Giant valley splitting and tunable anisotropic spin plasmons in a Janus ferrovalley monolayer

Manipulating the spin and valley degrees of freedom of electrons is crucial for next-generation information technologies. Altermagnets, as an emerging magnetic phase, provide a quantum platform with intrinsic spin-valley locking, enabling multi-state manipulation of both spin and valley. Here, we propose a Janus monolayer CaCoFeN2, achieved through in situ substitution of magnetic transition metal atoms in the two-dimensional (2D) altermagnet Ca(CoN)2 [Phys. Rev. Lett. 133, 056401 (2024)]. Our first-principles calculations identify CaCoFeN2 as an anisotropic spin-plasmon ferrovalley semiconductor, with a large valley splitting of 273 meV solely through crystal symmetry breaking, without any involvement of spin-orbit coupling (SOC). Furthermore, its anisotropic electronic structures facilitate highly directional spin plasmon propagation. Carrier-type switching (n-type ↔ p-type) reverses the anisotropy along orthogonal axes, yielding open equi-frequency contours in n-type CaCoFeN2. The integration of spontaneous spin and valley polarization within a single material without SOC, offers new opportunities for advancements in spintronics and valleytronics.