the bosonic mode in Ba0.6K0.4Fe2As2 and

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. According to the Bardeen–Cooper–Schrieffer theory, the su-perconducting state is achieved by the quantum condensa-tion of paired electrons, and the electron pairing is provided by the electron–phonon interaction. The compelling evidence for this model was the tunnelling spectrum dI/dV versus V, on which some fine structures, due to the electron–phonon coupling, appear at voltages of∆+Ω (or at−∆−Ω), where ∆ is the superconduct