29th Jan the prestigious British science journal Nature featured the latest research production of Professor Huiqiu Yuan from department of physics, Zhejiang University and his colleagues: isotropy at the superconducting state was discovered in the layered crystal structures with quasi-two-dimensional electronic properties. It was the first time that people found that the superconducting properties are quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. The review committee of Nature regarded the findings as unique and important discoveries in Superconductivity Research. It may be a milestone in the research of iron-arsenic-based compounds with a high superconducting transition temperature. Moreover Nature detailed the research findings in the column ‘News and Views’.

The project was accomplished by Zhejiang University, Los Alamos National Laboratory, USA and Institute of Physics, Chinese Academy of Science. The first author and the corresponding author is Professor Huiqiu Yuan, who is the first professor from the College of Science to publish his academic dissertation in the top science journal such as Nature and Science.

In 1911 Dutch physicist Heike Kamerlingh Onnes found that the resistance of mercury was zero at 4.2 Kelvin and he called this phenomenon superconductivity. So it was the first time that people discovered superconductivity. Superconductivity had some extraordinary electrical properties that appealed to man: if the superconductor at room-temperature were to be manufactured, it would lead to a new energy revolution, since there wouldn’t exist energy loss in transmission. To fulfill the dream scientists have been exploring the new materials for nearly a century. With the new superconductor found in succession, the record of the critical temperature was broken constantly. Every discovery stirred interest in Superconductivity Study. As is known to all, Maglev train and magnetic resonance imaging (MRI) technologies are the practical applications of the superconductivity technology.

Professor Huiqiu Yuan said,’ the development of the theory is always after the experiment in Superconductivity Study. Scientists first found a new kind of superconductivity material by accident, and then they did some research on the superconducting mechanism of the material, expecting to manufacture a new material with higher critical temperature‘. In 1986 Bednorz and Muller’s discovery of lanthanum-based cuprate-perovskite ceramic materials known as high-temperature superconductors, whose critical temperature were soon raised to approximate 160 Kelvin, spurred renewed interest and research in superconductivity all over the world. However, after 20 years the critical temperature is still there and people haven’t understood the formation mechanism of high-temperature superconductors, which remains one of the Physics Puzzles in the world. Scientists hope to manufacture room-temperature superconductors and to reveal the secrets of high-temperature superconductors by finding new materials except cuprate-perovskite ceramic ones. In early 2008 the Japanese scientists claimed that lanthanum oxygen fluorine iron arsenide (LaFeAsO1-xFx)’s critical temperature was below 26 Kelvin. Subsequent research from Chinese groups suggests that replacing the lanthanum in LaFeAsO1-xFx with other rare earth elements such as cerium, samarium, neodymium and praseodymium leads to superconductors that work at approximate 60 K, breaking MacMillan's formulation of the BCS theory. The researchers from department of physics, Zhejiang University, also contributed enormously to this work. These new findings indicate that the new research period of iron-arsenic-based compounds is around the corner.

It is very important for a physicist to ascertain whether the iron-arsenic-based superconductivity material is similar to lanthanum-based cuprate-perovskite ceramic one studied before. ‘If they are not the same, which means that the new findings are more important than expected, a totally new superconducting mechanism will be discovered.’ predicted Nobel laureate and Theoretical physicist Philip Warren Anderson from Princeton University, USA. It is possible that the iron-arsenic-based superconductivity material help scientists understand the superconducting mechanism with high temperature.

Special engaged professor of the Cheung Kong Scholars scholar Huiqiu Yuan, who devoted much time to studying superconductivity and physical property under extreme condition, paid close attention to the peculiar properties of the new superconductors after the iron-arsenic-based superconductivity material was founded. As the user of the National High Magnetic Field Laboratory operated by Los Alamos National Laboratory, USA, Professor Huiqiu Yuan got the permission to use the pulsed field magnets. He began to delve into the behavior of the iron-arsenic-based superconductors in pulsed fields in April 2008; meanwhile he kept working closely with several domestic Sample-preparation groups. The sample related to this research was provided by Group Nanlin Wang from Institute of Physics, Chinese Academy of Sciences.

Critical field is one of the important parameter in superconductivity. If the excitation energy of the external fields is higher than cohesive energy of the superconductor, the material will turn from superconducting state to normal state. And the external fields is named Critical field. However, Type II superconductors, including iron-arsenic-based superconductors, show two critical magnetic field values, one at the onset of a mixed superconducting and normal state and one where superconductivity ceases. As the external fields are lower than lower critical field, the magnetic field can penetrate while still maintaining zero electric resistivity paths through the material; as the external fields are at the range of critical fields, superconductors are mixtures of normal and superconducting; as the external fields are higher than upper critical field, superconducting state is totally destroyed. Previous studies on copper oxides showed that superconducting properties were anisotropic because of the two-dimensional electronic properties. As the iron-arsenic-based superconductivity material has the similar structure to copper oxides, it is believed that superconducting properties of the iron-arsenic-based ones were anisotropic, too. Besides, early work on the iron-arsenic-based compounds seemed to support this view that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity.

Through the experiments Professor Huiqiu Yuan proved that the previous presumption is far from comprehensive. The critical field of the iron-arsenic-based compounds (Ba,K)Fe2As2 are independent of the direction of the external fields at low temperature, namely isotropic in a extensive range of Temperature - magnetic field phase diagram on the iron-arsenic-based material. Superconducting properties of the iron-arsenic-based compounds were quite different from the copper oxides, which depended on the discrepancy of the underlying electronic structure. Professor Huiqiu Yuan said that the underlying electronic structure of the iron-arsenic-based compounds appeared to be much more three dimensional than that of the copper oxides. So the reduced dimensionality in these compounds is not a prerequisite for 'high-temperature' superconductivity. In addition there are a lot of similar properties between the iron-arsenic-based superconductivity material and heavy fermion, especially in the interaction between magnetism and superconductivity. Therefore Professor Huiqiu Yuan speculated that the iron-arsenic-based superconductivity material might be the bridge between low- temperature heavy fermion and high-temperature copper oxides.

The iron-arsenic-based superconductivity material has wide potential applications due to its special upper critical field (100T) and its isotropy. As a Chinese saying goes, strike while the iron is hot, scientists are filled with confident hope for the further research on the iron-arsenic-based superconductivity material.

It was learned that the research sponsored by the Ministry of Science and Technology, the Ministry of Education, Chinese Academy of Science and National Natural Science Foundation of China( NSFC).