The altermagnet (AM), characterized by zero net magnetization combined with momentum-dependent spin polarization, is regarded as a new type of collinear magnet and has attracted increasing interest since the fundamental concept of altermagnetism was established. Although many AM candidates have been theoretically proposed, most experimentally confirmed AM candidates are semiconductors. More recently, a metallic AM candidate of the bulk ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$ was experimentally identified to have Fermi surface and altermagnetic spin splitting features. It is anticipated that the two-dimensional (2D) monolayer counterpart can be fabricated by mechanical exfoliation of the ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$ bulk compound. Here, we present our first principles calculations to demonstrate that the corresponding 2D ${\mathrm{K}}_2{\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ monolayer has an increased lattice constant and excellent stability at room temperature. Most importantly, the ${\mathrm{K}}_2{\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ monolayer manifests as an altermagnetic semiconductor with an out of plane magnetization axis and its Néel temperature is approximately 580 K. The exfoliation of the ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$ bulk into the ${\mathrm{K}}_2{\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ monolayer probably leads to the 2D AM semiconductor consisting of spin-valley locking at the $X$ or $Y$ valley. It is anticipated that a giant piezovalley effect of 0.15 eV valley polarization can be obtained by using a 2% anisotropic strain to break the diagonal mirror symmetry. Our findings provide a promising AM candidate for valleytronics and spintronics at room temperature.

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• Magnetic order
• Spintronics
• Valleytronics
• 2-dimensional systems
• Altermagnets
• Magnetic semiconductors
• Density functional theory
• First-principles calculations