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

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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Figure 1: Topographic STM image and the tunnelling spectra of the Ba0.6K0.4Fe2As2 single crystal.
Figure 2: STS spectra and their temperature dependence for the Ba0.6K0.4Fe2As2 single crystal.
Figure 3: Spatial mapping of the LDOS and STS spectra across a vortex core on the Ba0.6K0.4Fe2As2 single crystal.
Figure 4: Spatially resolved STS spectra on Na(Fe0.975Co0.025)As single crystal.
Figure 5: Temperature dependence of the tunnelling spectra for the Na(Fe0.975Co0.025)As single crystal.
Figure 6: A universal relation between the mode energy and the superconducting transition temperature Tc.

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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.

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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.

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Correspondence to Hai-Hu Wen.

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Wang, Z., Yang, H., Fang, D. et al. Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As. Nature Phys 9, 42–48 (2013). https://doi.org/10.1038/nphys2478

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