Dirac Cones and Minigaps for Graphene on Ir(111)

Epitaxial graphene on Ir(111) prepared in excellent structural quality is investigated by angle-resolved photoelectron spectroscopy. It clearly displays a Dirac cone with the Dirac point shifted only slightly above the Fermi level. The moiré resulting from the overlaid graphene and Ir(111) surface lattices imposes a superperiodic potential giving rise to Dirac cone replicas and the opening of minigaps in the band structure.

Figure 1

(a) LEED pattern (69 eV) of graphene on Ir(111). First order Ir spots, C spots, as well as surrounding moiré spots are visible. (b) STM topograph ($75\mathrm{nm}\times 50\mathrm{nm}$) exhibiting moiré contrast (enhanced by an unsharp masking filter). Tunneling conditions 0.05 V, 32 nA. Inset: $5\mathrm{nm}\times 5\mathrm{nm}$ atomic resolution image of graphene lattice. Tunneling conditions 0.37 V, 32 nA.

Figure 2

(a) ARPES spectrum of clean Ir(111), $\phi =0.5°\pm 0.1°$. The positions of $K$ points of iridium and graphene are marked as $K_{\mathrm{Ir}}$ and $K_g$, respectively. $S_1-S_3$ are surface states. (b) ARPES spectrum of Ir(111) covered by graphene along the same azimuth as in (a). Horizontal arrows denote the minigap in the primary Dirac cone. A visible replica band is labeled as $R$. Detailed analysis in [34]. (c), (d) ARPES spectra for $\phi =1.4°\pm 0.1°$ and $\phi =3.0°\pm 0.1°$, respectively. The dashed lines are tight-binding-approximation-calculated bands for Dirac cones located at positions 0, 1, 2, and 5 defined in Fig. 3b.

Figure 3

(a) BZ of the Ir(111) surface and graphene lattices drawn to scale. Mini–Brillouin zone (mBZ) is indicated around the $K$ point of graphene. Open dots denote the vertices of Dirac cones replicated by moiré reciprocal vectors, ${\mathbf{G}}_m$. (b) Blowup of the mBZ. Central dot denotes the center of the primary Dirac cone, while 1–6 are replicas. (c) ARPES experimental plane (shaded) along the $\Gamma \mathrm{\text{−}}K\mathrm{\text{−}}M$ high symmetry direction cuts the primary and replica cones (only partially shown). Bright lines at the intersection mark the bands observed in Fig. 2b.

Figure 4

The minigaps determined from ARPES scans along the directions indicated by dashed lines fall on the edge of the mini–Brillouin zone. Their widths are annotated in meV. The dashed lines correspond to $\phi =-1.9°$, $\phi =0.5°$, $\phi =1.4°$, $\phi =3.0°$, and $\phi =3.9°$.