Long-lived spin plasmons in a spin-polarized two-dimensional electron gas

Collective charge-density modes (plasmons) of the clean two-dimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into single-particle excitations. Collective spin-density modes (spin plasmons) possess no similar protection and rapidly decay by production of electron-hole pairs. Nevertheless, if the electron gas has a sufficiently high degree of spin polarization (P>1/7, where P is the ratio of the equilibrium spin density and the total electron density, for a parabolic single-particle spectrum) we find that a long-lived spin plasmon - a collective mode in which the densities of up and down spins oscillate with opposite phases - can exist within a "pseudogap" of the single-particle excitation spectrum. The ensuing collectivization of the spin excitation spectrum is quite remarkable and should be directly visible in Raman-scattering experiments. The predicted mode could dramatically improve the efficiency of coupling between spin-wave-generating devices, such as spin-torque oscillators.