The spring of 2008 sees a special blossom of substantial breakthroughs in superconductivity research. Three groups of physicists with the CAS Institute of Physics (IOP) and one with the University of Science and Technology of China (USTC), one after the other, make remarkable progresses in the study of iron-based materials after the breakthrough made by H. Hosono's group in Japan, heralding renewed insights in the fundamental mechanism of high-temperature superconductivity (HTSC), a perplexing enigma on the frontier of condensed matter physics.
Enigma of high-temperature superconductivity
Superconductivity, the fascinating quantum phenomenon that occurs in certain materials at extremely low temperatures featuring the vanishing of electrical resistance and interior magnetic field, has attracted intense attention from scientists. The discovery of HTSC in copper oxides, or cuprate in 1986 has fueled new waves of enthusiasm for this topic and various cuprate high-temperature superconductors have been synthesized. For a long time, however, the underlying mechanism of HTSC remains elusive, though scientists have achieved a greater agreement with the mechanism of conventional superconductivity.
So far, a well-known effort to explain this magical effect in conventional superconductors is the BCS theory based on electron-phonon interaction coupling. This theory, which was developed by John Bardeen, Leon N. Cooper and John R. Schrieffer in 1957 and reaped the 1972 Nobel Prize for Physics, suggests that the disappearance of electrical resistance is due to the formation of electron pairs at extremely low temperatures, through their interactions with the crystalline lattice. Based on the understanding that electrical resistivity is resulted from the energy loss produced when traveling electrons collide with the vibrations of the crystalline lattices or defects in the material, it argues that at extremely low temperatures, no sufficient energy is available to separate these pairs, making it impossible to deflect them. In other words, no electrical resistance can arise in this case. Nevertheless, it is widely doubted that this theory can explain the superconductivity of cuprate materials, as the latter's electrical resistance turns into zero at a critical transition temperature (Tc) as high as over 138 kelvin (-135.15oC), which makes available the energy necessary for breaking the electron pairs (named Cooper pairs amongst the physics community). The limited materials available for theoretical exploration-the cuprate system had long been the only high-Tc superconductors known by human societies-made this mechanism even more mysterious. Against this background, the physicists have long been awaiting a non-cuprate system of high-Tc superconducting materials to gain new inspirations.
Breaking through
The latest confluence of discoveries of iron-based high-Tc superconductors by Japanese and Chinese scientists, as well as the ensuing debate is fostering new hopes for solving this predicament. Soon after the synthesis of the lanthanum oxygen fluorine iron arsenide (LaO1-xFxFeAs), a superconductor with a Tc of 26 kelvin (-247.15oC) by a Japanese group with the Tokyo Institute of Technology (TIT), three groups of researchers at the IOP and one group at the USTC reported their successes in inducing HTSC in different iron-based materials.
The IOP gained first limelight when two of its research groups, respectively headed by Profs. WANG Nanlin and WEN Haihu, confirmed the result obtained by the TIT group within one week after its publication. Then on March 25, a USTC group headed by Prof. CHEN Xianhui announced their successful identification of superconductivity at a Tc of 43 kelvin (-230.15oC) in the samarium oxygen fluorine iron arsenide (Sm1-xOxFxFeAs). Only one day later, Prof. Wang Nanlin's group reported another superconductor, the Ce(O1-xFx)FeAs, with a Tc of 41 kelvin. These two results, each independent from the other, mark a breakthrough, as the Tc went beyond 39 kelvin, the commonly assumed McMillan Limit and can be defined as unconventional superconductors. Subsequently, the later group also reported that the Tc could be further increased to almost 50 kelvin by substituting the element La with other smaller rare earth elements, for example, Nd.
The seemingly surprising blossom of significant discoveries at IOP is actually not so accidental as it looks. When asked about what has made it possible for IOP to respond so quickly and achieve so many breakthroughs in so short a time, IOP Director Prof. WANG Yupeng, a theorist specialized in condensed matter physics, attributes this to the IOP's build-up accumulated over its decades of efforts gone to research on cuprate high-Tc superconductors. ¡°We benefit from a powerful faculty, a very good environment for research and lasting support from the government which helps scientists focus on important issues on the disciplinary frontiers. This institute is emerging as a world known center of scientific excellence with a quite low annual budget of $25 million compared to those of western countries.
