A team mainly composed of researchers from the Institute of Physics recently discovered that at least three different kinds of iron(Fe)-vacancy order exist in one type of Fe-based superconductor, and showed that at least one of these three Fe-vacancy orders is non-superconducting and magnetic at low temperature. This new finding is exciting as physicists still do not know what causes some complex multicomponent materials to become superconducting, a gap in knowledge that is limiting real-life application of the phenomenon. The discovery was published in the Proceedings of the National Academy of Sciences (PNAS) on January 7.

         A superconductor is a material that can conduct electricity or transport electrons from one atom to another with no resistance. Superconductivity typically occurs in certain materials when they are cooled below a critical temperature. For many years, the highest critical temperature (superconducting phase transition temperature) known for a superconductor was about 23 K (minus 250°C, or minus 418°F), meaning that superconducting materials needed to be cooled by liquid helium for use, a process so complicated and expensive that it ruled out the practical use of superconductors in most applications. However, in 1986 a high temperature superconducting material – lanthanum barium copper oxide – was discovered. Then in 2008, more high temperature superconductors that are Fe-based were discovered. These so-called “high temperature” superconductors, especially the copper oxide system, have critical temperatures in a range up to around 130 K (minus 143°C, or minus 226°F), and were greeted with great excitement among physicists and potential manufacturers because they can be maintained in the superconducting state with liquid nitrogen (77 K) meaning that cooling the superconductor for real-life applications becomes much more feasible. However, although physicists believe they understand the mechanisms governing the earlier low-temperature superconductors, the mechanisms that determine high-temperature superconductivity – the origin of the superconductivity – remain a mystery.

         Professor Chien has won numerous awards during his 54-year academic career. Notably, in September 2011 he received the US 2011 National Medal of Science awarded by The White House, one of the highest honors bestowed by the United States government on scientists, engineers, and inventors.

         High-temperature superconductors exhibit an array of peculiar and unexpected behavior. Among the unresolved questions surrounding high temperature superconductor research are the exact chemical stoichiometry of superconducting compounds, why the superconducting transition temperatures exist over such a wide range in a single material system, and whether and how the superconductivity of Fe-based superconductors is related to the magnetism that normally comes with Fe element. All these questions have become the focus of current research in Fe-based superconductors.

         In the current study, the research team led by Academician Maw-Kuen Wu (Distinguished Research Fellow and the President of National Dong Hwa University), in collaboration with Professor Fu-Rong Chen of National Tsing Hua University, and Professor Dirk Van Dyck of University of Antwerp in Belgium showed that at least three different kinds of iron(Fe)-vacancy order exist in one type of Fe-based superconductor, an FeSe compound, and showed that at least one of these three Fe-vacancy orders is non-superconducting and magnetic at low temperature. In 2008, Academician Wu’s team discovered superconductivity in the FeSe compound for the first time. They used the same compound in their current study.

         The new study advances the field because, firstly, it is generally accepted that the origin of high temperature superconductivity is an antiferromagnetic insulating parent (original, non-superconducting) phase, which leads to the destruction of magnetism and then the appearance of superconductivity after the introduction of mobile electrons (or holes). The current new findings provide, for the first time, mapping of an exact phase diagram for the FeSe superconducting system. Secondly, the research discovery shows the presence of new types of Fe-vacancy order in Fe-based materials. This result gives scientists a better opportunity to understand the correlation between superconductivity and Fe-vacancy. In fact, this information may provide a new direction for the understanding of superconducting phenomena observed in all high temperature superconductors including non-Fe systems such as copper oxide systems.

         Superconductors are already used in several applications, notably magnetic resonance imaging (MRI) used in hospitals, maglevs (high-speed magnetic levitation trains), particle accelerator applications and NMR spectroscopy.

         If researchers can overcome the issues of cost, refrigeration and reliability that limit current superconducting materials, superconductors may have the potential to revolutionize electrical engineering with more efficient motors and generators and lossless power transmission. Promising future applications would include zero-loss electrical power lines, high-performance smart grids, electric power transmission, radar, transformers, power storage devices, high energy physics, high-end computing, nanoscopic materials and superconducting magnetic refrigeration.

         The complete article entitled "Fe-vacancy order and superconductivity in tetragonal β-Fe1-xSe" can be found at the PNAS website at: http://www.pnas.org/content/111/1/63.abstract .

         The complete list of authors is: Ta-Kun Chen, Chung-Chieh Chang, Hsian-Hong Chang, Ai-Hua Fang, Chih-Han Wang, Wei-Hsiang Chao, Chuan-Ming Tseng, Yung-Chi Lee, Yu-Ruei Wu, Min-Hsueh Wen, Hsin-Yu Tang, Fu-Rong Chen, Ming-Jye Wang, Maw-Kuen Wu and Dirk Van Dyck.