Marchenko, D.; Krivenkov, M.; Sajedi, M.; Fedorov, A.; Rader, O.; Varykhalov, A.: Revisiting the realization of artificial graphene in C60/Cu(111). Physical Review B 108 (2023), p. 115155/1-9
10.1103/physrevb.108.115155
Abstract:
Tamai et al. discovered an unusual electronic state near the Fermi level at the interface of Cu(111) and a molecular layer of C60, which was initially attributed to C60−Cu interfacial hybridization [Phys. Rev. B 77, 075134 (2008)]. Later on, Yue et al. suggested that the state was due to the reshaping of a two-dimensional electron gas hosted at the Cu(111) surface, into an artificial graphene with Dirac cones by cutting out muffin-tin potentials of adsorbed fullerene molecules [Phys. Rev. B 102, 201401(R) (2020)]. In the present paper, we introduce a different explanation using angle-resolved photoemission and show that the observed conical bands in the C60/Cu(111) system are neither Dirac cones nor hybridization states. Rather, they are formed by umklapp scattering of photoelectrons emitted from bulk and surface bands of Cu(111) and diffracted on the (4×4) fullerene superstructure. The circular contours near the Fermi level, which resemble the low-energy part of graphene Dirac cones, are a result of the backfolding of the bulk band of copper, while the triangular silhouettes observed at higher binding energies, which mimic the threefold symmetric higher-binding-energy part of graphene Dirac cones, are due to an umklapp effect governed by highly coherent photoelectron diffraction of steeply dispersing Cu(111) sp-type surface resonances. We also used density functional theory to study the behavior of the two-dimensional electron gas localized within the honeycomb net created by the electron-exclusive potentials of C60 and demonstrate doping above the Fermi level (p doping). The results show no presence of the electronic structure of artificial graphene in C60/Cu(111).