Minigap isotropy and broken chirality in graphene with periodic corrugation enhanced by cluster superlattices

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Minigap isotropy and broken chirality in graphene with periodic corrugation enhanced by cluster superlattices

J. Sánchez-Barriga, A. Varykhalov, D. Marchenko, M. R. Scholz, and O. Rader

Phys. Rev. B 85, 201413(R) – Published 24 May 2012

Abstract

The chirality of charge carriers in graphene determines its peculiar scattering properties and in particular the avoided backscattering, predicted to be observable in periodically corrugated graphene as the closing of the so-called minigaps. These are small gaps appearing in the graphene $π$ band at the crossings with umklapp-induced replica bands of the Dirac cone. By angle-resolved photoemission of corrugated graphene on Ir(111), we observe that the minigaps are instead isotropic and close nowhere in k space, unexpected for chiral charge carriers. Artificially enhancing the periodic superpotential by deposition of Au and Ir clusters confirms the minigap isotropy, indicating that the origin of this behavior is directly connected to the broken chirality in the system.

Received 29 February 2012

DOI:https://doi.org/10.1103/PhysRevB.85.201413

Authors & Affiliations

J. Sánchez-Barriga, A. Varykhalov, D. Marchenko, M. R. Scholz, and O. Rader

Helmholtz-Zentrum Berlin für Materialien und Energie, Elektronenspeicherring BESSY II, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany

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Vol. 85, Iss. 20 — 15 May 2012

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Images

Figure 1
(a), (b) STM images of Ir (a) and Au (b) clusters grown on moiré-patterned graphene/Ir(111). Tunneling parameters were $V_{t} = + 1.6$ V, $I_{t} = 0.5$ nA in (a) and $V_{t} = + 0.03$ V, $I_{t} = 6$ nA in (b). Inset in (a): High-resolution STM from Ir clusters revealing their hexagonal shape. (c), (d) Dirac cones measured by ARPES before (c) and after (d) deposition of 0.1 ML of Au. Red (light) points are fits to the peak positions, and black (dark) lines fits to the linear part of the dispersion. Inset in (c): The experimental geometry. Right panels: Spectra at $\bar{K}$ .Reuse & Permissions
Figure 2
(a)–(d) Evolution of asymmetric Dirac cones with coverage of Au clusters at low temperature. Minigaps are highlighted with horizontal yellow (light) lines. Spectra at (e) $\bar{K}$ point and (f) $k = 0.15$ Å $^{- 1}$ emphasize the progressive opening of a gap at the Dirac point and the enlargement of the minigap size, respectively. (g) Change in the linewidth, (h) minigap size, and (i) group velocity $v_{g}$ induced by the clusters. (j), (k) Dirac cones measured at RT for (j) graphene/Ir and (k) after deposition of 0.15 ML Ir clusters on top. (l) C $1 s$ core levels measured at $h ν = 400$ eV before [black (dark) symbols] and after [red (light) symbols] cluster growth. Inset: Zoom on the C $1 s$ peak showing a shift of 50 meV.Reuse & Permissions
Figure 3
Tomographic view of Dirac cones (a) before and (b) after Ir cluster deposition. Photoemission intensity appears as a color pixel when it reaches a threshold value. The threshold is fixed for the entire figure. (c), (d) Top: Corresponding minigap dispersions; bottom: minigap sizes. (e) Constant energy cut of graphene/Ir near the binding energy of the minigaps ( $h ν = 62$ eV). A scheme of the graphene MBZ is on top (black lines). (f), (g) First derivative of the ARPES intensity in the region of minigaps. (a), (c), (f) Before and (b), (d), (g) after growth of 0.15 ML Ir clusters.Reuse & Permissions
Figure 4
(a) Second derivative of the ARPES intensity showing dispersion acquired for bare graphene/Ir(111) along the $\bar{Γ K}$ direction (data is symmetrized). Red continuous lines show the dispersion of the bands and a typical avoided-crossing effect. The red (light) dashed circle emphasizes the position of the hybridization gap. The top arrow indicates the minigap at lower binding energy. (b)–(e) $π$ -band constant energy maps (b) before and (c) after Au deposition. Observation of asymmetric Dirac cones in (d) graphene on Ir(111) and (e) with 0.2 ML Au clusters grown on top. (f) Dispersions of the $π$ band extracted by fitting ARPES data shown in (d) [black (dark)] and (e) [red (light)]. Error bars are the peak widths obtained from fits to MDCs.Reuse & Permissions

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