  Coherent coupling of Dirac fermion magneto-excitons with an optical phonon is
observed in graphite as marked magnetic-field dependent splittings and
anti-crossing behavior of the two coupled modes. The sharp magneto-phonon
resonance occurs in regions of the graphite sample with properties of superior
single-layer graphene having enhanced lifetimes of Dirac fermions. The greatly
reduced carrier broadening to values below the graphene electron-phonon
coupling constant explains the appearance of sharp resonances that reveal a
fundamental interaction of Dirac fermions.
