Pressure has an essential role in the production1 and control2,3 of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature Tc to a record high of 55 K in these materials4,5. In the same way as Tc exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned Tc typically drops monotonically after passing the optimal pressure1,2,3. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the Tc drops from the first maximum of 32 K at 1 GPa. The Tc of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0–48.7 K for Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2.

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We thank I. I. Mazin, W. Bao, and T. Xiang for discussions and J. S. Schilling for the help with the alternating-current susceptibility technique. This work in China was supported by the NSCF, 973 projects, and Chinese Academy of Sciences. This work in the USA was supported as part of the EFree, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE-BES). The High Pressure Collaborative Access Team (HPCAT) is supported by CIW, CDAC, UNLV and LLNL through funding from the DOE-NNSA, the DOE-BES and the NSF. The Advanced Photon Source (APS) is supported by the DOE-BES.

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Author notes

    • Liling Sun
    •  & Xiao-Jia Chen

    These authors contributed equally to this work.


  1. Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China

    • Liling Sun
    • , Jing Guo
    • , Peiwen Gao
    • , Xiaolong Chen
    • , Genfu Chen
    • , Qi Wu
    • , Chao Zhang
    • , Dachun Gu
    • , Xiaoli Dong
    • , Xi Dai
    •  & Zhongxian Zhao
  2. Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA

    • Xiao-Jia Chen
    •  & Ho-kwang Mao
  3. Department of Physics, South China University of Technology, Guangzhou 510640, China

    • Xiao-Jia Chen
  4. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

    • Qing-Zhen Huang
  5. Department of Physics, Zhejiang University, Hangzhou 310027, China

    • Hangdong Wang
    •  & Minghu Fang
  6. HPSynC, Geophysical Laboratory, Carnegie Institution of Washington, 9700 South Cass Avenue, Argonne, Illinois 60439, USA

    • Lin Wang
  7. Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China

    • Ke Yang
    •  & Aiguo Li


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L.S., X.-J.C., H.M. and Z.Z. designed the project; X.-J.C., L.S., H.M., Q.W. and Z.Z. wrote the paper; P.G., J.G., C.Z. and L.S. performed the resistance and magnetic susceptibility measurements; X.-J.C., L.W., J.G., P.G., L.S., K.Y. and A.L. performed synchrotron X-ray diffraction measurements; and M.F., X.C. and G.C. synthesized the single crystals. All the authors analysed the data and discussed the results. All the authors read and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Ho-kwang Mao or Zhongxian Zhao.

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    Supplementary Information

    This file contains Supplementary Text, Supplementary Figures 1-2 with legends and Supplementary Tables 1-4.

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