In tetralayer graphene, three inequivalent layer stackings should exist, however, only rhombohedral (ABCA) and Bernal (ABAB) stacking have so far been observed. The three stacking sequences differ in their electronic structure, with the elusive third stacking (ABCB) being unique as it is predicted to exhibit an intrinsic bandgap as well as locally flat bands around the K points. Within our random phase approximation, these flat bands are predicted to promote correlated phenomena such as magnetic and unconventional superconducting order. Here, we use scattering-type scanning near-field optical microscopy and confocal Raman microscopy to identify and characterize domains of ABCB stacked tetralayer graphene. We reveal unique optical fingerprints of all three stacking sequences by addressing their infrared conductivities between 0.28 and 0.56 eV using amplitude and phase-resolved near-field nano-spectroscopy. Comparison with results from tight-binding calculations are in good quantitative agreement and allow for unambiguous assignments.
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