Giant Spin Splitting and Anisotropic Spin Polarization in 2D Altermagnet Cr ₂ O

In this work, we theoretically propose a novel 2D nonvan der Waals (non-vdW) monolayer Cr₂O, exfoliated from the bulk CrO along the [001] direction. Combining first-principles calculations, spin group symmetry analysis, and Monte Carlo simulations, we reveal that Cr₂O has exceptional altermagnetic metallic characteristics, featuring a record nonrelativistic spin splitting of 1.83 eV, a high Néel temperature of 507 K, and full spin polarization along specific Γ-X/Γ-Y paths. The spin splitting feature in Cr₂O exhibits an anisotropic <i>d</i>-wave-like pattern, arising from nontrivial spin-group symmetry operations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>[</mml:mo><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>∥</mml:mo><mml:msubsup><mml:mrow><mml:mover><mml:mi>C</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mn>4</mml:mn><mml:mi>z</mml:mi></mml:mrow><mml:mrow><mml:mo>±</mml:mo></mml:mrow></mml:msubsup><mml:mo>]</mml:mo></mml:math> that govern direction-dependent spin polarization. Moreover, Boltzmann transport calculations show that the spin polarization <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>S</mml:mi><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mover><mml:mi>n</mml:mi><mml:mo>^</mml:mo></mml:mover></mml:mrow></mml:msub></mml:math> reaches up to 95.2% along principal axes and follows a cosine-like angular dependence in the 2D plane. It remains above 80% across a wide energy range, ensuring robust spin filtering. These results establish Cr₂O as a promising platform for spin-symmetry-engineered transport in 2D non-vdW altermagnets.