Two-dimensional altermagnets (AMs) with tunable electronic and topological properties offer promising opportunities for next-generation spintronic and optoelectronic applications. Here, we predict that monolayer VO (ML-VO) is a two-dimensional AM semiconductor with collinear magnetic ordering and $d$-wave symmetry. In its pristine structure, there are two valleys characterized by spin-valley-lattice coupling at high-symmetry points in the Brillouin zone, leading to valley-selective absorption of linearly polarized light and pronounced linear dichroism. Strain engineering provides an effective method to manipulate a phase transition: Uniaxial strain induces valley polarization, whereas biaxial strain drives a topological phase transition from a gapped state to gapless phases featuring pairs of spin-polarized Weyl points emerging along distinct momentum directions. This transition is accompanied by the suppression of linear dichroism and the emergence of universal optical absorbance, which we demonstrate using a low-energy $k·p$ model. Moreover, we identify a critical regime hosting semi-Weyl points with linear dispersion along one direction and quadratic dispersion along the other direction. Our results reveal that ML-VO is an AM and demonstrate that it provides a promising platform for exploring optical responses and strain-induced phase transitions in two-dimensional altermagnets.

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• Valleytronics
• 2-dimensional systems
• Altermagnets
• Density functional theory