Japanese Researchers Superconduct Graphene

Scientists in Japan have developed superconductive graphene, according to research published in ACS Nano. Graphene is a single-atomic carbon sheet with a hexagonal honeycomb network. Electrons in graphene take on a special electronic state called the Dirac-cone, where the electrons behave as if they have no mass. This allows the electrons to flow at very high speed, giving graphene a very high level of electrical conductivity, and it is significant because electrons with no mass flowing with no resistance in graphene could lead to the realization of a high-speed nanoelectronic device.

The team, from Tohoku University and the University of Tokyo, fabricated bilayer graphene using a unique method—one that controls the number of graphene sheets in order to grow high-quality graphene on a silicon carbide (SiC) crystal—and then inserted calcium (Ca) atoms between the two graphene layers like a sandwich. The researchers then measured the electrical conductivity with the micro four-point probe method and found that the electrical resistivity rapidly dropped at around 4K (-269°C), indicative of an emergence of superconductivity.

Temperature dependence of electrical resistivity of Ca-intercalated bilayer graphene, measured by the micro four-point-probe method (inset). The resistivity shows a rapid decrease at around 4K and reaches "zero" at 2K, showing the emergence of superconductivity. Image: Tohoku University

Temperature dependence of electrical resistivity of Ca-intercalated bilayer graphene, measured by the micro four-point-probe method (inset). The resistivity shows a rapid decrease at around 4K and reaches "zero" at 2K, showing the emergence of superconductivity. Image: Tohoku University

The superconducting transition temperature (Tc) observed in this study on Ca-intercalated bilayer graphene is still low (4K), prompting further study into ways to increase Tc, for example, by replacing Ca with other metals and alloys, or changing the number of graphene sheets.

These researchers also found that neither genuine bilayer graphene nor lithium-intercalated bilayer graphene shows superconductivity, indicating that the superconductivity is driven by the electron transfer from Ca atoms to graphene sheets. It is currently not clear, however, what phenomenon takes place when the Dirac electrons with no mass become superconductive with no resistance.

Nevertheless, the group anticipates that its success in fabricating superconducting graphene will impact both basic and applied graphene research, and that further experimental and theoretical investigations will help unravel the properties of superconducting graphene. From the application point of view, the latest results pave the way for the further development of ultrahigh-speed superconducting nano devices, such as a quantum computing device that utilizes superconducting graphene in its integrated circuit.