Achieving high-temperature superconductivity
Truly, opportunities favor those who are well prepared. Prof. Wang Nanlin's group has engaged in growing single crystals of LaOFeAs since last December in a hope of investigating their electronic properties. Another IOP fellow, CAS Member Prof. ZHAO Zhongxian made significant contributions to cuprate high-Tc superconductor research as early as in 1987. Once again in this spring, Zhao's team scored -quick and arresting achievements. On Mar 28, his group reported a Tc of 52 kelvin (-221.15oC) in a new superconductor, the praseodymium-arsenide compounds (PrO1-xFxFeAs). This was the first time ever a Tc above 50 kelvin to be achieved in a non-cuprate system, suggesting the discovery of a new high-Tc superconductor. Soon after that they reported another new system, the Nd[O1-xFx]FeAs with a Tc of 51.9 kelvin, explicitly confirming the potential of the quaternary iron-arsenide compounds as a new family of high-Tc superconducting materials. Again on 13 April, they further raised the Tc to 55 kelvin (-218.15oC) in Sm[O1-xFx]FeAs by using a novel method- a high-pressure synthesis method, which could make more fluorine doping to the oxygen site and get higher Tc. So far this remains the highest Tc achieved in all non-cuprate systems. Just several days later, moreover, they reported similar superconductivity in five lines of materials from the same quaternary family, depicted as ReFeAsO1-d, where Re indicates rare earth elements Sm, Nd, Pr, Ce or La. Rather than F-doping, in this work the superconductivity is induced by oxygen-vacancies in the crystal lattice, which they proved to be applicable to all the five lines and actually more effective an approach. The discovery of this general applicability is expected to facilitate later experiments and theoretical research in this direction by providing a much simpler studying platform.
Naturally, the exceptionally high critical transition temperatures achieved in this wave of discoveries have gained due acclaims over the world. Although some of the compounds in the parent family of the new system were found to show superconductivity at extremely low temperatures, for example between 3 to 5 kelvin, none was ever found to undergo any superconductivity above 20 kelvin. What is more striking, however, is the fact that HTSC could even be induced in iron-based compounds, which was widely believed to be a mission impossible because of the ferromagnetism of these materials, the natural foe of superconductivity.This anomaly is especially interesting for theorists because of the implied potential of greater discoveries.
Working on the theoretical frontier
Particularly, the identification of the possible connection between the spin-density-wave (SDW) instability and the resistivity anomaly in a new system is deemed to be an encouraging effort to reveal the HTSC mechanism. In cooperation with another IOP group led by Prof. Fang Zhong, Prof. Wang Nanlin's group discerned that a phase, the antiferromagnetic SDW instability, was competing with superconductivity in the LaO1-xFxFeAs system. The two groups demonstrated that this SDW instability might be responsible for the resistivity anomaly occurring at about 150 kelvin in undoped LaOFeAs. When the samples are treated with F-doping, however, the instability is suppressed and hence leading to the superconducting ground state. They further show that similar competing orders exists in other rare-earth substituted systems. The SDW instability in the undoped compound was later confirmed by the neutron diffraction studies conducted by two independent groups in the USA. The surprisingly high Tc achieved by Wang's group in CeOFeAs samples, as high as 41 kelvin, has suggested intriguing prospects implied in the competing phenomenon. In fact, being consistent with this work, Prof. Zhao's group has proved that producing oxygen vacancies in the system is more efficient an approach than F-doping to inducing superconductivity in this kind of materials and higher Tc has been achieved. It is believed that this discovery will inspire further pursuit of the fundamental mechanism of HTSC.
On the other hand, the discovery made by Prof. Wen's group also attracts extra attention, as it directly challenged the latest hypothesis proposed by the TIT group led by chemist Hideo Hosono. The TIT group explicitly stated in February in their paper that a critical factor for the occurrence of superconductivity in the new system of superconductors was electron doping, rather than hole doping. This implies that superconductivity can only be obtained by introducing extra electrons rather than "holes" (provided by positive ions) into the structure of the material. In only a month, however, the new results from Wen's group subverted this hypothesis by successfully inducing superconductivity in hole-doped materials of the same system. If confirmed by further experiments, this result might also provide some novel thoughts for the related exploration.
Looking at the future
The newly made progresses in iron-based materials, the only high-Tc system other than cuprate chemicals to date, might lead to greater clarity in superconductivity mechanism, one of the most abstruse problems in condensed matter physics, as believed by the researchers and some renowned theorists, including Nobel Prize winner Philip Anderson with the Princeton University of USA, as reported by the Science Magazine. Some researchers, including some from the IOP groups, are more cautious, however, believing that there is still a long way to go before we can really knock on the door to the core mysteries of HTSC mechanism.
Nevertheless, the people are optimistic about the future because of something beneath the surface. "This time we are thought to have taken over the lead," remarks Director Wang: "China is becoming a strong country in science due to the increased investment in fundamental research and the excellent team of scientists. Some young scientists have made outstanding contributions during the few months since their joining us, like Dr. CHEN Genfu with Prof. Wang Nanlin's team and Dr. REN Zhi'an with Prof. Zhao's group. We still have problems, of course. Most scientists are expecting more stable support from the government and more reasonable an evaluation mechanism, given their essential roles in generating innovative results."(SONG Jianlan)
