Domain-Selected Spin Projection and Orbital-Filling Softening of Charge Plasmons in a Model Altermagnetic RbV2Te2O Coulomb Bilayer

Altermagnets combine compensated magnetic order with momentum-dependent spin splitting, offering spin-polarized states without stray fields. Plasmons in altermagnetic metals can carry a spin signal, but its distribution among the modes of coupled layers is unknown. Here, we study a Coulomb-coupled bilayer of the d-wave altermagnet RbV2Te2O, using a representative four-band tight-binding model with spin–orbit coupling and the random-phase approximation with density-channel Coulomb interaction. We find that, at equal filling in a symmetric environment, switching the relative magnetic domain leaves the charge spectra unchanged but exchanges the total-spin weight between the acoustic (187.4 meV) and optical (413.1 meV) plasmons at q = 0.2 nm−1. A missing spin peak arises from interlayer cancelation, which the layer-difference spin reveals, or from a symmetry node along the diagonal, where both spin channels vanish. Raising the chemical potential from 0 to 0.2 eV softens the acoustic plasmon by 15% and raises the relative spin amplitude, as the xy orbital loses low-energy current weight. With a dynamic hexagonal boron nitride (hBN) spacer, the plasmons move to 214.0 and 409.3 meV; the selection survives, and only about 2% leaks into the dark branch. Charge spectra are domain-independent, so testing requires a spin-sensitive probe.