Prof. Bang Yun-kyu, 48, in the Dept. of Physics has found that magnetic fluctuations appear to be responsible for superconductivity in a compound called plutonium-cobalt-pentagallium (PuCoGa5). The discovery of this "unconventional superconductivity" may lead scientists to a whole new class of superconducting materials and toward the goal of eventually synthesizing "room-temperature" superconductors. In research reported in the latest edition of the scientific journal Nature (March 31, 2005), Prof. Bang and the University of California scientists at Los Alamos Laboratory provide evidence of how magnetic fluctuations, rather than interactions mediated by tiny vibrations in the underlying crystal structure, may be responsible for the electron pairing that produces superconductivity in the mixture of plutonium, cobalt and gallium. A superconductor refers to a material that is a perfect conductor of electricity when cooled to a very low temperature, called critical temperature, at around absolute zero of 0 Kalvin (minus 273 degrees Celsius). Because a superconductor has exactly zero electrical resistance, it carries a direct current with 100 percent efficiency, prompting many to dub it as future technology in many fields. Although the temperatures at which superconductivity is observed are usually quite low, a handful of compounds like PuCoGa5 have been found to possess superconductivity at temperatures warmer than minus 427 degrees Fahrenheit. Even though that temperature seems low, PuCoGa5 possesses highest superconducting transition temperature among actinide-based compounds found so far. This "unconventional superconductivity" suggests that PuCoGa5 may be one of a very small handful of superconductors whose superconductivity actually derives from magnetic correlations. Scientists theorize that having found one unconventional superconductor like PuCoGa5, they may find more in the future. Making the research even more intriguing is the fact that plutonium is a base actinide material of the compound. This new class of magnetically mediated superconductors might encompass a broad range of materials, metals to oxides, and be the path toward superconductor science
s ultimate goal to someday synthesize a "room-temperature" superconductor that would be the basis for the dissipation-less flow of electric current through power lines, and for an even more minute generation of computer chips. In plutonium, the substance that occurs in the creation of uranium-derived nuclear power, superconductivity was first observed in 2002 by John Sarrao at the Los Alamos National Laboratory of the United States. Although a plutonium superconductor has a transition temperature of relatively low 18.5K, Bang categorized it as a high-temperature superconductor because both have the same structure of electrons pairing. Since the 1980s it has been known that heavy-fermion systems lose their resistance to electric currents when cooled below about 1 K, whereas high-temperature copper oxides can remain superconducting at temperatures of 100 K or higher. Now Bang and scientists at the Los Alamos National Laboratory have shown that a plutonium-based superconductor first discovered in 2002 has properties that appear to bridge these two extremes. In a conventional or low-temperature superconductor the total orbital angular momentum of the two electrons in a Cooper pair is zero -- a so-called f-wave state. By contrast, the Cooper pairs in a high-temperature superconductor have two units of orbital angular momentum, which is known as a d-wave state. By measuring the spins of cobalt and gallium nuclei in the plutonium alloy at different temperatures, Bang and co-workers have now confirmed that the plutonium compound is an unconventional superconductor. The Ministry of Science & Technology announced that Prof. Bang’s discovery should be called as a triumph succeeding to two previous outstanding discoveries of Prof. Lee Sung-ik’s “MgB2 Superconducting Thin Films with a Transition Temperature of 39 Kelvin” and Prof. Choi Hyung-joon’s “The origin of the anomalous superconducting properties of MgB2.” “Prof. Bang’s study revealing the mechanism of plutonium-based superconductor is a tremendous achievement. We expect the substance will play a crucial role in areas such as high-performance motors or transformers, power storage devices, particle accelerators or blazingly fast computers. The experiments with plutonium have been rigidly restricted, therefore, Korean scientists have been working in other countries with world’s leading researchers,” said Prof. Lee Sung-ik, Pohang University of Science and Technology,” Prof. Bang graduated from Seoul National University (SNU) and received his master’s degree from the Dept. of Physics, SNU. He received a doctor’s degree from Rutgers University and worked at Max-Planck Institute in Germany as a researcher. In 1995, he joined the CNU faculty. Bang’s study has been supported by the Center for Strongly Correlated Materials Research at SNU (CSCMR), one of the Science Research Centers of Excellence designated by the Korea Science and Engineering Foundation.