulation of the magnetic order. The symmetric exchange parameters reveal a dominant AFM nearest-neighbor coupling ($J_{12} \approx -23$ meV), while Dzyaloshinskii–Moriya interaction (DMI) is present but weak ($|D_{12}| < 0.2$ meV), precluding spontaneous skyrmion formation in the absence of external fields and confirming collinear ordering.

Monte Carlo simulations parameterized with the computed exchange interactions and anisotropy yield a high Néel temperature $T_N$ of approximately $400$ K. This is a crucial metric for practical devices, ensuring magnetic order persists well above ambient conditions.

Electronic Structure and Altermagnetic Features

The electronic band structure reveals momentum-dependent spin-splitting up to $v \approx -0.10$0 eV, observed in the absence of SOC—a hallmark of altermagnetic symmetry. Crucially, this spin splitting originates from the non-relativistic symmetry of the buckled Lieb lattice and its associated magnetic space group operations, confirming the decoupling of real and spin space. The bands possess Dirac-like crossings at the Fermi level, protected by mirror symmetries; these are subsequently gapped by SOC. Projected density of states highlight dominant V-d character near the Fermi energy, with moderate hybridization with O-p states at lower binding energies.

Berry curvature calculations identify sharp hotspots at SOC-gapped Dirac points, exhibiting a quadrupole symmetry consistent with the underlying crystal structure. These features have profound consequences for band topology and transport coefficients.

Spintronic Transport Signatures

Transport calculations utilizing the Wannier interpolation and Kubo-Greenwood formalism yield a vanishing anomalous Hall conductivity (AHC), imposed by the magnetic point group and the out-of-plane Néel vector orientation (as dictated by Landau theory). In contrast, a pronounced intrinsic spin Hall conductivity (SHC) is observed, peaking near the Fermi energy at approximately $v \approx -0.10$1. The experimental implication is the possibility of generating pure spin currents without concurrent charge Hall signals, facilitating detection and exploitation in two-dimensional spintronic architectures.

Theoretical and Practical Implications

The identification of monolayer V₂O as a thermodynamically and mechanically stable 2D altermagnet with $v \approx -0.10$2 K represents a significant advancement toward viable altermagnetic spintronic platforms. The large SOC-free spin splitting, auxetic mechanical response, and robust intrinsic SHC distinguish V₂O from existing FM, AFM, or weakly correlated 2D systems.

The coexistence of strong symmetry-protected altermagnetic features and compatible mechanical properties positions this material as a leading candidate for next-generation, robust, scalable spintronic and valleytronic devices. The out-of-plane easy axis coupled with in-plane isotropy enables versatile manipulation via electric fields, strain, or heterostructure engineering. The lack of measurable AHC at the Fermi level, simultaneous with a high SHC, provides a clear experimental avenue for isolation of pure spin currents—a critical requirement for low-dissipation logic and memory elements.

Beyond immediate applications, the work motivates further theoretical investigation into phase control, domain engineering, and interactions with topological superconductivity. V₂O offers an unprecedented platform for exploring strongly correlated and topologically nontrivial electronic phenomena in practical 2D settings.

Conclusion

This study establishes V₂O as a highly stable, buckled Lieb lattice altermagnet with a room-temperature AFM ground state, large symmetry-driven spin splitting, negative Poisson’s ratio, and strong spin Hall transport response. These intrinsic properties collectively indicate substantial potential for future high-speed, scalable spintronics and as a testbed for emergent quantum phenomena in low-dimensional correlated systems. Further experimental realization and device integration of monolayer V₂O are warranted as the next step in 2D altermagnet research.