Angle-Resolved Photoemission Spectroscopy Studies of Flat Bands in Rhombohedral Graphene

The emergence of interaction driven phases in the twisted two-dimensional van der Waals heterostructures is due to the presence of van Hove singularities or flat bands where Coulomb repulsion energies dominate the kinetic energy of electrons. However, the correlated phenomena in twisted systems are extremely sensitive to angle disorder and strain, making them hard to achieve for experimental reproducibility. In this regard, graphene multilayers with rhombohedral or ABC stacking offers a different route to achieve flat bands. In this work, we have systematically studied the flat band electronic structure of ABC stacked multilayer graphene via high resolution angle resolved photoemission spectroscopy (ARPES). Few-layer (4 - 120) graphene flakes are directly exfoliated on to highly conducting Si (100) substrates using blue tape. We then identified ABC stacked flakes via Raman spectroscopy measurements. The thickness of the flakes is determined via atomic force microscopy (AFM). Synchrotron-based micro-ARPES experiments revealed intense flat bands around the <i>K</i> point, close to Fermi level (<i>E</i>_F). The presence of the flat bands over a large, measured photon energy range (44 -235 eV) confirms their surface origin. From the energy distribution curves (EDCs) stacks a curvature in the ΓK and KM direction exists, and this dispersion extends over 25 meV. The width of the dispersion shrinks to ~10 meV, when the layer number is reduced to 4-5 layers, suggesting an increase in the correlation strength with the decrease of layer number in ABC stacked graphene. We have also performed temperature (12 – 300 K) dependent and polarization ARPES studies of the flat bands, to determine the magnetic origin of the curvature. Since RG is thermally more stable than twisted bilayers and naturally available, avoiding the need for fabrication, our work provides critical insights in understanding the correlated phenomena in chemically simple systems devoid of disorder.