Superconductivity of K-Intercalated Epitaxial Bilayer Graphene

Graphene-based materials are among the most promising candidates for studying superconductivity arising from reduced dimensionality. Apart from doping by twisted stacking, superconductivity can also be achieved by metal-intercalation of stacked graphene sheets, where the properties depend on the choice of the metal atoms and the number of graphene layers. Many different and even unconventional pairing mechanisms and symmetries are predicted in the literature for graphene monolayers and few-layers. However, those theoretical predictions have yet to be verified experimentally. Here, it is shown that potassium-intercalated epitaxial bilayer graphene is a superconductor with a critical temperature of T-c = 3.6 +/- 0.1 K. By scanning-tunneling microscopy and angle-resolved photoelectron spectroscopy, the physical mechanisms are analyzed in great detail, using laboratory equipment. The data demonstrate that electron-phonon coupling is the driving force enabling superconductivity. Although the consideration of an s-wave pairing symmetry is sufficient to explain the experimental data, evidence is found for the existence of multiple energy gaps. Furthermore, it is shown that low-dimensional effects are most likely the cause of a gap ratio of 6.1 +/- 0.2 that strongly exceeds the Bardeen-Cooper-Schrieffer (BCS) value of 3.52 for conventional superconductors. These results highlight the importance of reduced dimensionality yielding unusual superconducting properties of K-intercalated epitaxial bilayer graphene.

Automated summary

The intercalation of potassium into epitaxial bilayer graphene is shown to enable superconductivity with a critical temperature of Tc = 3.6 +/- 0.1 K. The physical mechanisms are analyzed in detail using scanning tunneling microscopy and angle-resolved photoelectron spectroscopy. The data suggest that electron-phonon coupling is the driving force for superconductivity, and multiple energy gaps are observed. Additionally, the low-dimensional effects lead to a gap ratio exceeding the BCS value for conventional superconductors. These findings highlight the significance of reduced dimensionality in determining the superconducting properties of K-intercalated epitaxial bilayer graphene.