  Since the discovery of high temperature superconductivity in the iron
pnictides and chalcogenides in early 2008, a central issue has been the
microscopic origin of the superconducting pairing. Although previous
experiments suggest that the pairing may be induced by exchanging the
antiferromagnetic spin fluctuations and the superconducting order parameter has
opposite signs in the electron and hole pockets as predicted by the S+- pairing
model, it remains unclear whether there is a bosonic mode from the tunneling
spectrum which has a close and universal relationship with superconductivity as
well as the spin excitation. In this paper, based on the measurements of
scanning tunneling spectroscopy, we show the clear evidence of a bosonic mode
with the 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 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 [mode energy]/kBTc ~ 4.3, which underlines the unconventional mechanism of
superconductivity in the iron pnictide superconductors.
