As an emerging category of magnets, altermagnets are distinguished by their alternating spin-splitting band structures and a zero net magnetization. In this study, we explore the collective excitation modes within two-dimensional (2D) altermagnets, revealing unique modes of coupled charge density and spin density fluctuations, known as spin plasmons. We find that fluctuations in charge density can induce corresponding fluctuations in spin density, and vice versa. The momentum-dependent spin splitting inherent in altermagnets leads to significant anisotropy in spin plasmons. Through first-principles calculations, we demonstrate the existence of the anisotropic spin plasmons in altermagnetic ${\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ monolayer. These spin plasmons establish a connection between spintronics and plasmonics, offering exciting possibilities for advancement in spin transport within spintronic devices.

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• Plasmons
• Altermagnets
• First-principles calculations
• Linear response theory