Gapless Spin-1 Neutral Collective Mode Branch for Graphite

Using the standard tight binding model of 2D graphite with short range electron repulsion, we predict a gapless spin-1, neutral collective mode branch below the particle-hole continuum with energy vanishing linearly with momenta at the $\Gamma$ and $K$ points in the Brillouin zone. This spin-1 mode has a wide energy dispersion, $0$ to $\sim 2\mathrm{eV}$, and is not Landau damped. The “Dirac cone spectrum” of electrons at the chemical potential of graphite generates our collective mode, so we call this “spin-1 zero sound” of the “Dirac sea.” Epithermal neutron scattering experiments and spin polarized electron energy loss spectroscopy can be used to confirm and study our collective mode.