Anomalous c-axis shifts and symmetry enhancement in highly oriented pyrolytic graphite at the magic angle. (September 2019)

• Record Type: Journal Article
• Authors: Boi, Filippo S.; Liu, Mengjiao; Xia, JiaChen; Odunmbaku, Omololu; Taallah, Ayoub; Wen, Jiqiu
• Is Part Of: Carbon. Volume 150(2019)
• Page Start: 27
• Page End: 31
• Publication Date: 2019-09
• Subjects: Carbon -- Periodicals; Carbone -- Périodiques; Koolstof; Toepassingen; Electronic journals; 546.681
• DOI: 10.1016/j.carbon.2019.05.003
• Languages: English
• ISSNs: 0008-6223

Abstract

Abstract: 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 θ misfit ∼ 1° (first-magic-angle). We report a novel investigation of HOPGs with θ misfit 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 θ misfit 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 ( θ misfit ∼ 0.8° and ∼0.5°). Two diffraction-peaks present at ∼23.5° and ∼48.5° 2 θ (detector angle, in typical Bragg- Brentano configuration) for θ misfit ∼1° are also found to vanish for θ misfit ≫ 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). Graphical abstract: Image 1