Article

Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As

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Abstract

Since the discovery in 2008 of high-temperature superconductivity in the iron pnictides and chalcogenides, a central issue has been the microscopic origin of the superconducting pairing. In particular, it remains unclear whether there is a bosonic mode from the tunnelling spectrum, which has a close and universal relationship with superconductivity as well as with the observed spin excitation. Here, on the basis of measurements of scanning tunnelling spectroscopy, we show clear evidence of a bosonic mode with energy identical to that of the neutron spin resonance in two completely different systems, Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As, with different superconducting transition temperatures. In both samples, the superconducting coherence peaks and the mode feature vanish simultaneously inside the vortex core or above the transition temperature Tc, indicating a close relationship between superconductivity and the bosonic mode. Our data also demonstrate a universal ratio between the mode energy and superconducting transition temperature, that is Ω/kBTc≈4.3, which underlines the unconventional nature of superconductivity in the iron pnictide superconductors.

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Acknowledgements

We acknowledge useful discussions with A. V. Chubukov, S. Kivelson, M. Norman, I. I. Mazin, I. Eremin and A. Balatsky. We especially thank A. A. Golubov, J. Schmalian and I. I. Mazin for their preliminary calculations in helping us to understand the data based on the Eliashberg theory. This work was supported by the NSF of China, the Ministry of Science and Technology of China (973 projects: 2011CBA00100, 2012CB821403, 2012CB21400, 2010CB923002, 2011CB922101) and PAPD. The single-crystal growth effort at UTK is supported by the US DOE BES No. DE-FG02-05ER46202.

Author information

Author notes

    • Zhenyu Wang
    •  & Huan Yang

    These authors contributed equally to this work

Affiliations

  1. Center for Superconducting Physics and Materials, National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

    • Zhenyu Wang
    • , Huan Yang
    • , Delong Fang
    • , Qiang-Hua Wang
    •  & Hai-Hu Wen
  2. National Laboratory for Superconductivity, Institute of Physics and National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China

    • Zhenyu Wang
    • , Bing Shen
    • , Lei Shan
    •  & Pengcheng Dai
  3. Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA

    • Chenglin Zhang
    •  & Pengcheng Dai

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Contributions

The low-temperature STS measurements were performed by Z.W., H.Y. and D.F., with help from H-H.W. and L.S. The samples were prepared by B.S. and C.Z. The simulation based on the Eliashberg theory was performed by Q-H.W. H-H.W. coordinated the whole work and wrote the manuscript, which was supplemented by Q-H.W., Z.W. and H.Y., and revised by P.D. All authors have discussed the results and the interpretation.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Hai-Hu Wen.

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