Phonon-assisted indirect transitions in angle-resolved photoemission   spectra of graphite and graphene
  Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single-crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for $k$ points along the $\Gamma$-$K$ and $K$-$M$-$K'$ directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy of about 11.1 eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy of about 6.3 eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed. 
Pourya Ayria, Shin-ichiro Tanaka, Ahmad R. T. Nugraha, Mildred S.   Dresselhaus, Riichiro Saito
cond-mat.mtrl-sci cond-mat.mes-hall