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

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. Different experimental probes have found different bosonic modes in the iron-based superconductors. A scanning tunnelling spectroscopy study of two separate superconductors now links the tunnelling mode with the ‘neutron resonance’, both of which vanish when superconductivity disappears.