Superconductivity of potassium-intercalated epitaxial graphene

Abstract

The properties related to superconductivity of metal-intercalated, graphene-based, layered systems exhibit a clear dependence on the number of adjacent graphene layers and the intercalant species. In particular, superconductivity of potassium-intercalated mono- and bilayer graphene has not been proven yet. This work provides a detailed investigation of the evolution of structural and electronic properties of epitaxial monolayer graphene on SiC(0001) upon K intercalation. It is shown that the well-known (2x2) superstructure of the K atoms with respect to the graphene lattice forms below the topmost layer. Moreover, the intercalants accumulate as well below the buffer layer and induce its effective decoupling from the underlying SiC substrate, enabling the sample to behave like K-intercalated, quasi-freestanding epitaxial bilayer graphene. Via local and area-averaging experimental methods, it is determined that the presence of K atoms causes not only a filling of the Dirac bands of graphene, but also an occupation of two parabolic interlayer bands. By means of tunneling spectroscopy measurements of an emerging temperature-dependent energy gap around the Fermi level, it is shown that K-intercalated quasi-freestanding epitaxial bilayer graphene is a superconductor below a critical temperature of 3.65(2) K, which is also verified by determination of the average electron-phonon coupling strength on the Dirac bands using angle-resolved photoelectron spectroscopy. Although a strongly elevated gap ratio of 6.19(7) compared to conventional superconductors is determined, strong-coupling mechanisms appear to be unlikely considering the related electron-phonon coupling strength. Hence, the unconventional behavior is most likely a consequence of low-dimensional effects. In particular, this study provides the first investigation of the temperature dependence of the energy gap related to superconductivity among metal-intercalated thin films of graphene-based, layered systems.