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arXiv · 2609.00083

Fermi-Level Metal-d Character of Group-6 Bis-Hexahapto Bilayer Graphene

Mingguang Chen
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Abstract

Superconductivity in graphite intercalation compounds requires occupancy of a carbon-derived interlayer band filled by charge from an electropositive intercalant. Bis-hexahapto (eta6-eta6) coordination binds transition metals covalently to two graphene sheets with little charge transfer: the mechanism defining this chemistry removes the donation that superconductivity rests on. We ask, for group-6 Cr, Mo and W in bilayer graphene, whether Fermi-level states retain the interstitial, carbon-p character of ionic references, with bulk CaC6 and same-cell C12Ca/C12Li controls. Electron-phonon coupling is not computed. At one metal per gallery (C12M), the ionic controls retain 68 and 45 per cent of gallery spectral weight in atom-masked interstitial regions at EF; the group-6 systems retain 15, 22 and 23 per cent. The contrast is compositional: group-6 galleries carry more absolute interstitial weight than C12Li, but projected density of states within +/-0.15 eV of EF is carbon dominated in the ionic references and metal dominated (~73 per cent, of which 89-98 per cent is d) in all three group-6 bilayers. Every system studied is metallic; they differ in what carries the Fermi surface. Zone-centre phonons are clean for all three ordered bilayers; the pristine AA bilayer is not, its unstable mode being the AA-to-AB shear. Geometrically, eta6-eta6 coordination requires AA stacking and reverses the intrinsic Bernal preference by 400-600 meV wherever metal is present. Ordered phases are metastable against bulk metal (+2.9-5.0 eV per atom) but bound against isolated atoms (-1.6-4.1 eV); instability ordering (Cr least, W most) matches Cr > Mo > W reactivity. C12Cr is metallic; tungsten alone carries 0.65 muB. Bis-hexahapto intercalation delivers strong interlayer bonding and a definite AA registry, but not the interlayer-band character of superconducting graphite intercalation compounds.

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Mingguang Chen. 2026-09-07. Fermi-Level Metal-d Character of Group-6 Bis-Hexahapto Bilayer Graphene. https://arxiv.org/abs/2609.00083

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