Neutral Triplet Collective Mode in Doped Graphene

Abstract

Particle-hole continuum in Dirac sea of graphene has a unique window underneath, which provides a unique opportunity for emergence of a pole in the susceptibility of the {\em triplet} particle-hole channel in the entire Brillouin zone (BZ). Here we use random phase approximation (RPA) to study such collective mode at zero temperature, in a single layer of doped graphene. We find that due to the chiral nature of one-particle states, in undoped graphene, the wave function overlap factors do not lead to qualitative differences, while in doped graphene they will kill small momentum part of the branch of magnetic excitations by pushing it to touch the lower part of the continuum. The pole corresponding to magnetic excitations survives for for larger momenta in the BZ.