Angle-resolved photoemission study of the graphite intercalation compound ${\text{KC}}_8$: A key to graphene

Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here, we unravel the electronic structure of noninteracting, doped graphene layers by revisiting the stage one graphite intercalation compound ${\text{KC}}_8$. To this end we apply angle-resolved photoemission spectroscopy and ab initio calculations. The full experimental dispersion is in excellent agreement with calculations of doped graphene once electron correlations are included at the $GW$ level (Greens function $G$ of the Coulomb interaction $W$). This highlights that ${\text{KC}}_8$ has negligible interlayer coupling allowing us to access the full experimental Dirac cone.

Figure 1

(Color online) (a) Crystal structure of ${\text{KC}}_8$. (b) Photo of intercalated ${\text{KC}}_8$ single crystals with golden color. (c) Region close Fermi level measured from the $\Gamma$ point of the BZ to put in evidence the lack of photoemission signal. Lines denote LDA and $GW$ calculations. (d) As measured EDC cuts through the corner of the 3D BZ. (e) Equienergy contours of the photoemission intensity taken at 48 eV photon energy for four binding energies along with calculations at the LDA and $GW$ level and a 3NN TB fit (see text).

Figure 2

(Color online) (a) ARPES scan measured close to the $k_y$ direction along with the TB fit of the bare-band dispersion (black) and the $GW$ calculation for rigidly doped graphene (green). (b) Symmetrized equienergy contour for $E=0.24\text{eV}$ and maxima (crosses) along with the TB fit and the $GW$ ab initio calculations.

Figure 3

(Color online) (a) Experimental Dirac cone from the observed photoemission intensity maxima (denoted as dots). Measured and calculated $\left(GW\right)$ values of $v_F$ for (b) electrons and (c) holes around the Dirac point.