Superconductivity in graphene induced by atomic intercalation

Abstract

We report the emergence of superconductivity (SC) in graphene via atomic intercalation with calcium (Ca) and ytterbium (Yb), each producing distinct electronic environments. In the case of Ca-intercalation, SC arises below 5 K due to electron doping that lowers an interlayer band (ILB) to the Fermi level (E_F), enabling hybridization with graphene’s π* bands around the K point in the Brillouin zone, which was confirmed by angle-resolved photoemission spectroscopy (ARPES). This results in a high density of states at E_F conducive to Cooper pairing. Systematic tuning of carrier density reveals a dome-shaped dependence of the SC transition temperature on the carrier density, suggesting involvement of van Hove singularity (vHs) around the M point with appropriate doping. In contrast, Yb intercalation leads to SC at 0.85 K, accompanied by a markedly different band structure. ARPES shows that Yb 4f levels lie near E_F and clearly hybridize with graphene’s π and π* bands, while no distinct ILB is observed around E_F. X-ray photoelectron spectroscopy (XPS) and photoelectron diffraction reveal that divalent Yb ions reside between the graphene layers, acting as the primary contributor to SC. If the anisotropic nature of Yb 4f orbitals is introduced into the graphene band relevant to SC, it enables us to realize potentially unconventional pairing mechanisms in the two-dimensional limit. Our comparative study highlights how the characteristics of the intercalant—its valence, orbital character, and stacking structures—profoundly influence the superconducting state. The Ca system follows a more conventional band-filling picture except for possible electron correlation at vHs, while the Yb system exhibits hybridized bands and enhanced correlations. These results suggest that introducing heavy elements such as Yb into graphene, a light electron system, to induce hybrid orbitals can transform it into a heavy-electron system, thereby enabling the extraction of a wealth of physical phenomena.