A potential way to make a single layer of carbon atoms superconducting could be the answer to a long-sought search to engineer materials for super-efficient nanoelectronics, scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have discovered.
Researchers have found a way to make a layer of carbon atoms -- graphene -- superconducting by using a beam of intense ultraviolet light to look deep into the electronic structure of a material made of alternating layers of graphene and calcium.
Scientists have known for nearly a decade that the combined material is superconducting, but the new study offers the first compelling evidence that the graphene layers are instrumental in the process, a discovery that could transform the engineering of materials for nanoscale electronic devices, they said.
"Our work points to a pathway to make graphene superconducting -- something the scientific community has dreamed about for a long time, but failed to achieve," said Shuolong Yang, a graduate student at the Stanford Institute of Materials and Energy Sciences (SIMES) who led the research at SLAC's Stanford Synchrotron Radiation Lightsource.
The researchers observed 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. They reported their findings March 20 in the journal Nature Communications.
Graphene, a single layer of carbon atoms arranged in a honeycomb pattern, is the thinnest and strongest known material to date and a quality conductor of electricity, the scientists said. Researchers hope to eventually use graphene to make very fast transistors, sensors and even transparent electrodes.
Graphene is usually made by peeling atomically thin sheets from a block of graphite, a form of pure carbon that is used as the lead in pencils. But scientists can also isolate these carbon sheets by chemically interweaving graphite with crystals of pure calcium. The result, known as calcium intercalated graphite or CaC6, consists of alternating one-atom-thick layers of graphene and calcium.
In nearly a decade of trying, researchers were unable to tell whether graphene's superconductivity came from the calcium layer, the graphene layer or both.
"These are extremely difficult experiments," said Patrick Kirchmann, a staff scientist at SLAC and SIMES. But the purity of the sample combined with the high quality of the ultraviolet light beam allowed the researchers 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.
"With this technique, we can show for the first time how the electrons living on the graphene planes actually superconduct. The calcium layer also makes crucial contributions. Finally we think we understand the superconducting mechanism in this material," said SIMES graduate student Jonathan Sobota, who carried out the experiments with Yang.
Applications of superconducting graphene are still speculative and far in the future, but they could include ultra-high frequency analog transistors, nanoscale sensors, electromechanical devices and quantum computing devices, the scientists said.
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 researchers from SLAC, Stanford, Lawrence Berkeley National Laboratory and University College London. The work was supported by the Department of Energy's Office of Science, the Engineering and Physical Sciences Research Council of UK and the Stanford Graduate Fellowship program.