A sheet of four pairs of molecules less than one nanometer wide can be superconducting, scientists have found. The Ohio University-led study provides the first evidence that nanoscale molecular superconducting wires can be fabricated, which could be used for nanoscale electronic devices and energy applications. “Researchers have said that it’s almost impossible to make nanoscale interconnects using metallic conductors because the resistance increases as the size of wire becomes smaller. The nanowires become so hot that they can melt and destruct. That issue, Joule heating, has been a major barrier for making nanoscale devices a reality,” said Saw-Wai Hla, an associate professor of physics and astronomy with Ohio University’s Nanoscale and Quantum Phenomena Institute. Hla is also lead author on a paper about the work published today in an advance online publication of the journal Nature Nanotechnology. The smallest superconductor ever found is a sheet of four pairs of molecules only .87 nm wide. (Images courtesy of Saw-Wai Hla and Kendal Clark, Ohio University) Superconducting materials have an electrical resistance of zero, and so can carry large electrical currents without power dissipation or heat generation. Superconductivity was first discovered in 1911, and until recently, was considered a macroscopic phenomenon. The current finding suggests, however, that it exists at the molecular scale, which opens up a novel route for studying this phenomenon, Hla said. Superconductors currently are used in applications ranging from supercomputers to brain imaging devices. In the new study, which was funded by the US Department of Energy, Hla and co-workers deposited molecules of an organic charge-transfer salt on a silver surface and used a technique called scanning tunnelling spectroscopy to examine the molecular islands and chains that formed on the surface. They found evidence for a superconducting gap — a classic signature of superconductivity — at temperatures within ten degrees of absolute zero. The size of this gap depended on the length of the chains, and the gap could still be seen in wires that contained just four molecules. To observe superconductivity at this scale, the scientists needed to cool the molecules to a temperature of 10 K. Warmer temperatures reduced the activity. In future studies, scientists can test different types of materials that might be able to form nanoscale superconducting wires at higher temperatures, Hla said. “But we’ve opened up a new way to understand this phenomenon, which could lead to new materials that could be engineered to work at higher temperatures,” he said. Several nanoscale superconducting molecular wires on a silver substrate. The study also is noteworthy for providing evidence that superconducting organic salts can grow on a substrate material. “This is also vital if one wants to fabricate nanoscale electronic circuits using organic molecules,” Hla said. Collaborators on the paper include Kandal Clark, a doctoral student in the Russ College of Engineering and Technology at Ohio University; Sajida Khan, a graduate student in the Department of Physics and Astronomy at Ohio University; Abdou Hassanien, a researcher with the Nanotechnology Research Institute, Advanced Industrial Science and Technology (AIST) and the Japan Science and Technology Agency’s Core Research of Evolutional Science & Technology (JST-CREST) in Japan who conducted the work as a visiting scientist at Ohio University; Hisashi Tanaka, a scientist at AIST and JST-CREST who synthesized the molecules; and Kai-Felix Braun, a scientist with the Physikalisch Technische Bundesanstalt (National Metrological Institute of the Federal Republic of Germany) in Braunschweig, who conducted the calculations at the Ohio Supercomputing Center. For more information, visit: www.ohio.edu