Effect of particle-hole continuum on two-stream instability modes in graphene

Recent theoretical studies [1,2] have shown that under conditions in graphene that are analogous to the two-stream instability in plasma physics, some plasmons may be unstable in the sense that their amplitudes increase with time. These studies used approximate forms of the dielectric functions that excluded the effects of the particle-hole excitation continuum (PHEC), which typically leads to decay of plasmons. For parabolic band systems the electron density can be tuned so that the unstable mode lies outside the PHEC. However, in graphene, it is not possible to do this (because the fermi velocity does not change with electron density) and therefore it is important that theoretical studies take into account the effects of the PHEC. We report theoretical studies of these unstable plasmon modes in single-layer graphene in which we utilize an approximation of the dielectric function which includes the effect of the PHEC. Our preliminary results suggest that while the PHEC does indeed suppress the formation of the unstable plasmons, there is a range of experimental parameters where unstable plasmons still exist.

[1] Chinta M. Aryal, Ben Yu-Kuang Hu, and Antti-Pekka Jauho, Phys. Rev. B 94, 115401 (2016)
[2] Tiago A. Morgado and Mário G. Silveirinha, Phys. Rev. Lett. 119, 133901 (2017)