The researchers saw how electrons scatter back and forth between graphene and calcium, interact with natural vibrations in the material’s atomic structure and pair up to conduct electricity without resistance.

The classic way to make graphene—a single layer of carbon atoms arranged in a honeycomb pattern—is by peeling atomically thin sheets from a block of graphite. But scientists can also isolate these carbon sheets by chemically interweaving graphite with crystals of pure calcium. The result—CaC₆—consists of alternating one-atom-thick layers of graphene and calcium.

After nearly a decade of trying, researchers were unable to tell whether CaC₆’s superconductivity came from the calcium layer, the graphene layer or both.

For this study, samples of CaC₆ were made at University College London and brought to the Stanford Synchrotron Radiation Lightsource (SSRL) for analysis.

The purity of the sample combined with the high quality of the ultraviolet light beam allowed them to see deep into the material and distinguish what the electrons in each layer were doing, revealing details of their behavior that had not been seen before.

Our analysis of the superconducting gaps and EPC [electron–phonon coupling] strengths reveals that superconductivity of CaC₆ cannot be attributed to either the π* or IL band alone. Instead, our discoveries demonstrate the critical role of the interaction between the two bands.

An important implication of this understanding is that the substantial contribution to the total EPC strength from the interband interaction can be readily explored in monolayer graphene. In this case, the IL band can be created by forming an array of surface adatoms with the resulting interband interaction then enabling superconductivity. … If this material can be fabricated, it will allow an efficient integration of various nanoscale technologies, such as superconducting quantum interference devices and single-electron superconductor-quantum dot devices. Importantly, in this proposal the IL pocket size and the EPC strength contributed by the interband interaction are the keys to realizing the superconducting phase transition.

Our work demonstrates the significant role ARPES will have in the implementation of this proposal, as it allows for direct measurement of not only the size of the IL band, but also the strength of the interband electron–phonon interaction. These results set a solid foundation for future exploratory activities in the pursuit of fabricating superconducting graphene devices.

—Yang et al.

Although applications of superconducting graphene are speculative and far in the future, the scientists said, they could include ultra-high frequency analog transistors, nanoscale sensors and electromechanical devices and quantum computing devices.

The research team was supervised by Zhi-Xun Shen, a professor at SLAC and Stanford and SLAC’s advisor for science and technology, and included other researchers from SLAC, Stanford, Lawrence Berkeley National Laboratory and University College London. The work was supported by the DOE’s Office of Science, the Engineering and Physical Sciences Research Council of UK and the Stanford Graduate Fellowship program.

Resources

• S. L. Yang et al. (2014) “Superconducting graphene sheets in CaC6 enabled by phonon-mediated interband interactions,” Nature Communications doi: 10.1038/ncomms4493