Anomalous c-axis shifts and symmetry enhancement in highly oriented pyrolytic graphite at the magic angle

Observation of superconductivity in twisted-bilayer-graphene and highly-oriented-pyrolytic-graphite (HOPG) for certain magic-angles-of-rotation has recently attracted an important attention. Unusual temperature-(T)-induced-shifts in the graphitic c-axis have also been reported in HOPG in conditions of θmisfit ∼ 1° (first-magic-angle). We report a novel investigation of HOPGs with θmisfit of 0.5°, 0.8°, 1.5°, ≫ 1.5° and of water-treated-turbostratic-graphite in the T-range from 298.15 to 673.15K. Presence of magic angles of rotations corresponding to the reported θmisfit values is demonstrated by repeated HRTEM and Moiré pattern analyses of HOPG lamellae extracted from individual samples with scotch tape methods. Interestingly in our measurements the c-axis-shift is found to depend strongly on the misfit-angle, with the highest-values of 0.00428 and 0.00426 nm in proximity of the first-magic-angle (θmisfit ∼ 0.8° and ∼0.5°). Two diffraction-peaks present at ∼23.5° and ∼48.5° 2θ (detector angle, in typical Bragg- Brentano configuration) for θmisfit ∼1° are also found to vanish for θmisfit ≫ 1.5°. These findings imply existence of additional symmetry elements at the magic angle, which are not present in the standard space group notation used for structural characterization of graphite (P63/mmc). [Display omitted]