Superconductivity in some materials can be induced or tuned by chemical substitution, external pressure and magnetic field. Among these control parameters, pressure is a "clean" way to produce or manipulate superconductivity of matters without changing their chemistry. 

Recently, a new finding from Prof.ZHAO Zhongxian's group at Institute of Physics, Chinese Academy of Science, demonstrated an unexpected superconducting phenomenon in a newly discovered iron selenide based superconductors under pressure condition. In this investigation, Prof. SUN Liling, a member of this group, and her colleagues in collaboration with Prof. Ho-Kwang Mao and Dr. CHEN Xiaojia at Carnegie Institute found that the superconducting transition temperature (Tc) in this kind of superconductor decreases with increasing pressure from the maximum of 31 K to vanishing at around 10 GPa, then reappears at pressure above 11.5 GPa, which was identified by in-situ resistance and magnetic measurements in a self-integrated system developed by ZHAO's group. The Tc of the re-emerging superconducting phase reaches 48 K which is considerably higher than the first maximum (31 K), setting a new Tc record in iron selenide based superconductors. Interestingly, the basic structure of these compounds was not changed when the superconductivity reemerged. They believe that further studies can help to determine what is happening on a closer structural level in the pressure-induced superconducting state and shed insight on the underlying mechanism of high-Tc superconductors.

The work is the first observation on the pressure-induced reemergence of superconductivity among the high-Tc superconductors. It is also demonstrated that high pressure is an effective tool in manipulating quantum state of matters. This work is published by Nature on March 1, 2012 [Nature 483，67 (2012) ] and highlighted by Nature's news online.

The samples studied were provided by Prof. Xiaolong Chen and Prof. Genfu Chen from Institute of Physics, Prof. Minghu Fang from Zhejiang University. High-pressure x-ray diffraction measurements were performed at Shanghai Synchrotron Source and Advanced Photon Source. This project was supported by the NSCF, 973 projects, and Chinese Academy of Sciences.

http://www.nature.com/nature/journal/v483/n7387/full/nature10813.html
http://www.nature.com/news/superconductor-breaks-high-temperature-record-1.10081

Pressure dependence of theTc for Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2. The symbols represent the pressure–temperature conditions for which Tc values were observed from the resistive and alternating current susceptibility measurements; symbols with downward arrows represent the absence of superconductivity to the lowest temperature (4 K). All samples show two superconducting regions (SC-I and SC-II) separated by a critical pressure at around 10 GPa. NSC, the non-superconducting region above 13.2 GPa. The maximum Tc is found to be 48.7 K in K0.8Fe1.7Se2 at a pressure of 12.5 GPa. At higher pressures above 13.2 GPa, the samples are non-superconducting. Error bars are one standard deviation.（Image by Prof. SUN Liling etc.）